so welcome to the class we will review cell cycle and cell division in this lecture perhaps it's important that we start with the definition of cell cycle we define cell cycle as self-regulated sequence of events that occur in a cell and this self-regulated sequence of events are aimed at two things they are aimed at controlling the growth of that particular cell but they also aimed at leading that cell to its division so that the cell eventually divides into two daughter cells so that self-regulated sequence of events is what you are calling the cell cycle it is a continuous process if you get it right because a cell will be doing this then the cell divides and the data cells begin the journey again we routinely describe the cell cycle as having two major phases one of the phases of the cell cycle is known as the interface if you understood that it is a sequence of events that lead to its growth and its division then we can define the interface as the period between two successive cell divisions the period between two successive cell divisions during that period the cell largely grows and also replicate this genetic material and then the other phase is what you call the m phase the letter m standing for mitotic phase which is now the period of actual division and during mitosis the cell separates there is division of the nucleus as well as division of the cytoplasmic contents so the nucleus will divide aimed at going to two data cells and then the cytoplasm also de flows the nuclei that are going apart and that is what happens during the mitotic phase so the cell is either dividing or is in that period of preparing to divide so interface is that period of preparation for division then emphasis is the actual period of division perhaps let's say something more about the interface we have already given the definition the period between two successive cell divisions so if you look at this image if this is the period of cell division this period between one division and the next division is what you're calling the interface during that time immediately a cell has been formed from cell division the cell begins to grow and it is during that time that the cell will also replicate its genetic material perhaps you're asking why the cell needs to replicate its genetic material let's think for a moment that it is possible to count the dna that a cell has then let us assume that we are able to count and we found that each cell has 200 or let's say the cell that we have in equation the cell that need to divide has 200 dnas and i'm taking this literally so that we are able to understand what i want to put across if this cell has 200 dnas and this cell divides would expect the daughter cells then to have 100 each now do you imagine what will happen when the daughter cells then divide the grand daughter cells will have 50 each now think through that over several generations then you'll realize that cells will actually be losing their genetic content now we don't want that to happen others will not be in existence in our generation so a cell that is intending to divide this cell we are starting with that has 200 dns that is intending to divide for it to make sure that the daughter cells will also have the same amount of genetic material that it has the cell that is intending to divide must replicate its genetic material so in the example given if this cell has 2200 dnas it will replicate that so that it has now 400 dns to replicate just to make another copy of itself the dna is synthesized that makes another copy of itself so that instead of 200 now we work with 400. and inevitably then this cell divides when it divides the daughter cells will each have 200 just like the original genetic makeup of the parent cell now this is the rule of genetics this is the rule that a cell that needs to divide has to replicate its dna before that division this is to aim that ensuring that the little cells have the same amount of genetic material as the foreign cell so during interface that's when the cell will actually do that it's part of preparation for cell division routinely we divide interface into three stages there's what we call the g1 phase s phase and the g2 phase let's start with the g1 first the g1 phase of the cell cycle that letter g stands for gap so we call the gap one first so this is the period immediately after cell division the period immediate after cell has been formed a new cell immediately a cell has been formed the cell will begin to grow because remember could be slightly tinier than what we intend it to be because cytoplasmic division has taken place so the cell will grow and perhaps accumulate nutrients but remember this cell is intending to divide later some days so because it is intending it's looking forward to division it means that at some point it will have to replicate its genetic material and if this cell is going to replicate genetic material then it needs the correct machinery that will regulate the process of dna replication so this is the time that the cell prepares the necessary machinery that is required to control dna replication there are some molecules which are formed in this stage which are vital in checking a number of things in the cell cycle to ensure that things are okay perhaps it's important for you to also know that if a cell successfully goes through the g1 phase most likely that cell will divide and so if the process of the g1 phase is not correctly checked there could be some problems that can be seen as with regard to for example the development of cancer can be just because of some defects which occur in the g1 phase of the cell cycle okay let's assume normal that the cell has gone beyond the g1 phase successfully it now has the necessary regulatory molecules that will help it to go through dna replication then the cell will then take up that particular activity dna replication and the period of dna replication is what we call the s phase the letter s in this case stands for synthesis this is the synthetic phase and what are we synthesizing the dna is what is being synthesized so the s phase is a period of dna replication at this time the genetic material is doubled for the reason that i've just given you a few minutes ago in that case therefore it means that when you're going to have chromosomes a chromosome will be having two chromatids because now we've made an extra set of the genetic material so each chromosome at this stage will be later on as you can see the chromosomes will be having two chromatids each great there are some checkpoints again in the s phase that just confirmed that the dna that has been replicated is normal and in case there's need then some repairs actually take place and perhaps even cell death can take place if those repairs are very grievous once a cell has replicated its dna remember it's aimed at going to divide eventually this cell will then go to the next phase when the image here shows you the process of dna replication so the original dna usually first must unwind itself once it has unwound itself then each strand that has been formed then forms another complementary strand with that in mind therefore usually the dna that is being formed the new dna will actually be an exact copy of the original genetic material so in dna replication we don't change things the dna that we have is exactly similar with the original okay after the s-phase the cell goes into the g2 phase as i told you g stands for gap so the gap two phase the gap two phase of the cell cycle this part here is a period after dna replication it is also a period when the cell will continue to grow and importantly also it is a period when organelles will also be formed and perhaps be reorganized because we are preparing for cell division one of those organelles which will be reorganized will include the centrioles really remember they are formed from microtubules which are important in cell division so basically gap two phase is that period when it's actually preparing for division well there's also some check that occurred during the gap two phase of the cell cycle basically checking whether the dna that the cell has is normal and things like that after the g2 phase that means that interface is over now the cell enters the mitotic phase we call it the m phase m for mitosis the mitotic phase is a period when the nuclear divides and after the nucleus divided the contents of the cytoplasm of the cell also divide this is a period of nuclear division as well as cytoplasmic division we routinely divide mitosis into stages is the prophase metaphase anaphase and telophase stages of mitosis perhaps a reminder of what happened in those phases will be good so i'm not going too deep because you maybe you already know this from high school the key thing that occurs during prophase is that the genetic material condenses into visible chromosomes so we now have chromosomes the genetic materials condense into chromosomes because the genetic material was replicated we expect therefore that we have two copies of the genetic material in each chromosome and those two copies are the ones we are calling the chromatids apart from the condensation of the genetic material to form chromosomes we also have the disintegration of the nuclear envelope so the nuclear envelope disintegrates during this period well there are other things that may also happen as concerning the microtubules which from the essentials they may be moving apart in preparation for this particular process the hallmark of the process is that the genetic material condenses into visible chromosomes after my thought after process the next stage is metaphase this image displays the hallmark of metaphase during metaphase those chromosomes which have already been formed or are now visible during process now realign themselves in the equator of the cell so the chromosomes align themselves in the equator of the cell that's the hallmark at this point you note that the essentials have now gone away on extreme end because they are preparing now to split this particular chromosomes so that time when the chromosomes align in the equator perhaps could also be the best time that carrier typing can be done remember karyotyping is is a graph karyotype is a graphic representation of the chromosomes within the nucleus of a cell so this is the best time that stereotyping is done you are able to study the chromosomes at this time after metaphase the next step is anaphase anaphase is that period when the chromatids are separated as we can see in this third image the chromatids are being separated so one chromatid is pulled to the other pole while the other chromatid is pulled together pulled remember each the chromatids were of a single chromosome but this is because the genetic material was already duplicated so now they're being separated sister chromatids are being separated at this time once the sister chromatids have been separated and now we have telophase during telophase now the cytoplasm is the one that is now trying to separate as you can see here now the cytoplasm is time to separate also the nuclear envelope is now reconstituting again that's telophase so those are the stages of mitosis of course eventually the two cells are going to go apart in what we call while completing the process of cell division so why do cells undergo mitotic cell division cells undergo mitotic cell division one because we desire to achieve some numerical increase in the number of cells think about blood cells in your bone marrow we just want numbers and numbers and faster for that matter so you want numbers and faster for that matter mitosis is a good way of increasing the number of cells we just wanted for proliferation cells also undergo mitosis because through this the organism or an organ will actually grow and develop think about the size you are when you're born and right now you've increased in size you've developed thanks to mitosis so mitotic division is a mechanism of growth and development either of an organ or an organism as a whole importantly also mitosis is a mechanism of repairing worn out tissues or one out parts the mechanism of replacing what is gone through wear and tear think of a wound that could be there later on there is some healing my thoughts is important for this importantly note that in mitosis the original genetic makeup is maintained so the dota cells have the same amount of genetic material as the parent cell the deuter cells will have 46 chromosomes just like the parent cell having 46 chromosomes great how about if it was meiosis meiotic cell division is unique it only occurs in the germ cells so it will affect the sperm and the oocyte only negative cell division involves two phases as well there is meiosis one and messes two in the first phase of meiosis the main thing is separation of homologous chromosomes we know that we have 46 chromosomes put in other words we have 23 pairs of chromosomes during meiosis one the homologous chromosomes are the ones which are separated which means that we separate the pairs of chromosomes so that each daughter cell after meiosis one has only 23 chromosomes that's what we call haploid so the aim of mesis one is to achieve haploid then meiosis ii is very similar to mitosis the only difference is that in meiosis ii we are working with 23 chromosomes and like mitosis where you're working with 46 chromosomes now you need to ask yourself why cells need to divide meiotically meiotic cell division serves two key roles one meiotic cell division achieves haploid [Music] the daughter cells are haploid cells they have 23 chromosomes instead of 46 chromosomes they have only one set of chromosomes instead of two sets of chromosomes that we usually see in the diploid cells and perhaps you're asking yourself so why is it important to then have hyploidy remember these are the germ cells the cells that will eventually meet at fertilization so that now we then form a zygote and yet yes another human being being the reason why we make these cells to have haploidy to be in the haploid state is so that when fertilization takes place we can restore the normal diploid number of chromosomes [Music] so we need to have the genetic material so that that fertilization will restore to normal otherwise if we didn't do this then it means that every time human beings get offspring the offsprings will be having more genetic material than the parent cell so again it's a mechanism of ensuring that we have the same number of chromosomes throughout the generations now the second reason why cells undergo meiotic cell division is so that we can have genetic variation the daughter cells in meiosis are not genetically identical unlike in mitosis where the two daughter cells are actually genetically identical in meiosis the four daughter cells are not genetically identical because there's some crossing over of genetic material during meiosis and because of that will have genetic variations so that is meiotic cell division you need to distinguish it from mitotic cell division now how do you account for those cells which have lost their capacity for cell division yes we've talked about the cell cycle and we've said that there is mitotic phase and interface and that that interface has gap 1 s phase and the gap two phase and then mitosis again how about those cells which no longer have the capacity for cell division the cells which have lost their capacity for cell division are described as being in the g0 phase of the cycle now the zero here actually means o not zero and that o means outside the cycle we believe that the cells have now left the cycle so g zero phase is a face of terminal differentiation the cells have lost the capacity for cell division they can no longer divide again well in some circumstances the cells which are in the g zero phase may be reintroduced into the cycle but that induction so that they can be reintroduced into the cycle is in most cases an abnormal induction like in the case of cancer cells in most cases in a normal scenario they will have lost the capacity for cell division and so they rest we call the quiescent stage some examples of cells which are in this phase include mature red blood cells you know they don't even have a nucleus so they can't even divide skeletal muscle cells they don't divide most nerves they don't divide a number of cardiac muscle cells if not all of them they don't divide so they are considered to be in the g0 phase the cell the mature cells of bone we call them osteocytes they don't divide right so that is cell cycle and the cell division that's what i had for you in that particular lecture