Transcript for:
Understanding Cell Cycle and Division

hello everyone and once again welcome to the amazing platform of physics voila where i ayushiya garwal your botany teacher is going to teach you the next chapter of class 11 cell cycle and cell division very important chapter very interesting chapter but before starting with the chapter i had a thought in my mind i would like to share that thought maybe this thought is going to change your perception towards life the thought says a winner is a dreamer who never gives up what is this lining means a winner is a dreamer who never gives up that means the people who succeed in life maybe like mukesh ambani rathan tata these all people are successful because they not only dream but they work hard to even achieve those dream simply dreaming not working hard is not going to become fruitful so winner are those dreamer first of all who never give up and also put hundred percent of their efforts to achieve their dream to achieve their goals so students it's a high time it's a very high time that now all your dreams are going to come true on this very day of your neat examination so now if you really want to achieve your dream and if you really want to fulfill your goals then already dreaming part is done now your hard work is going to pay off so please student everybody stay focused keep working hard till the time you do not achieve your target and with the simple thought of the day let's start with today's class that is cell cycle and cell division so this part is done moving ahead with the introduction part first of all the chapter says cell cycle and cell division already you have done the chapter cell the unit of life and now we are moving ahead with the next chapter which is just the continuation of the last chapter that is cell cycle and cell division in the last chapter you have studied about what a cell the structures of cell and the functioning of cell but now in this chapter you have to study that how the cells increase their number already rudolph varchau has told you that all the cells they arise from pre-existing man from the pre-existing cell by the manner of division so that line now we are going to study that how a parent cell divides to produce new daughter cell now division of a cell is not a random process it's not like if any egg cell wants to divide so without preparation without any activity it will just get up and divide and produce number of cells if this is happening then surely there is some trouble in your body but in normal condition there is whole sequence of events taking place in order to let a cell divide and cell division will only take place whenever and wherever it is required clear and those sequence of events which takes place prior to cell division together are termed as cell cycle so what a cell cycle how can you define the cell cycle the cell cycle means the sequence of all those events like growth of the cell dna replication duplication of cell organelles and etc so sequence of all the events which take place during the time of cell division is termed as cell cycle first point cell cycle cell division is completely under the control of genetics this whole process is genetically controlled that means nowhere cell will divide automatically whenever and wherever the cell division is required then only that cell is going to divide that means it is completely under the control of your genes am i correct or not okay very good so let's read one by one what the statements are written these statements are directly taken from ncrt hence it becomes very important to read and to understand that what each statement wants to convey you never know that any of the question can be framed from any of these sentences so reading ncrt understanding ncrt and then solving question is very very important part so now let's read the sequence of events by which a cell duplicates its genome that means dna replication apart from dna replication organelle duplication take place energy synthesis takes place cell grows in size all these things together is known as cell cycle right now next point these events are under the control of genes so whole cell cycle is controlled by your genetic system right okay now these two points i'll discuss while i'll go deep inside the cell cycle now you know what is cell cycle cell cycle can broadly be divided into two phases if i want to study the events of cell cycle then it's a sequential manner no random so cell cycle can broadly be divided first into two phases as interface and m phase right interphase is actually the long-lived phase of the cell cycle where the cell prepares itself for division during whole interface the cell growth is taking place suppose there is a parent cell who is x amount of size but if the cell if that parent cell will enter into interphase then its size is going to increase why because the amount of cytoplasm during interphase is increasing clear so during whole interface cell grows in size due to various events and once the cell receives or achieves the complete size once the cell gets and achieves the full and fledged size then it escape interface and enter into m phase where actual division takes place correct so now read the second statement although cell growth in terms of cytoplasmic increase is a continuous process during whole interphase the cell growth in terms of cytoplasmic increase is taking place however there is one phase in the interface that is only restricted to allow dna replication so during complete cell cycle there is only one step stage where the dna replication takes place otherwise cytoplasmic growth is taking place throughout the interface however dna synthesis occurs only during one specific stage in the cell cycle while i'll discuss the details of interface i'll tell you its respective stages and where what happens then you will notice that actually cell growth is taking place in all the substages of interface but specifically if i talk about dna replication then it is only taking at one specific stage of the interface in the cell cycle clear next the chromosomes dna synthesis is taking place the amount of dna during cell cycle is doubled it is increased so it is saying that replicated chromosome are then equally distributed to their daughter cells and for this for equal distribution of genetic material to the daughter cell the next phase that is m phase is required that means cell cycle is a sequential event so you can define cell cycle as the sequence of events which are taking place to initiate and to help in cell division right cell cycle can be broadly divided into two phases interface m phase during interphase cell prepares itself for division whereas during m phase cell actually divides whatever it has prepared inside inter in during interface now during m phase it is the time to divide all those material into their daughter cells equally right now during interface whole interface cell growth is taking place in terms of cytoplasmic increase but the genetic material is only replicating doubling during one specific stage of the interface right now if i talk about time then different cells they have different timing of their cell cycle like for example if i talk about our human cell then our human cell the total time it takes to divide is about 24 hours and like this if you talk about some other species cell then their timing will be variable so you have to remember that human cell divides in every 24 hours and this time is not fixed for every cell it is variable and from species to species it varies clear okay very good so now moving ahead with the phases of cell cycle let's start with the first interface i'll explain whole interface and then we'll write about m phase interphase is also known as resting phase or preparatory phase during interphase actually the cell is not dividing but it is preparing itself to undergo division like for example if your mother wants to prepare vegetable like for example palak paneer at your home so what will she do will she go in the kitchen and directly will she start preparing and just come and serve it to you or before starting the gas and keeping the vessel on the gas she'll go to the market she'll bring paneer she'll bring pallak and then she's going to wash them chop them right so before just keeping the vessel on the stove there are so many preparations done behind it right so then only your vegetable your curry can be prepared so similarly the cell if wants to divide it cannot simply get up and undergo division then it will be a wrong division daughter cells will not be appropriate hence every cell before dividing has to first prepare itself and that preparatory phase where the cell is only preparing itself for division is known as interphase so interface also known as preparatory phase right it's a preparatory phase as cell prepares to divide now there is a misnomer that interface is also known as resting phase simply resting is called because division is not taking place but actually the cell is resting in interface no it is working even more because it is preparing itself it is duplicating the organelles replicating the dna producing proteins rna nucleotides so many work is being done by the cell when it is an interphase but still it is known as resting phase which you can call that it is a misnomer or even if you want to justify that yes interface is resting phase then your answer should be only in terms of lacking division in this space correct so second point is also known as resting phase maybe a misnomer because cell is not dividing during this stage only preparation is being done correct otherwise it is a misnomer because cell is not actually resting it is doing lot of work third point now preparation take lot of time right division see when your mother has to go to the market bring spinach bring paneer and she has to wash chop prepare the curry and it takes lot of time but when she serves it to you you hardly take five minutes and you fat off what you eat and then you go and play in the ground right so preparation always takes lot of time and actual work takes limited time so total time of cell division if it is 24 hours like for example in human cell so 95 percent of those 24 hours will actually be occupied by interface m phase only requires five percent of the total time so that means interface is the longest one which consumes about 95 percent of the total cell division time correct now this interface can be divided into three sub stages and one by one you have to study all these three sub stages in detail they are in sequence g one s and g2 hamesha always remember these stages in sequence you cannot keep g2 above then s or s below than g2 no the sequence has to be followed g1 s and g2 once g2 is done then that means interphase is completed cell has to escape interface and enter into m phase m phase is the actual dividing phase it is called as the most dramatic phase of the cell cycle because actually here division is going to take place it takes only five percent of the total cell cycle duration so first point you will write dramatic phase it is the most dramatic phase of the cell cycle why because actual cell division takes place during m phase right it only takes five percent of the total time right that means if 24 hours are there then 95 percent will be occupied in interface and 5 will only be occupied during m phase now this m phase can be further divided into two steps m phase can further be divided into two steps which is first nuclear division called as karyokinesis and then cytoplasmic division called as cytokinesis correct yes or no so this m phase can also be divided into two broad categories as karyokinesis where nuclear division takes place right and then followed by cytokinesis where the cytoplasm undergoes division finally dividing the parent cell into its respective daughter cells this karyokinesis is again divided into four stages in sequence they are prophase metaphase anaphase and telophase so overall how many stages are there in cell cycle starting from g1 then s g2 once g2 is completed cell will enter into m phase m phase means prophase metaphase anaphase telophase and then finally division of the cytoplasm completing the cell cycle so if you want to represent this cell cycle in the form of a diagram then how will you depict it very important two years back question on this diagram was asked this diagram i have taken from ncrt which i'm going to draw it in front of you please pay attention and yes it is very very important all the sub stages one by one we will discuss in detail so for example you are having one cell parent cell it is going to undergo cell cycle and thus it will divide finally once it completes the cell cycle it divides into daughter cells right now if the cell cyc if this cell wants to enter into cell cycle it is first going to join the g1 phase after completing the g1 phase it is then going to enter into s phase once completing the s phase it will enter into g2 phase so you can see that major portion of the cell cycle is actually being completed in interphase which is approximately 95 percent of the total time correct now remaining five percent of the time is completed in m phase which is divided into two steps that is karyokinesis followed by cytokinesis so this is your cytokinesis karyokinesis can be subdivided into four stages prophase metaphase anaphase antillopro meta ana and telo correct and once cytokinesis is completed the cell ultimately divides into two daughter cells so this is how the cell cycle so this is your m phase yes or no clear to everybody which takes only five percent of the total time so this is how the cell cycle has to be depicted now my dear students is the cell always dividing no is my skin cells right now dividing maybe no if not then where are they present so such cells who do not want to proliferate who do not want to divide escape the cell cycle through g1 phase and enters into a sort of resting phase called a question stage called g naught now some cells they remain in g naught permanently but some cells can also again come back to the cell cycle as per the requirement through g1 state so the cells who wants to divide keep on rotating in the cell cycle the cells who do not want to proliferate further rather they want to perform function they escape the cell cycle through g1 phase and enter into a question stage called g note here the cells can rest temporarily or even permanently so this is how the cell cycle seems to be like in a diagram form now we are going to study all these stages one by one starting from g one so sub stages of interface first is g1 g stands for growth or gap so this is growth one or also you can say gap one phase this is the first phase of interface where actually cell growth is taking place in terms of cytoplasmic increase why because there is a synthesis of proteins rna nucleotides organelle duplication also takes place during g1 phase so g1 phase is characterized by cell growth cell growth in terms of cytoplasmic increase right how because during this phase certain organelles like endoplasmic reticulum is undergoing duplication so here you can write organal duplication takes place apart from this like endoplasmic reticulum apart from this then there is also synthesis of proteins required for cell division nucleotides like in the form of energy atp gtp and etc so thus g1 phases characterized is simply a growth phase but during g1 there is no dna replication for dna replication the next very important stage is s phase s stands for synthesis phase s stands for my dear students synthesis now what happens during s phase during space in the cytoplasm there is duplication of centrosome cell organelle as well as synthesis of specific protein called histone other proteins are also uh getting duplica are getting increased but the specific one which i am mentioning over here is a stone protein so you have to remember that so during s phase in the cytoplasm of the cell what is happening organelle duplication which organelle centrosome centriole is getting duplicated apart from this synthesis of proteins takes place right which protein i should mention specifically histone proteins correct now coming to the nucleus here what happens very important students if you understand this little bit small concept now it is going to become very easy once we start with m mitosis and meiosis see once the cell is was in g1 phase the genetic material the chromosomes are actually present in the form of thin thread like chromatin fibers you remember cell cell chapter i told you that during interphase the genetic material is represented by thin thread like chromatin fibers right suppose i am talking about a diploid cell so suppose this is a deployed cell right a deployed cell cell means having two complete sets of chromosome like if i talk about my skin cell which is a deployed cell it is having two complete sets one complete set have 23 chromosome the other complete set have another 23 and total it becomes 46 chromosome okay so i was talking about a diploid cell suppose a deployed cell i have for example as i told you my skin cell that is having two complete sets of chromosome now the two complete sets of chromosome i am going to represent it like this see suppose this is one complete set and this is another complete set both the sets the genetic material is present in the form of thin thread like chromatin fiber there is upper arm lorem attached with centromere but those arm are thin thread like now what happens during g1 phase these deployed cell these two complete sets remains as it is without any change in them but once the cell escapes g1 phase and enters into s phase what will happen to the genetic material dna replication will take place the amount of dna the amount of dna is going to double without changing the number of chromosome right the number of chromosomes will remain 46 but however if initially the dna content is 2c after s phase it will become 4c the number of chromosomes will remain 46 but suppose initially the amount of dna was 20 picogram so after s phase it will become 40 picograms see how a deployed cell now dna replication will take place each chromatin fiber is going to produce one more copy of itself attached to the same centromere right now it is going to become like this initially one complete set was represented by single thread from each centromere but after dna replication one more thread will be attached complementary to the existing one so number remains same one complete set another complete set so still the number is 2n only 46 but now the amount of dna has doubled initially it was 2c so here it becomes 4c clear to everybody that what happens during s space cell growth is taking place s phase is also characterized by cell growth in terms of cytoplasmic increase as well as nuclear increase cytoplasmic increase synthesis of protein and duplication nucleus may dna replication but without changing the number of chromosome now moving ahead after s the cell is going to enter into g2 phase what will happen g2 phase is also just like g1 phase where cell growth is taking place only in terms of cytoplasmic increase by alloying protein synthesis specifically tubulin protein is synthesized during g2 phase along with organal duplication like mitochondria and chloroplast they duplicate during g2 phase and then align the cell growth but without changing the number of chromosome or amount of dna so g2 will also be represented by deployed cell and amount of dna as 4c because already replication has been done in s phase so once the cell escapes s phase enter into g2 phase the amount of dna as compared to g1 has become double but the number of chromosome remains the same clear yes or no very good so what is g2 phase g2 phase means first of all it is characterized by cell growth cell growth takes place in terms of cytoplasmic increase how letting the synthesis of proteins specifically tubulin protein is synthesized during g2 phase then organelle duplication which could not duplicate in g1 and s like mitochondria will duplicate in g2 so organal duplication takes place like for example mitochondria which could not duplicate in g1 phase will duplicate in g2 phase and the amount of dna as compared to in comparison to g1 is double is twice that means if g1 is having 2c then it will have 4c clear so this is the amount of dna which was present in g1 and this is the amount of present in g2y because in mid there was s phase i hope the thing is clear however the number of chromosomes will remain the same so this is about your interface interface is done now moving ahead there is one more stage but not the part of cell cycle that is g naught or question i already have shown you in the diagram so come back to the diagram once so g naught is not the part of cell cycle when the cell wants to suspend its division or does not want to further divide or want to exit the cell cycle then it enters into question stage or g naught stage so talking about g naught phase first and then we will come to m phase g naught phase means also known as question stage right now when the cell does not want to divide it does not want to proliferate so when cell do not want to divide when the cell do not want to divide escapes the cell cycle right through which phase through g1 stage through g1 phase and enters into g naught phase during g not phase the cells are not dividing they are sort of permanent cells which have lost their capacity of division but now they are rather performing some specific function now there are two types of permanent cells you already know you are students of anatomy that some cells who have temporarily lost their capacity of division like for example my skin cells and some cells have permanently lost their capacity of division those who have permanently lost their capacity of division my dear student they remain in g not phase once they enter into it right they cannot re-enter into the cell cycle but the cells who have temporarily lost their capacity of division as and when required they can re-enter the cell cycle through g1 divide complete the job again exit the cell cycle suspend the cell cycle and enter into g naught phase like your skin cell suppose right now my skin is perfect but if some damage occurs then healing is required now and for that new cells are required then my existing skin cells who are currently in g naught phase will enter into g1 produce new cells heal my injury and again come back to g naught right yes or no very good and one more thing my dear student g naught phase is also referred to as resting phase sometimes but this is also a misnomer because during g naught phase also cell may not divide but still it is metabolically active because it is performing function so here you will write cells are even metabolically active during g naught phase so calling g naught also a resting phase is misnomer like that of interphase i hope till here interphase g one s g two and what is g naught is clear in everybody's mind and if yes then let's start with m phase clear now m phase as i have already told you it is the most dramatic period of the cell cycle that only requires five percent of the total time of cell division most dramatic means preparation is already being done now the main job of division main job of dividing parent cell into new daughter cell is going to happen during m phase correct so it is the most dramatic phase where you can see all the drama happening condensation of genetic material and so and so we will teach you one by one everything clear very good now this m phase as i have already told you can be divided into two broad stages karyokinesis where nuclear division takes place cytokinesis where cytoplasmic division take place now first talking about karyokinesis they can be two ways of nuclear division and sometimes we also say there can be two types of dividing phases to be more specific there are two types of nuclear division or karyokinesis that is mitosis and meiosis correct so the two ways how nuclear division can take place once nuclear division has taken place ultimately cytokinesis comes which is same in both the cases so first we are going to study about mitosis type of karyokinesis and then we will study meiosis type of karyo kinases and the here from here onwards the importance of the chapter becomes even more better so why do waste time and blood let's directly jump into mitosis oh ho i have given heading as meiosis please please just change this and make it as mitosis my error human errors humans are full of errors and it's always good that when you rectify your error you accept your mistake and you have courage to get them even corrected right so never be shameful of your mistakes accept it with full heart and try to rectify it and try not to repeat them so like here i wrote meiosis but actually i was about to start mitosis right okay so what is mitosis mitosis is a type of karyokinesis where equational cell division takes place now what is equational cell division why are we using the term equation for mitosis so before i explain you these thing let me tell you one more point mitosis can take place in both haploid as well as diploid cell a cell who's having one complete set of chromosome can also show mitosis and surely a deployed cell a cell which is having two complete sets of chromosome can also show mitosis right chalo so before i start with the main process of mitosis first i will explain you that why it is known as equational cell division so if i am having a parent cell i think white color will be better okay so now first i'm having a diploid cell which is having how many complete sets of chromosome two this is a cell which is in g1 phase right an amount of dna is also suppose to see now after g1 phase the cell will enter into s phase where you know dna duplication takes place so amount of dna becomes 4c but since the number of chromosome does not change so the [Music] ploidy remains 2c 2n sorry right now it is going to enter into g2 phase again same 4c 2n same as s phase now my dear student after completing g2 phase this cell is going to enter into m phase suppose mitosis type of karyokinesis is taking place then in that case this parent cell will divide and produce two new daughter cells so during the time of mitosis type of karyokinesis at the end one parent cell produces two daughter cells which are genetically identical to each other how because both the daughter cells receives the same amount of genetic material as present in their parent cell correct yes or no okay so what happens after mitosis both the daughter cells will become 2n 2c 2n 2c they will receive two complete sets of chromosome they will receive two complete sets of chromosome having 2c amount of dna so your initially parent was also 2 and 2c and finally both the daughters which you achieved they also have 2c amount of dna with deployed condition so you can see that the parent and the daughters are identical to each other that is why this division is known as equational as both the daughters both the daughters are receiving equal amount of genetic material so you can say that in case of mitosis why it is equational division because both the daughters at the end of division receives equal amount of dna as that of parent clear yes or no so that is why this is known as equational cell division till here you have studied now you have to study that how this event is taking place so now let's start with mitosis in detail you know mitosis is a type of karyokinesis which is divided into four stuffed substages prophase metaphase anaphase and telophase so first we are going to study prophase my dear students prophase is the stage which directly comes after the cell completing g2 phase so imagine if the cell has just entered prophase you can also called as early prophase so how will it look like if you are observing it under the microscope if the cell is present in early prophase it has just entered the prophase completing g2 then if you're looking it under the microscope how will it appear to be see so what will happen there will be a suppose i'm talking about an animal cell so there will be a plasma membrane well genetic material called nucleus apart from the nucleus cytoplasm will be there in the cytoplasm all the cell organelles will be found but in the double amount suppose initially there was one er so in prophase two er if it was suppose one centriole then two centriole if there's one golgi then two all the organelles will seem to be doubled and the genetic material will also be the amount the number will remain same but the amount of genetic material will also get doubled but the genetic material will appear to be like all the threads will be intermingled and you cannot just identify can you see how many genetic how many fibers how many chromatin fibers have i drawn in this circle in the nucleus no so now we are talking about prophase which is the first step of karyokinesis that is the first step of mitosis now as i was telling you if the cell has just entered the prophase then you can see how the cell would appear like under microscope it will have genetic material the double the amount of genetic material but all those genetic material will be present in entangled manner there will be no clear distinction that how many fibers how many chromatin fibers are present in this so this is how the early prophase cell will look like in the cytoplasm all the cell organelles would have doubled and the genetic material in the nucleus would have also doubled in terms of dna replication but not in terms of number but nothing is as such visible because everything is entangled now as prophase cells start so it starts with certain events one of the major event of prophase is condensation of the genetic material the genetic material which is present in the form of thin thread like it start getting condensed to form thick rod like structures correct so these one of the major step that takes place during prophase is condensation of genetic material apart from condensation of the genetic material the nuclear membrane which you can see the nuclear membrane which you can see along with other duplicated cell organelles they also start getting disintegrated the cell organelles along with the nuclear membrane will get disintegrated second event third event centrosome which has duplicated its number now will start moving towards the opposite pole third event fourth event now the microtubules which are present in the cytoplasm they will join together to form thread like structures called spindle fibers so these are certain events that takes place during prophase now let's try to read them from here number one profis actually is the first stage of karyokinesis that follows the g2 phase of the interface after s space you know this thing second it is marked by the beginning of the as i told you condensation of the genetic material right now genetic material are present in thin thread like manner now both the chromatin fibers of the centromere will undergo condensation to form chromatids and two chromatids on one centromere forms your chromosome and these two chromatids of the same chromosome are known as sister chromatid correct so condensation of both the threads will take place to form full-fledged chromosome so this even takes place during prophase apart from this you can see that centromere centrosome that is a cell organelle it's which is already duplicated during the s phase it also start moving towards the opposite pole along with the cell organelles as well as the nuclear membrane disintegrate during the end of prophase as well as spindle fiber formation also takes place so if i have to draw the complete cell once it has completed the prophase then see how my cell will look like this is the condition of the cell when it has just entered the prophase now i'm going to show you the cell which is present in its late prophase or you can say towards the beginning of the next stage that is metaphase so during late prophase first of all centrosomes the cell organelles which duplicated during the s phase will occupy the pole region right these centrosome once they reach towards the opposite pole they they give rice some micro tubular asters proper star-like appearance so they produce micro tubular asters and these asters helps in the attachment of spindle fibers which are also made up of micro tubules so asters as well as spindle fibers together they make mitotic apparatus clear nuclear membrane and your centrosome and other cell organelles will get disintegrated they will not appear toward the late prophase or towards the beginning of early metaphase and the genetic material which is now present in the form of chromosome will hang themselves to these spindle fibers with the help of their kineto cores so with the help of the kinetochores the chromosomes will hang themselves all 46 chromosomes will hang themselves to the spindle fibers clear so this is how the actual cell will look like once the late prophase or early metaphase is going to begin so what are the events movement of centrosome towards the opposite pole formation of aster that lets the attachment of spindle fiber together called mitotic apparatus condensation of genetic material now my dear students once the condensation is done of the genetic material then actually you can count the total number of chromosome which was initially not possible as genetic material was not having its specific identity they were all entangled they were forming a mass of threads but now once the condensation is done then actually you count you can count the total number of chromosomes present in a diploid or if it is a haploid cell but here i am taking an example of diploid cell clear cell organelles they get disintegrated nuclear membrane nucleolus also disappears now this is the events that takes place during late prophase now late prophase means marks the beginning of metaphase see my dear students there are no clear-cut distinctions between the different stages of karyokinesis it's just like for our convenience scientists in order to study they have made they have made some boundaries that okay here prophase ends then metaphase then anaphase and then telophase but actually in reality there is no such distinction that here prophase ends then metaphase start why because all the four steps are taken in such a manner one after the other that clear cut tube to make clear-cut distinction is very tough so now once the prophase ends it marks the beginning of metaphase and metaphase is characterized by complete condensation of genetic material during metaphase the genetic material will be present in their thickest and shortest form called chromosome their condensation began in prophase almost done in prophase but complete condensation complete thick rod like chromosomes are appeared in metaphase stage so metaphase stage are characterized by complete condensed chromosome first point second complete disintegration of nuclear envelope and other cell organelles also mark the beginning of metaphase so the complete disintegration of the nuclear envelope marks the start of the second phase of mitosis so actually this diagram you can say late prophase or transition to metaphase because here you can see that there is no nuclear envelope there are no cell organelles proper chromosomes are present attached to the spindle fiber clear yes or no so this diagram can also be drawn as early metaphase so early metaphase is characterized by complete disintegration of the nuclear envelope and other cell organelles and now what will happen if the nuclear membrane will completely get disintegrated so the genetic material will surely come in the cytoplasm of the cell will they float in the cytoplasm no the genetic material which are now represented by chromosome right which are represented by chromosome which are now represented by chromosome come in the cytoplasm the genetic material is present in its thickest and shortest form highly condensed form right and are they floating in the cytoplasm no they're attached to the chromosome with the help of spindle fibers now we also say that metaphase is the best stage to study the morphology of the cell if you want to study their sizes their number etc whatever morphology you want to study metaphase is the best stage to study the morphology of chromosome chromosomes they are represented by two chromatids on one centromere having kinetochore disk shape structure on its side so with the help of kinetochore spindle fiber they attach and then they help the chromosome hang and hang in the cytoplasm clear now this diagram which i have drawn for late prophase can also be considered for early metaphase so centrosomes they have reached the pole they have asters coming out to which spindle fibers have attached right chromosomes are now what they are present in their thickest and shortest form and they're hanging with this chromosome of their hanging with the spindle fiber with the help of kinetochore like this but however that distribution is right now random during early metaphase the distribution of chromosome in the cytoplasm on the spindle fiber is random towards the end of metaphase all these chromosomes the spindle fibers they pulls and pushes the chromosome so that all the 46 chromosome if it is a human deployed cell to come and align in the center so what happens towards late metaphase all the chromosome they will come and get attached to the they get aligned to the center so right now you can see that chromosomes are haphazardly arranged on the spindle fiber but once the metaphase is going to get completed all the 46 chromosome will align themselves on one plate one after the other and this is known as metaphasic or equatorial plate so actually the the plane of alignment of the chromosome at metaphase is termed as metaphasic plate so because of this metaphasic plate the stage is known as metaphase so what is the main characteristic feature of metaphase formation of this plate formation of this equatorial plate will all the chromosome align in a row one after the other clear so this is how your metaphase stage is completed you should remember that it is the best stage to study the morphology of chromosome each chromosome is made up of two chromatids which are held together by a centro mir i hope all these points are clear and metaphase is also done now after metaphase is completed the next stage is anna phase anna face my dear students it is characterized by the splitting of the centromere now see what was the condition in metaphase this was the condition all the chromosomes they were hanging on the spindle fiber and they all were arranged in a sequence one after the other right so this is what the condition was when i talk about metaphase so all the chromosomes are aligned at the same equatorial plate first thing now what happens during anaphase the centromeres they split these are the centromeres can you see the orange color which i'm marking now if centromere is going to split from the center that means the two sister chromatids which are held together by a centromere they will also get separated see this is the condition initially now centromere is going to split so what will happen both the sister chromatids will get separated from each other here right now 46 chromosomes that means 92 chromatids are there correct 46 chromosomes were there it is a deployed cell 2 and 46 so 46 chromosomes are there that means how many chromatids 92 as one chromosome have two sister chromatids so if splitting takes place then what will happen all the 92 chromatids will get separated from each other 46 chromatid will be pulled on one side and remaining 46 chromatid will be pulled on the other side how because imagine there is a rope correct in which you try to keep your rig in which you keep your clothes there's a rope your mother has tied a rope under the sun for drawing the clothes and you have kept the clothes for drying suppose if that rope gets cut from the center what will happen half of the rope carrying the clothes will go towards one end and remaining half of the rope carrying the clothes will go to the other end this is only going to happen now because pressure is being applied from two ends so after the rope gets cut off it gets chopped up from the center the pressure which is applied from left will pull the half of the rope towards left and the pressure which is applied from the right is going to pull the rope towards itself so same thing happens during the splitting of centromere when the centromere splits from the center actually spindle fiber is getting cut from the center right now the spindle fiber is complete but if the centromere splits that means along with the splitting of the centromere the spindle fiber is also getting cut off from the center and if this is going to happen then you know the ends of centromere the ends of spindle fiber are attached to the centrosome right so half of the spindle fiber this will be pulled upward and this remaining half will be pulled downward yes or no so as soon as centromere split the spindle fibers they move towards the opposite pole carrying each chromatid getting my point yes or no so now what will happen soon after splitting of the centromere soon after the splitting of centromere see what will happen this is the centrosome cell organelle having asters to which spindle fibers are attached now splitting took place because of which because of which spindle fiber also got cut from the center so see i've cut it i've just cut the spindle fibers from the center right now what will happen the centromere splitting key versus say the chromatids which got separated now they will start moving towards the opposite pole like this with the centromeres moving ahead and arms trailing behind correct so now here chromosomes are no more present chromosomes were only there till metaphase in anaphase due to splitting now you will use the term chromatids so total 92 chromatids are there due to splitting of 46 chromosome out of which 46 chromatid move towards one pole and remaining 46 chromatid move towards the other pole getting my point with centromere leading centromere leading and arms trailing behind so 46 chromatids are going towards one pole and remaining 46 chromatids are moving towards another pole as spindle fibers are attached to the poles so poles are going to pull the remaining part of the spindle fiber as it gets cut from the center correct so this is anaphase centromere splits the chromatids separate and chromatic they move towards the opposite pole now what will happen chromatid will reach towards the pole thus marking the end of anaphase and beginning of telophase what happens during telophase telophases actually can call the reverse of prophase whatever whatever happened in prophase is going to just get reversed in telophase so telophase first of all here the genetic material which is now present in the form of chromatid will get decondensed once again the genetic material is going to lose its identity and again they will become in thin thread like manner right first point second nuclear membrane nucleolus and other cell organelles which actually got disintegrated will now reappear third spindle fibers which were formed asters which were formed all those will now get disintegrated so what happened during anaphase what was the diagram of anaphase c splitting of centromere took place because of which spindle fiber also got cut from the center carrying each chromatid towards the opposite pole with centromere leading and arms trailing behind like this so now what will happen it is going to move on one side and 46 chromatids will move on one side now during telophase first of all these 46 chromatids will move towards one pole now all these spindle fibers and asters will get disintegrated 46 chromatids they will decondense to form again original chromatin fibers as it was present in their parent cell nuclear membrane will start appearing so one nuclear membrane will be produced around this 46 chromatids and one nuclear membrane will be produced around this 46 chromatid so one nuclear membrane another nuclear membrane nucleus will reappear spindle fibers will get disintegrated this nucleus will get its own centrosome this is going to get its own centrosome here 46 chromatids will disintegrate to again form thin thread like chromatin fiber again in time guild here also 46 chromatids will lose their identity will get the condensed to reform the chromatin fibers now towards these are the characteristic features of telophase this is how the cell will look like in telophase see chromosome cluster at opposite spindle poles and they lose their identity by again getting decondensed second nuclear envelope reappear along each cluster one cluster upper and one cluster towards the lower end and then all other reorgans are also re-formed right now with the end of telophase soon the next cardiokinesis is done now next step is division of the cytoplasm so soon after ending telophase the cytokinesis also begins and during late telophase in animal cell the plasma membrane also start constricting inward so telophase may he within telophase you will see the beginning of cytokinesis by the constriction of plasma membrane in case of animal cell so this is all about telophase now karyokinesis is done you can see two nuclei per cell now two nuclei per cell each having 46 46 chromatin fibers you started the same with parent cell correct parent cell was also having 46 chromatin fibers in the beginning due to dna replication the amount of dna got doubled but here you can see the doubled genetic material you have separated and you have equally distributed in two nuclei each again having 46 46 chromatids or chromatin fibers so can you see that how this mitosis is equational cell division ultimately at the end of telophase you obtain two nuclei per cell each having same amount of genetic material as that of parent cell that is 46 46 chromatin fibers each now it's the time to separate these two nuclei and establish themselves as independent cell and for this cytokinesis is important so now next and the final step of mitosis is cytokinesis which is division of cytoplasm now my dear students for cytoplasmic division there are different methods in animal cell and plant cell for example an animal cell due to the absence of cell wall and outer delimiting membrane being cell membrane which is living and can show constriction so what happens in animal cell plasma membrane constricts inward simply like this the plasma membrane is going to constrict inwards this is one nuclei and this is another nuclei so this is plasma membrane which is constricting inward this constriction will keep on growing towards the center called centripetal movement and finally meeting each and finally meeting each other thus dividing the parent cell into two daughter cell so animal cells they show constriction in the plasma membrane to allow cytokinesis and this type of movement from outward to inward is centripetal movement correct but if i talk about plant cells my dear students so plant cells the outermost membrane is not cell membrane they have cell wall which is dead and cannot grow inward therefore in case of plant cell there cannot be centripetal type of cytokinesis rather they show centrifugal type of cytokinesis where the division of cytoplasm begins from the center and then that moves towards the opposite pole and for this for plant cell there is establishment of cell plate so slell plate is formed in the center of the two nuclei of the plant cell this cell plate then grows outward and thus finally divides the plant cell into two new daughter plant cells cell plate is formed with the help of different mitochondria golgi bodies and other cell organelles in the plant cell suppose like this this is a plant cell parent plant cell which has just completed its karyokinesis so per plant cell there will be how many nuclei two nuclei now they cannot show constriction because the plasma membrane is not the outermost layer it is cell wall and cell wall being non-flexible it cannot come inward therefore cell plate is formed with the help of cell organelles like er golgi body they get deposited in the center and then this is known as cell plate which start growing towards the opposite side showing centrifugal movement finally when the cell plate meets the end of the wall the parent cell also divides into two daughter cells later my dear students the cell plate behaves as middle lamella it shows deposition of calcium and magnesium and then it is known as middle lamella clear that how cytokinesis also takes place in case of animal and plant cell i hope with this mitosis concept is clear in each and everybody's mind so what is mitosis it is basically equational cell division where one parent cell divides to produce to two daughter cells each receiving same amount of genetic material first point how this happens because there is only one round of dna replication during s phase where the amount of genetic material is doubled without changing the number of chromosomes right ultimately when during anaphase when splitting of centromere takes place all 46 chromatids in a deployed human cell they separate and they produce 92 chromatids right out of which 42 move towards one pole 46 move towards one pole and remaining 46 move towards another pole so these 46 46 chromatids then decondense to form 46 chromatin fibers towards telophase and once nuclear envelop reappear everything reappears cytokinesis takes place producing two new daughter cells each having 46 chromatin fiber as that of parent cell so with this mitosis equational division one of the type of karyokinesis is done i hope you have understood till here and please revise it from ncrt check the diagram from ncrt and then please try to copy paste them in your mind it is very very important chapter some theoretical and some numerical based questions are asked from this chapter which i will also do so now we are going to solve some questions both numerical and theoretical based from this chapter we are going to see some questions related to it so the first question which i am going to discuss related to mitosis is which of the following duplicates in the cytoplasm during s phase s space is one of the preparatory phase it which comes after g1 phase and s phase is characterized by the duplication of centriole cell organelle in the cytoplasm and replication of the genetic material inside the nucleus so the question is asked that what happens during s phase in the cytoplasm question specifically mentioned the during s phase in the cytoplasm so you do not have to mention about those things that happens during s phase in the nucleus right so dna replication dna doubling do take place in the s phase but this is not taking place in the cytoplasm rather it is taking place in the nucleus so this option becomes wrong according to the question chromosome that only chromosome number remains unchanged even if it is s phase so again this becomes wrong and chromatid option is also wrong hence the answer of this question becomes a centriole hope you have understood this question so now moving ahead the next question that centrifugal cytokinesis is centrifugal means growth of any substance from inward to outward growth that is known as centrifugal growth so centrifugal growth now let's see centrifugal type of cytokinesis takes place in which cell animal cell or plant cells you know this already told you just five minutes back just look according to the options occurs in animals no in animals due to the presence of plasma membrane constriction occurs and that constriction that grows inward this type of movement from outward to inward is known as centripetal movement so first option is wrong constriction no during centrifugal cytokinesis there is no constriction rather there is cell plate deposition so wrong cell plate is laid down as i was telling you that during centrifugal cytokinesis in plant cell cell plate is formed in the center which grows towards the outside and dividing the cytoplasm of the plant cell hence the answer of the question becomes c hope this thing is clear with this mitosis is completely done now let's move ahead towards the second type of karyokinesis that is meiosis right very very important almost similar to mitosis but yes with few differences so let's try to see what is mitosis sorry what is meiosis so the term meiosis was coined by farmer and murray term itself means reduction to reduce so what happens in meiosis meiosis specifically takes place only in diploid cells correct this meiosis process cannot take place in haploid cells so meiosis in diploid cell they reduces the chromosome number to half in their respective daughter cells unlike that of mitosis which was equational cell division as the daughters were receiving the same amount of genetic material as that of parent but here in meiosis you will see reduction in the total number of chromosome from the parents to their offspring parents are deployed but after the end of meiosis there will be four haploid daughters produced that means meiosis is also known as reductional division the term meiosis itself means to reduce correct so the term meiosis was coined by farmer and murray first point this term itself means to reduce so it is basically a reductional division where the number of chromosome received by four daughter cells not two meiosis may at the end of meiosis there are four daughter cells produced and each daughter cells are haploid as compared to the deployed parent correct now to study whole myosis if i want to teach you the process of meiosis then meiosis takes place in two steps as meiosis one and meiosis two so we can divide meiosis in two steps meiosis one and meiosis two meiosis one is actually the reductional division once you will see reduction of meiosis one once i'll teach you meiosis one then there you will see that once you complete meiosis one then the number of chromosomes are reduced whereas meiosis 2 is also same as mitosis hence meiosis 2 is equational type of division whereas meiosis 1 is reductional type of division after completing both the steps of meiosis there are four haploid daughter cells produced correct now meiosis one is actually a type of karyokinesis which is characterized by four substages prophase one if you write one in front of prophase that means you're talking about meiosis one if you don't write any number in front of prophase that means we are talking about mitosis so these numbers have great significance prophase one followed by metaphase one anaphase one and telophase one then coming to meiosis two it is keryokinesis only so it is also divided into four substages same prophase metaphase anaphase telophase but here to specifically signify that we are talking about meiosis 2 we will write 2 in front of each substage so it is prophase 2 metaphase 2 anaphase 2 and telophase 2 clear to everybody yes now there are two rounds of division taking place in meiosis that is meiosis 1 and meiosis 2 so you can say that during meiosis there are two rounds of nuclear division but there is only one round of dna replication which takes place in the beginning of the process during interphase right now let's read the points which i have mentioned on the screen they are directly taken from the ncrt so reading these points also become equivalently important see the production of offsprings by sexual reproduction involves the fusion of two gametes each with a complete haploid set of chromosome now my dear students production of gamete gametes are produced by the process of meiosis from a deployed cell so this meiosis is very very important for the process of sexual reproduction for alternation of generation this meiosis process helps in switching from have deployed stage to haploid stage in our ovaries in our testes the cells on maturity they undergo meiosis to produce haploid sperm and haploid egg so production of gametes during sexual reproduction in higher organism is done with the help of meiosis i hope first point is clear moving ahead gametes are formed from specialized deployed cell in higher organisms like your implants if i talk about than in gymnosperms and angiosperms gametes are produced by meiosis in their deployed mother cells so gametes are produced by the meiosis process in a deployed cell third this specialized kind of cell division this type of cell division meiosis reduces the chromosome number by half if parent is deployed then the daughter cells which will be produced are haploid and from one parent cell from one parent cell four haploid daughter cells are produced which are not genetically similar correct yes or no okay very good and one more point i have told you that during meiosis there are two rounds of nuclear division but only one round of the dna replication that takes place just in the beginning of the process like interphase so like that of mitosis also if it's okay so now moving ahead now read the statements meiosis involves two sequential cycles of nuclear division that means in one end meiosis one another in meiosis two but only single cycle of dna replication next point meiosis one is initiated after the parental chromosome have replicated to produce identical sister chromatids at the s phase now this i'll show you with the help of a diagram so just focus on this part now initially i'm having a deployed cell this is a parent cell which want to undergo meiosis to produce gametes so initially it is deployed that means having two complete sets of chromosome so even before initiating meiosis the cell first had to undergo preparation preparation is again important that means the cell will first undergo interphase g1s and g2 once after completing g2 then it will enter into meiosis one will complete meiosis one perform cytokinesis one then we'll undergo meiosis ii and then resulting in four haploid daughter cells so right now i'm having a parent cell which is deployed and having only 2c amount of dna correct now this is g1 stage now this cell will go into s phase what will happen in s phase in the nucleus dna is going to replicate without changing the number of chromosome so now my cell will be deployed but the amount of dna will become 4c so how will you represent each chromatin fiber will produce one more thread attached on its own centromere which will be complementary to the existing one right like this okay so this shows what that the dna has undergone replication and it has produced its copy which is attached to the same centromere hence the number is not increasing but the amount is increasing s phase done now this cell will enter into g2 phase where different organelle duplication and protein synthesis will take place however the dna amount will remain as 2n and 4c correct yes or no so this is your g2 phase now once g2 is completed that means interface is done one round of dna replication is already being done after this now this cell will enter into meiosis one meiosis one is actually reductional division after completing all the steps of karyokinesis of meiosis one at the end after completing even cytokinesis one there will be two daughter cells each receiving half the amount of chromosome so now after completing meiosis one two daughter cells will be obtained each having only one complete set of chromosomes that means now the daughter cells will will become haploid and the amount of dna will remain to see correct so this is reductional division it's only meiosis one which you have completed now these two cells before entering into meiosis ii they need one more round of preparation called as inter-kinesis or intra-meiotic preparatory phase but in that preparatory phase everything is going to be re-synthesized except dna replication so as it is the cells will remain synthesis of organelles and proteins will surely take place now after this it will enter into meiosis ii once completing intra meiotic interface and once it completes the meiosis 2 then now each parental cell is going to perform mitosis because meiosis 2 is same as that of mitosis it is equational cell division and it is going to produce each cell is going to produce two two daughter cells having half the amount of dna as that of parent but if this becomes the parent then these daughter cells will be haploid but the amount of dna received will be only half of the parent so at the end of complete meiosis 2 4 daughter cells are obtained which are having half the amount of genetic material as that of parent parent was 2 and 2c and 4 daughter cells are obtained which are nc now how whole this process takes place now we are going to discuss that so i hope that this chart is clear to everybody now how things are happening during meiosis now we are going to study all those steps one by one so first let's start with meiosis one so meiosis one is actually your reductional division where the deployed cell is going to become reduced to haploid cells meiosis one is characterized by four different phases prophase one metaphase one anaphase one and telophase one so first let's see what is prophase one prophase one first of all it's the most complex and elaborate phase of meiosis one which is again subdivided into five stages that is leptotene zygotine pachytine diprotein and dichinesis what i have said my dear students now we are starting with meiosis one meiosis one which is actually reductional division is completed in four substages prophase one metaphase one anaphase one and telophase one so let's first start discussing prophase one this prophase one is the most complex longest and elaborate phase of meiosis one which is subdivided into five stages leotine zygotene pechitine diplotine and dichinesis correct so there are how many substages five we are going to see all these five stages sub stages of prophase one one by one so the first stage is leptoteen leptoteen is the stage which just comes after g2 so cell has just completed interphase and it has just entered into leptotene so what happens in leptotin suppose this is a cell right cell which is having well-defined nuclear membrane a nucleus is nucleolus is there all doubled cell organelles are there why doubled because already interphase has been done so all cell organelles are present which are double right now apart from this two centrioles and all everything is there now in the nucleus there is 46 if i'm talking about a human cell then total 46 chromosomes are there out of which 23 is definitely contributed by mother that is through egg and remaining 23 one complete set is actually contributed by father so here i am going to represent two different complete sets of chromosome by two colors one is with green green suppose represents one complete set given by mother and orange represents one complete set given by father so total 46 chromosomes are represented by green and orange thread now this is how the cell comes in leptotene stage now leptotene stage marks the condensation marks the beginning of the condensation of genetic material leptoten is just the start of prophase one correct where the activities which have to take place in prophase one just begins like condensation of genetic material begins disappearance of nuclear membrane nucleus and organelles begin then centriole movement begins towards the opposite pole spindle formation begins so everything is just beginning in the prophase one leptoteen correct so leptotene stage is just characterized by the beginning steps so one we can write leptoten is characterized by the beginning of the condensation of genetic material correct however it won't be visible nothing will be visible all the process they have just started so when you will look a cell which is at leptoten stage then you will just find it like this only there will be no such specification which you will see during leptote now once the cell completes leptotene it enters into zygotene stage zygotene stage is a stage next after leptotin so surely some amount of condensation would have been done some amount of nuclear membrane and nucleus other organelles would have been disintegrated centriole must have little bit moved apart towards the pole all those things but still they will not be visible things are happening things are happening in the zygote stage but they are not very clearly visible so see what actually will be visible during zygote stage so this is a cell right still nuclear membrane nucleus are disintegrating but right now they will be visible to you other organelles they have began disintegrating but still they will be visible centriole are moving apart but still they have not reached the poles now talking about the genetic material which is very important condensation has began see initially the genetic material is like this yeah so genetic material right now is like this orange one are already s phase has been completed dna has replicated so all 23 chromatin fibers are present like this and remaining 23 chromatin fibers are present like this right yes or no so now condensation has just began so in zygote out of the two sister chromatids which you can see one of them gets condensed and remaining another thread is still waiting to get condensed so what happens in zygote how are you going to represent them so like this partial condensation has been done as a result one of the chromatid is visible but still another chromatid is present in the form of thread like like this so like this 23 chromosomes must be present for green and 23 must be present for orange whose one chromatid has been condensed but another chromatid is still waiting to get condensed as it is still the beginning of prophase one you have so many three stages still left for the condensation to happen now so partial condensation is done that is the first point after partial condensation these 23-23 chromosomes like chromosome one from mother chromosome one from father two from mother to from father they are homologous that means that exactly in shape size and morphology such homologous chromosome my dear students they start pairing the chromosomes which are exactly in shape size and morphology they find each other and they start pairing this pairing of homologous chromosome is known as synapses so first point zygotene is characterized by partial condensation correct second point homologous chromosomes they are paired and this is known as synapsis now homologous chromosomes have come they have come close to each other but to keep them close to so to not let them move apart again there is a zip-like structure formed between them made up of ribonucleic acids and proteins nuclear protein issues structure is produced called synaptonemal complex so what i'm saying in order to keep the paired homologous chromosome together there is a zip-like structure nuclear protein issue structure called synaptonemal complex is formed so third steps take place is formation of synaptonemal complex which is nuclear proteinaceous zip like structure correct now the paired homologous chromosome they are known as bivalence so bivalence are basically formed during zygotene stage so now how will i draw all homologous chromosome i'm going to pair like orange will be 23 and similarly green will also be 23 and all homologous chromosomes will find their partner and will pair up once they pair up to keep them paired a zip like structure nuclear protein issues synaptonemal complex will be formed so this is how the structure is going the cell is going to appear under leptoten sorry under zygotene stage correct apart from these three now these structure they are known as bivalence so bivalence are clear in zygotene stage this is how what happens in zygotene stage coming to the next pacchitine now what happens during pectin now you are more proceeding towards the end of prophase one so more disintegration of nuclear membrane other cell organelles more movement of centriole towards the opposite pole spindle fiber formation as well as more condensation of genetic material will take place in the next stage that is piccatin so if i have to draw the diagram of pectin see how it will look like so this is a cell right nuclear membrane is disintegrating but still it will be quite visible so it is still visible however it is disintegrating nucleolus other cell organelles etc and centrioles are trying to reach to the pole but still they have not reached to the pole right clear okay now what will happen we are having bivalence where one of the chromatid has got condensed of the chromosome but another is still remaining it is still present in the form of thread like so now during pachitine the another thread will also get condensed to form two sister chromatids of the chromosome so 95 percent of the condensation is done in pekitine state so how your chromosomes will appear now see now actually the term chromosome can be used because initially they were thread like but now you can see both the chromatids of the chromatin fiber have got condensed right so same goes for orange one also so all these 23 or have also got condensed and now these two homologous chromosome having all four chromatids this structure is known as tetrad so pacchitine stage is characterized by the formation of tetrad where all the four chromatids of the homologous chromosome are visible now synaptonymal complex was there that was keeping these paired homologous chromosomes together right now on the synaptonymal complex recombination nodules are there which are having the enzyme recombinase so these are recombination nodules these nodules they are having the enzyme recombinase correct so now what happens now see these two are sister chromatids these two are sister chromatids but one orange and one green of homologous chromosome are non-sister chromatids so if i talk about this and this then these two are non-sister chromatids of homologous chromosome now these non-sister chromatids are homologous chromosome on the recombination nodule they will perform the exchange of genetic material called crossing over so pectine stage first point it is characterized by tetrad formation right it is characterized by tetrad formation first point second point due to the recombinase enzyme present on the synaptonemal complex there is exchange of genetic material between the non-sister chromatids of homologous chromosome and this phenomena is known as crossing over correct so what is crossing over and which stage does it takes place i hope it is clear now once crossing over is done now the two strands they're going to meet on this point of recombination nodule now this is about pekka tea once pectine is done then let's move to the next stage of prophase one that is diplotine diplotine is more moving towards end of prophase one so almost now if i have to draw the diagram of diplotine stage then my almost my nuclear membrane has got disintegrated so i can draw it dotted correct all the cell organelles they have almost disintegrated so i can draw them dotted right nucleolus has almost got disintegrated i can draw it in dotted manner centrioles are almost towards the opposite pole but still not reached correct spindle formation is also parallely taking place now let's talk about the genetic material which is very important now what happens in the protein stage the synaptonemal complex dissolves the synaptonemal complex which was formed during leptoten stage its work is done crossing over is done and now it has been dissolved in the protein state now once the synaptonemal complex has dissolved so the paired homologous chromosome will start moving apart however they will not be able to because they have now got attached at the point of crossing over so like this see suppose these are my sister non-sister chromatids of homologous chromosome this is one homologous chromosome this is another homologous chromosome and these two other non-sister chromatids and these two are the sister chromatids so crossing over takes place between the non-sister chromatids of homologous chromosome so now imagine there is a synaptonemal complex in between what will happen sc is having node the two sister chromate the non to the non sister chromatid will come on the recombination nodule and will allow crossing over now this is about pekitine now as the cell enters into diplotine stage the synaptonemal complex which is present in between dissolves so these paired homologous chromosomes now they want to move apart but they are not able to because they are attached at the point of crossing over and this forms an x-shaped structure called chiasmata so diplotine is characterized by the formation of x-shaped chi is mata what kai's meta represents it represents the point of crossover right so due to dissolution of synaptonemal complex homologous chromosome they try to move apart but they fail to do so as they are still attached at the point of crossing over so how will you draw them see now now these chromosomes they are just trying to move apart but they fail to as their non-sister chromatids are attached to each other so this point this point this point represents chiasmata in some vertebrates the in oocytes of some vertebrates this diplotine stage yahitak mios is one and then it gets suspended for few years and then it becomes the longest phase of prophase one diplotine stage is one of the longest phase of prophase one in case of oocytes of some vertebrate where the cell division gets suspended at this stage and in that case this stage will be known as diction correct but now we are moving ahead we are not going to suspend our class so this is about diplotine now now coming to the last step of prophase one which is dichinesis dichinesis is actually marking the transition to metaphase one so where nuclear membrane disintegrated genetic material highly condensed organelles got disintegrated poles centriole have reached spindle fiber almost completed now these paired homologous chromosomes still they are paired enough they are still attached to the point of crossing over they are trying to move apart so what happens the kaia's matter starts slipping pressure is being applied they are attached and after nemal complex has been dissolved chromosome are trying to move apart but they're attached but pressure is being applied on the sky's matter so kai's matter just starts slipping and it reach to the point of their meeting that means at the end so this is known as terminalization of kaya's matter so dichinesis marks the transition to metaphase one during dichinesis terminalization of chiasmata takes place so two points are going to write for kinases first it marks the transition to metaphase one and second it also shows terminalization what is terminalization it is slipping of chiasmata such that now the two homologous chromosome they only remain attached to the ends so how will you show the diagram now no nuclear membrane need to be drawn right there will be no nuclear membrane there will be no nucleus organelles poles centriole aster formation with the help of microtubules then there will be spindle formation correct now homologous chromosome will hang with the help of these spindle fibers and how see right so this is one complete set represented by orange and green will also be attached to it but only to their tips like this correct so like this 23 pairs will now be hanging rather than 46 separate chromosomes which you saw in mitosis in meiosis one 46 chromosome will convert themselves into 23 pairs right and all those 23 pairs are now hanging with these spindle fibers happy deadly and this marks the transition to metaphase one so once this is the transition to metaphase one so let's also jump into tran metaphase now in metaphase one it's almost similar as i was telling you to your mitosis metaphase what happens during the mitosis metaphase the oppositely arranged chromosome they try to come and they arrange on the same equatorial plate now same thing is going to happen in metaphase one also but the major difference is because the chromosome right now are paired up rather than 46 chromosomes separate the chromosomes are present in the form of 23 pairs so now rather than forming one metaphasic plate there will be two metaphasic plates that will be formed so now see what happens during metaphase one the metaphase one the tetrads which are there which are still attached to that terminal points even after terminalization of kai's mata which are arranged happily on the spindle fiber they all will be pulled and they will be made to align on the same equatorial plate and there will be two equatorial plates produced one for green chromosomes and another for orange chromosome so the diagram will appear to be like this so spindle fibers are there on which the chromosomes are attached in paired manner and they all will come and align on the same line center equatorial plate right but still their ends are attached like this so here one this is one equatorial plate having 23 chromosomes and this is another equatorial plate or metaphasic plate having 23 chromosome why this is happening why two metaphasic plates are produced because chromosomal still paired so they align themselves in a parallel manner so this is what happens during metaphase one once metaphase one is completed now next stage is anaphase one now during anaphase one normally splitting take place in anaphase one also splitting will take place but not of centromere now rather these paired homologous chromosome will separate from each other this wire is going to break this wire this point of attachment it is going to break spindle fibers are going to break during anaphase one because of which now the paired homologous chromosome will be separated the paired homologous chromosome which are still attached now they will tend to get separated and 23 chromosome will start moving upward and 23 chromosome will start moving downward that means we are almost reducing the chromosome number to half rather than 46 chromosome moving towards one pole or another pole 20 23 are moving and then telophase one will come reappearance of nuclear membrane two nuclei will be produced per cell each having 2323 chromosome so anaphase one of meiosis one is actually marking the reduction in the total chromosome number so anaphase one is actually marking the reduction in the chromosome number correct how by separating the paired homologous chromosome apart from each other so what happens during this c suppose this is a cell so the spindle fibers will break as as a result these spindle fibers will read rectile they will obviously pole is going to pull if you cut the rope from the center then half of the rope will move towards one end and remaining half of the pole or rope will move towards the another end so similar happens with the spindle fibers as well because of which some 23 chromosome move towards one pole having little bit of orange why orange because exchange of genetic material has took place right and some 23 chromosome they are moving towards another opposite pole having little bit of green why because marking the exchange of genetic material that took place during the caitine stage so 23 chromosomes are moving towards one side of the pole and remaining 23 chromosomes are moving towards another pole this is anaphase one that marks the reduction in the total chromosome number now followed by telophase one which you already know is just the reverse of prophase one so during telophase 1 what is going to happen the nuclear membrane and nucleolus along with cell organelles will reappear spindle fibers will disintegrate and your genetic material will also get decondensed to form thin thread like chromatin fibers so one nuclear membrane will appear over here another nuclear membrane will appear over here all the cell organelles will again come back and they will be distributed one for above another four below this is the centrosome for this cell this is henderson for this nuclei right now genetic material inside one nuclei will be now they will also get decondensed right only 23 threads having some part of orange as well showing recombination and here also 23 chromosomes will decondense to form to form what to form haploid cell so this is also 23 so this is one haploid cell this is another haploid cell but the haploid nuclei haploid nuclei now soon after telophase one cytokinesis one will take place which will just begin in the telophase one stage only if it is an animal cell then constriction in the plasma membrane will occur which will grow grow grow and divide this cell into two daughter cell this is one daughter cell and this is going to be the second daughter cell each daughter cell receiving half the chromosome number as that of parent 23 23 but these genetic material they are produced after recombination after crossing over clear so have you seen that how meiosis one is reduction division and how two daughter haploid daughter cells are produced from a deployed parent cell right now you are having two haploid daughter cell with n2cn2c now these two haploid daughter cells wants to undergo what they want to undergo meiosis ii this is the second round of division in meiosis itself which is similar to equational cell division however but before entering into meiosis ii these two haploid daughter cell will first enter into a short-lived preparatory phase called as intra meiotic interphase so soon after completing telophase one and cytokinesis one now these two cells will enter into intra meiotic meiosis intra meiotic interphase right it is a kind of interface it is a kind of preparatory phase which is characterized by all the events that take place in actual interface except dna replication that means protein synthesis rna synthesis energy synthesis organelle duplication everything take place but there is no dna replication so intra meiotic interface is a short-lived preparatory phase right it is a short-lived preparatory face where all the things takes place everything is synthesized rna proteins energy organelles everythings are re synthesized except dna replication but no dna replication so it is almost similar to interphase but in this difference no dna replication no histone protein synthesis no dna replication otherwise intra meiotic interface is same as that of real interface it is also a preparatory phase now this phase this intra meiotic is responsible for preparing these two cells to get ready for meiosis 2 for next round of equational cell division so now this intra meiotic interface is clear to everybody now you are having two daughter cells each haploid with 2c amount of dna now meiosis 2 which is actually same as equational cell division so what you are having now you are having two haploid daughter cells with 2c amount of dna yes or no now these two daughter cells have undergone intra meiotic interface but dna replication did not took place so there is no change in the amount of dna nor in the change in the amount of chromosome so it's already n that means total 23 chromosome with 2c amount of dna now they will undergo meiosis 2 meiosis 2 means first prophase 2. prophase 2 is exactly same as normal prophase what will happen nuclear membrane nucleus disintegrate centriole again move to the opposite pole condensation of genetic material these thread like will convert themselves into thick rod like chromosome then all the organelles will also duplicate spindle fiber will be formed so everything will take place during prophase two same as prophase one a normal prophase once prophase two is completed then comes your metaphase two now in metaphase two only one equatorial plate will be produced as all the 23 chromosome they have not paired they are separate from each other and they will align themselves on only one equatorial plate clear so only one plate will be produced same as that of metaphase but with 23 chromosome anaphase 1 anaphase 2 anaphase 2 is also same as anaphase that means now the centromere will separate now the two chromatids of the chromosome will move apart marking half in the amount of dna now 2c will become c due to splitting of centromere one chromosome will split into two chromatids that means 23 chromosome will split into 46 chromatid each 23 chromatid will move towards the opposite pole so number of chromosome will be 23 2323 but the amount of dna will become c from 2c coming the telophase 2 telophase 2 again the nuclei will be formed all the organelles will reappear spindle fiber will disintegrate and decondensation of genetic material will also take place followed by cytokinesis 2 cytokinesis 2 is also same by plant cell cell plate formation centrifugal movement animal cell centripetal movement constriction of plasma membrane exactly same everything goes same and at the end we obtain four haploid daughter cells from each parent cell it will divide into two haploid daughter cells receiving half the amount of dna this is also going to produce two heploid daughter cells receiving half the amount of dna so after completing meiosis ii we get four daughter cells correct so actually you started your meiosis with 2 and 2c and at the end you received four daughter cells having n and c condition so can you see the reduction in the amount of chromosome as well as amount of dna so that is why this is red overall reductional division and specify current agar if you want to specify then in meiosis meiosis 1 is actually reductional and meiosis 2 is equational clear to everybody yes so with this whole meiosis process is also over you have studied the two types of karyokinesis that takes place in eukaryotic cell mitosis and meiosis i hope both the things are clear to everybody just once read it from ncrt as well as go through the recording very well moving ahead significance of meiosis and mitosis why are we actually doing so much why our body needs these two processes mitosis and meiosis care reason here so first if i talk about meiosis meiosis is very much important for sexual reproduction sexual reproduction is characterized by alternation of generation where haploid deployed deployed haploid switch has to take place and there is only one way to switch from deploy to haploid that is meiosis so meiosis helps in producing gametes that are required for sexual reproduction so first significance of meiosis is it helps in sexual reproduction how by forming gametes gametes which are haploid right now to restore haploid to deploy the fertilization is done so whatever meiosis does it reduces the number of chromosome to half now to restore the condition fertilization takes place and again haploid becomes deployed the two haploid gametes they fuse to again restore the number of chromosome so the cycle keeps on going and this cycle is very important for sexual reproduction so first significance of meiosis second meiosis is the result of meiosis leads to variation there is exchange of genetic material not you're not receiving exactly the same genetic material as of your parent otherwise you would have been identical we are not identical to our parent however we are similar because we are receiving the same genetic material 23 23 from mother and father however it is a recombination chromosomes the gametes which are produced by my mother and by my father has already gone recombination so whatever gametes i am receiving in my zygote are actually the result of recombination so those recombination creates variation that changes that does not produces offspring identical to their parents so meiosis helps in introducing variation then these variations they keep on accumulating accumulating for many many years and then it leads to evolution so meiosis is one of the major reason for evolution as well clear so the two main significance of meiosis are it helps in gamete formation during sexual reproduction and second it introduces variation that leads to evolution now coming to the significance of mitosis if i talk about mitosis then mitosis is required for repair and regeneration it is required for growth of the body it is required for healing purpose suppose some damage happens in my skin right so these skin cells they will try to regenerate they will try to heal that injury by producing new cells and that new cells are produced by the process of mitosis in mitosis identical cells are produced first thing second mitosis helps in growth how growth growth because of organism who is small the size of an organism is only going to increase if the number of cells are going to increase and identical cells are going to increase and for that again mitosis is required clear so mitosis is mainly required for repair regeneration for growth purpose for healing purpose and etc so these are certain significance which i have told you otherwise you can read it directly from your ncrt little bit is given and there you can do it so with this finally your cell cycle and cell division is done from my side i hope you have enjoyed the lecture if you find any trouble in understanding any of the concept then please please feel free to ask in the comment box i will surely surely respond now before we end the session let's do certain questions the question number one says crossing over occurs due ring so when does crossing over occurs you already know it occurs during the pectine stage of prophase one so even without looking the options i have studied so well that i know the answer is going to be c correct zygote in synaptonemal complex is formed and bivalence are produced leprotein only condensation begins diplotine dissolution of synaptonymal complex so answer becomes c hope you have understood this next question longest phase of meiosis one is so the longest phase of meiosis or mitosis is actually prophase so your answer of this question becomes a so prophase one is the longest most elaborate and complex phase which is subdivided into five stages so with this i hope the session for the today is done if you face any trouble students then please please surely share your issues and your troubles in the comment box and soon i am going to take your marathon marathon means we are going to quickly revise the whole syllabus of class 11th and 12th with the help of questions and some very very important concepts so please students if you are listening to me then do join that marathon session it is just it is kept now just to help you for your neat examination within three or four hours whole of your 11th and 12th syllabus i will revise and i'm going to tell you the tips and tricks to how to remember the things rather than mugging up remembering is very important if you are going to crack your need so a marathon is going to happen on 8th of july and please do connected remain connected on the channel for the recent updates thank you everyone see you in the next class bye bye take care