welcome back everybody and in today's lesson we are going to be looking at meiosis now just a quick reversion meiosis is a sister type of cell division to mitosis and so both of these phases have things in common and first of all the thing that they have in common is that they both produce cells now mitosis is only one stage in other words we only do the process once and we end up with two cells now these two cells that we produce are identical and they are deployed and essentially what that means is that all the cells that we produce have this full set of chromosomes so if the cell started off with 46 chromosomes at the end of mitosis you will have still 46 chromosomes in each cell produced now meiosis on the other hand is slightly different meiosis happens in two phases meiosis one and meiosis ii and each of those stages is responsible for doing something slightly different and the outcome is different in meiosis we're producing four cells at the very end of meiosis ii each of these four cells are non-identical and they are haploid which means that they carry half the genetic information that we started with in other words during meiosis we can take a cell that perhaps has 46 chromosomes and at the end of meiosis 2 you should have 23 which is half of the 46 in each of the cells but to take it one step further each of those 23 are slightly different from the original 46 that they came from and that's essentially why meiosis is so important is because we want to make cells that are non-identical and that are genetically different now before we start any of the meiosis phases we need to remember that the cell spends the majority of its time in interphase and it's important to be able to identify interface based off of what you can see in the diagram and that's a skill that you need you're either going to be asked to label the diagram identify the phase or you're going to supply a reason why you think that cell is in that phase based off of what you can see in the diagram so let's just recap a couple of things about interphase so interphase is essentially the phase in which the cell spends the majority of its time now why is it important and how do we identify it out of the diagrams now what i've done for you in this picture here is i have labeled what is the most important structures that you need to know and how they translate through the rest of the diagrams we have the centrioles and we have the centrosome the centrosome is essentially this structure that sits around our um centrioles what houses them then we have the nucleus the nucleus is where we find obviously the dna which will eventually become our chromosomes now the microtubules is perhaps something that not everybody is familiar with and essentially the microtubules are what we also call spindle fibers so for studying purposes let's have a look at what is the purpose of the interphase so interphase's purpose is related to dna replication dna replication if we remember from our previous lessons is when you take a set of dna and you want to replicate it you want to make a full new set and to do this we go through dna replication and we take our 46 chromosomes and we turn them into chromatids the second thing we want to do and why is we need to ensure that there is the correct chromosome number and basically at the end of meiosis you want to transform a diploid cell into a haploid cell and the only way to do that is to ensure that you have the right number of chromosomes at the beginning so that at the end you have enough in every cell now if you were given a picture of interface how do we identify it so first of all we're going to look for the chromatin network and as you can see in this diagram the chromatin network is this long spaghetti structure that we see inside of the nucleus it's also going to be perhaps transitioning meaning that it might not look perfectly like the chromatin network it might even be forming the chromosomes or starting to condense and pull together what's very important is that at this point homologous pairs have yet to appear and the nuclear membrane is still very visible and so these are answers that you can give in terms of how do you know that it's interface now moving on to pro phase one now prophase one is generally pictured in two different ways i'm going to show you the first way and then the second way and essentially prophase is responsible for a really important function of crossing over to create genetic variation and so in the diagram alongside here you will notice that our chromosomes have condensed and you can see them very clearly here as individual strands they are replicated which if you remember is when we have one strand or one chromatid with a centromere in the middle and another chromatid next to it that then gives us our chromosome structure and that is one chromosome two chromatids now this picture that i've given you here is one way to identify prophase you'll notice like i mentioned that the chromosomes have condensed and you can also see that the outer layer sitting around here which is the nuclear membrane is starting to disintegrate these are our centrioles that are going to stop moving it to the poles and this is early prophase it's going to go into mid and late prophase later on but i want to show you the difference between the two so just in case you might get one of these two pictures you know for certain that this is prophase and then we'll go into how to identify it and what its purpose is this second diagram that i've now included is the alternative view of what prophase might look like and as you can see here we've got some labels on this diagram so we have our homologous chromosomes which are now present and i will explain what those are you can see that the nuclear membrane has pretty much completely disappeared the centrioles are still on their way to moving around to the poles the spindle fibers are forming and our chromosomes are touching each other and the most important part of prophase is um the process of crossing over and i'm going to explain this a little bit more detail but just so that we know what's happening in prophase prophase is when our homologous partners which a homologous partner is when we have a set of chromosomes that carry similar information alongside you i have a red and a blue one but one of them is a maternal chromosome the other one is a paternal chromosome in other words one chromosome from mom one chromosome from dad and we call this a homologous pair essentially homologous pair is a set of chromosomes that carry corresponding information in other words if we were to talk about your eye color for example we'll have a maternal chromosome that carries your eye color and a paternal chromosome and this is one pair one homologous pair now during prophase one these homologous pairs as we can see in the second diagram they cross over with one another and so if i just point that out to you you can see here that our red and blue chromosomes are touching each other at different points and they are doing a genetic variation process called crossing over so if we first look at the purpose of prophase one it is summarized into the fact that we want to have genetic variation so we cross over and that allows one chromosome to share information with its homologous partner i am going to explain this in detail in the next part of the video now when we want to do crossing over we go through something called synapsis synapsis is the movement of the homologous partners towards each other in other words it's how they float together in order to have crossing over now when they're actually touching each other we call them by valence we can no longer call them homologous partners or homologous pairs so to round that up before crossing over we call them homologous pairs but after and during crossing over we call them by valence so how do we identify prophase so prophase is easily identifiable because the nuclear membrane has broken down there is the formation of spindle fibers and most importantly by valence have formed a reminder that a bivalent is when you have a homologous pair that is now touching each other during crossing over exchanging genetics now crossing over is the most important part of prophase because essentially it is where we're going to exchange information and it creates genetic variation and the more genetic variation you have the more likely it is that that organism will survive so i just want to reiterate what exactly is happening so we know what to do with all of these words and what crossing over actually means so in this diagram we have a homologous pair a blue and a red chromosome one of them is the maternal chromosome one is the paternal in other words you inherited one of them from your father and one of them from your mother now they move towards each other in a process called synapsis so in order to get synapses to happen synapsis is the movement towards each other in other words our two homologous chromosomes are moving to each other and now they are going to touch and they form a bivalent so in other words this structure here is a bivalent it is when they are touching now the point at which they touch is called chiasmata and it is where they are exchanging information with one another so that they can create variation the product of that is now what we call recombinant chromatids collectively the whole thing we can call a recombinant chromosome understanding this step of meiosis is extremely important and it's often the thing that is tested most regularly to understand and i know that there are many new words that you have to be familiar with at the very end of the video i always do a vocabulary checkup so that you know exactly how to use the terminology correctly moving into metaphase one we now see that our our chromosomes have now relocated and as you can see here our bivalence have now lined up on the equator which is this is the center point of the cell so if we were to draw a sort of dotted line through the middle that's where they're going to line up and the way in which they line up is very important because when they line up they do something called random arrangement which essentially means the way in which they line up on the equator is random we don't know if we're going to have the maternal on the left or the south side or the paternal on the right or the north side of the cell as an alternative point of view it's really important to know how you're looking at the cell and that's why i've included the second diagram is because they're both metaphase it's just that the poles have been moved around because we've just orientated the picture differently it's still the same phase the only thing we've done now is instead of moving from the north and south we just are moving from the west and the east of the cell so just make sure that you orientate yourself with the picture very well but now what's the importance of metaphase how do we identify it and what is really important so metaphase is very important for introducing even more variation through this process of random arrangement to reiterate that one more time when we speak about random arrangement we are speaking about the way in which the maternal and the paternal chromosomes align at the equator in other words you don't know if you are going to be the cell that gets the maternal partner or the paternal partner and this is very important to increase genetic variation because of this variation of which one you're going to get you're uncertain of the combinations that you could possibly produce but now how do we identify metaphase if we're looking at a picture so what are we looking for when we are trying to identify metaphase we are looking at the most defining quality is that chromosomes align at the equator in other words the midpoint of our cell whichever way we're looking at the cell that's our midpoint and that's our why metaphase is one of the easier ones because meta makes you think of middle so when we have our chromosomes lining up at the middle the second thing we can look for is the bi-valence still present in other words they're still touching each other and lastly and this is a very important one that we mustn't forget because it will influence how we identify phases later and that is that the double stranded chromosomes are still present in other words if we think of the structure of a chromosome we still have one chromatid with a centromere in the middle and another chromatid on the other side this is what we're referring to as a double stranded chromosome in other words it has two strands to it as long as we have two strands and we're meeting in the middle we are in metaphase one moving on to anaphase one so this is following metaphase one and anaphase is also simply an a phase in which we are separating our homologous partners from each other and so it's an easy one to identify because of the space that seems to occur in between our partners now and that growing space might be a little bit bigger or a little bit smaller in a diagram it just depends on the diagram that you have been given but you're looking for this empty space in the middle here that's the easiest way to identify it and so when it comes down to purpose this is what the anaphase one is for anaphase one's purpose is extremely important along with all the ones that we've already done but this one is important because we are separating the bar valence now which means we're going to half the chromosome number in this instance perhaps we are going to go from 46 chromosomes if this was a human and we're going to half it now and we're going to make it 23 chromosomes and that's the goal of anaphase one the last uh thing associated with anaphase one or the last purpose is independent assortment now this refers to something based in genetics and perhaps you will revisit this when you do genetics but essentially there's a law that governs how you inherit your characteristics in other words independent assortment is all about how you inherit your hair color separately from your eye color in other words the color of your eyes does not dictate automatically what hair color you have or skin color they are independent of one another now how do we identify anaphase one so the two key things you can look for firstly our double-stranded chromosomes are being pulled to opposite poles which we can see very clearly here as our spindle fibers are shortening they are pulling apart our bivalence the second thing that you are going to need to look out for is the recombinant chromosomes and their visibility in other words you can see in this picture here that the chromosomes have little bits of the other chromosome attached to it in other words we've got a blue chromosome here with some purple parts to it and that itself is called a recombinant chromosome and we are um separating it from its partner the final step in meiosis one is telophase one and this is one of the easiest phases to identify it's essentially where we finally made our two separate cells now this picture here has two very defined cells but sometimes telophase in diagrams and in exams will still have the cells joined together and if we remember back to what we've learned about telophase in mitosis this separation of the cytoplasm that's happening here has a name it's called cytokinesis and cytokinesis is when the cytoplasm pinches off and we take one cell that we were previously working with and we're going to form it into two cells which eventually will give us the product that we see above here in this diagram where we have two distinct cells now it's important to remember that we're doing meiosis not mitosis so at the end of this we want two cells that are non-identical so these two cells that we see here may look very similar but they have some very important differences the first difference is that they do not have the same chromosome number as they did before in other words if this was a human cell they should have 46 each but now that they've gone through meiosis one they have 23 chromosomes in each of these cells and also the chromosomes have changed they've gone through crossing over and they've changed their genetic information so what is the purpose of taylor phase one we are separating the cytoplasm via cytokinesis and that's what we spoke about down here where we are pinching off we call this cell invagination it's where the cytokinesis pinches so you pinch the cytoplasm in in the middle and that then forms two most importantly non-identical individual cells and they will have half the genetic information that we started with now how do you identify this phase we are looking for identification in the cell cleavage which i mentioned earlier is this little pinching off area this little indentated area over here is called the cell cleavage you can also look for the reforming of the nuclear membrane which we can see in this top image up here there's our nuclear membrane that's reformed and last but not least the fibers they have disappeared you can see here there's no more fibers in either one of these cells they've both disappeared they've gone back inside their centrioles and that's the easiest way to identify taylor phase one so let's do a quick terminology recap for meiosis one so meiosis one has something called synapsis and synapsis is when our two homologous pairs they the partners of that pair move together and that movement is called synapsis and when they're touching each other we call them by valence now when we talk about a cell's number of chromosomes we can refer to it as having a full set of chromosomes which means it's diploid but if we have half the number of chromosomes we call it a haploid cell so for example humans have 46 chromosomes that's our diploid number and our haploid number is 23. ultimately that's how many we will have in the end of meiosis 2 which is where we make our gametes then we had a process called crossing over this is the process whereby a homologous pair touch each other and they exchange genetic information to increase genetic variation random arrangement is when our chromosomes randomly align at the equator in a non-specific ratio of male and female inheritable characteristics for example your paternal and your maternal characteristics they are randomly lining up at the equator recombinant chromosomes refers to when two chromosomes of the pair of the homologous pair have touched each other and exchange genetics and they look slightly different homologous pair is when we have a set of chromosomes that carry similar genetic information but there might be slight variations in them and one of them is a maternal copy and the other is a paternal so essentially you have two for every set the chiasmata is the point at which a bivalence touches and cytokinesis is the separation of the cytoplasm at the end of telophase and that's how we form two separate cells now this is meiosis one i'm going to have to put meiosis two in a separate video i know it's reasonably long video but i need to be as specific as possible um and so that's where i'll dive into meiosis two i hope you enjoyed this video and i'll see you again very soon bye