hi everybody welcome back and today we are going to be looking at meiosis ii we've already looked at meiosis one so we're just going to pick up where we left off in our previous video and so meiosis ii we're going to focus in on is this second stage running along here we have already done meiosis one so if you haven't watched that video it's really important that you watch that first and then you can return to this video now that has also a recap in it of what exactly meiosis is and how it's similar to mitosis so looking at prophase two which is the next stage in our meiosis journey is you will notice now that we're dealing with two cells at the same time remembering that we started with one cell we took its chromosomes we exchanged the dna and now we've divided it into two separate cells we now need to take each of these two separate cells and we need to um separate the chromatids from one another because then we'd have too many pieces of information or too many pieces of dna in the cell and so we still need to do that now prophase two is a very short phase um it doesn't have a lot of detail in it there's no crossing over taking place now and that is because the chromosomes are no longer homologous partners you'll notice in the diagram alongside that the homologous partners have actually been separated from each other and we know that based off the picture because we find one of our homologous partners in this cell which is this purple chromosome and we find the other one in the other cell and because they're in separate cells now they can't actually cross over with each other anymore and so now we're going to do now is in prophase we're going to look at what is its purpose and how to identify prophase 2 so the overall purpose of prophase 2 is to allow for our spindle fibers to form and our centrioles to move to opposite poles very importantly it's to note the following things about what to look out for and how to identify it by what's not happening in prophase two so how do we identify prophase two we are looking for the fact that number one there is no crossing over taking place and that is because the homologous pairs are now in two different cells so they can't get to one another the second thing that we can use to identify prophase two is the recombinant chromosomes of visible and what we mean by that is if we look at the diagram that we have alongside here you can see that the chromosomes have little bits of another chromosome on them they've got a little bit of purple on them a little bit of blue and now that means that they're recombined they're not their original form the last thing that we're going to look out for is obviously our spindle fibers forming which is what we also mentioned in the purpose but it's a nice indication to show how far along we are on the phase because the fibers are now forming now moving into metaphase two our chromosomes are now doing their characteristic movement to the equator of the cell and so now they're lining up along the equator the spindle fibers have now attached to our centromeres and what we're going to do now is we need to pull apart the chromatids right now if you remember we have one chromatid centromere in the center and then we have the other chromatid on the other side and what we're trying to do is we want to pull apart these two chromatids so that they separate from one another and the reason why we're doing that is because we want to make sure we keep the number of chromosomes from this point onwards consistent we don't want to half the amount again in other words we don't want to take 46 then into 23 and then half that 23 which would end up ourselves having uneven amounts of chromosomes so what we need to do is we need to ensure the correct lining up at the equator and then separating so what is the purpose of metaphase 2 so at this stage we want to look for random arrangement once again but this time of our recombinant chromosomes because now they're lining up as individuals you'll notice in the picture that they're not touching each other like they were in metaphase two they were touching their homologous partner they were still by valence but here they're individuals and so what we want to do is we now want to arrange them at the equator once more we're not really sure which side of the chromosome we're going to get in other words what i mean by that is are you going to get this top section or are you going to get this bottom section that is another form of random arrangement now how do we identify metaphase 2 we identify metaphase ii by looking for our double stranded chromosomes aligning at the equator which that's exactly what they're doing in this picture here they're lining up in the middle of the cell and secondly we're looking for those spindle fibers attaching to our centromere that structure that holds the chromosome together and remember we ultimately want to now pull it apart now this then brings me to anaphase two which we're going to look at now but i want you to be very cautious a lot of students get anaphase 1 and anaphase 2 confused with each other now starting with anaphase 2 we can already see some dramatic differences the first thing is you will notice that there is our characteristic space that is appearing in between our now separate chromosomes and now some terminology gets a little bit tricky because previously we had one chromatid with a centromere in the middle and the other chromatid on the other side and this whole thing the whole structure we called a chromosome at the moment you pull two chromatids apart like we're doing here in this diagram we now refer to each individual strand as a separate chromosome so technically in this diagram i've labeled them that number one is one chromosome and that is number two a separate chromosome i know that they used to be called chromatids but the moment you pull the centromere apart they're now individual chromosomes from one another so how and what is the purpose of anaphase ii and how do we identify it so looking at the purpose we want to separate our recombinant chromosomes remember recombinant chromosomes where now they have these little extra bits of other chromosome attached to them these little areas of different color and we want to form single stranded chromosomes chromosomes cannot exist in their duplicated form forever they have to be single strands at some point now in order to identify anaphase two we need to look for spindle fibers that are contracting in other words they're getting shorter and that these double stranded chromosomes are becoming single and we can see that as i mentioned earlier where i labeled it one and two those are single stranded chromosomes they used to be attached to each other but now they are separate and now finally we enter into the final phase telophase 2. as you can see here we've had a lot that has changed the first thing you'll notice is that a nuclear membrane is starting to form around each of our new sets of chromosomes that we have freshly separated from each other and so essentially what that means is we've taken two cells and we've now turned them into four each one of these will produce a separate cell and so as we can see here the nuclear membrane is reforming and all we need to do now is to pinch off the cytoplasm if you remember what that looks like it's when the cell does this and we pinch off the cytoplasm via cytokinesis and we will make four separate cells so the purpose of telophase is really straightforward it is simply to go through cytokinesis where we pinch off the cytoplasm via the cell cleavage and then how do we identify telophase ii we're going to look for four separate cells that are non-identical and you can see that these cells are not identical in front of you here because of the colorations that they've used and last but not least we are going to look for single stranded chromosomes in other words we are going to look for structures that are one single chromosome we don't want to see two chromatids attached with the centromere so in other words this is not what we're looking for we are looking for individual pieces now my diagram here still has the cytoplasm attached to one another but essentially once the cytoplasm has pinched off you should have four cells in the picture as well sometimes they give you one like my diagram here sometimes they'll give you the four individual cells so let's make a quick summary of meiosis 1 and meiosis 2 collectively at the beginning of meiosis 1 we started off with the cell that was sitting in interphase you'll see that we have two single stranded chromosomes they went through dna replication and they doubled the amount of genetic information inside of them remember we do this because we have to double the genetic information at first so that we can half it later we also go through interphase to double the genetic information because we want to make sure that each of the cells gets the correct chromosome number remember we want to go from a cell that has a full set of chromosomes that is diploid and we're trying to make a haploid cell we now move into meiosis one and here we see our chromosomes moving towards each other they are going to go through the process of crossing over random arrangement and at the end of meiosis one we've done two very important things the first thing you'll notice is that our chromosomes now look a little bit different and that's because of crossing over the second thing you'll notice is that we started with two chromosomes originally in the cell we now have one each and so at the end of meiosis you will produce two haploid cells now we can't leave our cells with these chromosomes in them while the chromosomes are still replicated in other words you'll see that these cells here still have their centromeres and we need to get rid of those we need to essentially take the chromatids apart we need to move them away so now we enter meiosis ii the purpose of meiosis ii is to take these chromosomes which we call recombinant chromosomes and we want to separate the individual strands the chromatids from one another and so what we do is we take each one and we place it in individual cells and you will notice that each of these four cells contain one chromosome so it's important to remember that we originally started off with two chromosomes we finished meiosis one and now each cell has one chromosome in it now even though we separated the chromatids from each other into separate cells which we don't call chromatids anymore we call them single stranded chromosomes at the end of meiosis 2 every cell still only has one chromosome in it often that's the thing that gets most confused about because when is it a chromosome when is it a chromatid remember if it is a if there's a centromere it's still a chromatid no centromere we now call it a chromosome and so at the end of meiosis ii we are going to have four unidentical cells and they are haploid from the original diploid cells that we started with so to round off a lesson let's do our terminology recap so random arrangement is when our chromosomes line up at the equator and we are going to separate them in this instance in meiosis ii we're not separating them from the homologous partner instead we are separating them from their chromatid partner one chromosome therefore goes into each cell cytokinesis is the separation of our cytoplasm and at the end of meiosis ii you should have four separate cells diploid is when you have a full set of chromosomes that have not been changed whereas haploid means it's a cell that has half the amount of the original chromosomes an example of this is the diploid number of chromosomes in humans is 46 the haploid number is 23. recombinant chromosomes or chromosomes that have gone through crossing over and now they need to be separated from one another in meiosis two because we need to separate the individual arms of the chromosomes which in all instances we can also call them recombinant chromatids and last but not least single-stranded chromosomes that is the goal of meiosis ii you want to take a double-stranded chromosome that has been recombined and you want to transform it into a single strand if you don't do this your cell will have too much information inside of it and you can end up having a abnormal meiosis results which is not what you want thank you for watching everybody i hope this is very helpful informative video this one's a little bit shorter than the previous one if you liked it please let me know and i'll be back very soon again to do more explanations make sure you go through the other videos look at perhaps the exam questions that go with these topics so that you can prepare for tests and i will see you again bye