this is the first lesson of chapter 10. we're going to be continuing our discussion of how cells divide making new cells but unlike in mitosis meiosis results in cells that are haploid and these are the specifically these are cells that become the eggs and the sperm and so let's take a look at meiosis foreign to the mitotic cell cycle meiosis involves several sub stages also like the mitotic cell cycle there are interphase and then the phases of meiosis as shown here in fact as you can see in meiosis one on the left [Music] interphase is the actually before meiosis the first phase if you could say there is a first phase because it's a cycle of course it kind of depends on where you're starting what stage that cells in but there is one interface throughout the entire meiotic cell cycle notice that that's a very important detail and then that is followed by meiosis 1 which includes prophase one metaphase 1 anaphase one telophase one and cytokinesis and then there's a second round of meiosis but note that there's no interphase in meiosis II it goes right from telophase 1 and cytokinesis to prophase two metaphase 2 anaphase 2 telophase II and cytokinesis so now we're going to take a closer look at each of these phases like we did with mitosis we're going to go over some of the key events and structures that would happen in that cell at that stage and then make a drawing of a hypothetical cell foreign so when we look at the model cell drawn here we're going to be modeling like in mitosis the animal cell with a diploid number of four or two n equals four that means that a normal body cell of that animal has four chromosomes however the resulting cells for meiosis are going to have half the diploid number and that is called the haploid number and that will be two but we'll see that at the very end of the process so remember that interphase involves three substages G1 s and G2 and they occur and have the same exact events as we learned about in mitosis so if you want to review those you go back to the lesson on mitosis and see the details of g1s and G2 so the cell that I've shown here would have already gone through Gap Gap 1 and S so this would be specifically in G2 and the reason I'm emphasizing that is because there's already been a duplication of the the centrosomes with the pairs of centrioles in them and also although you won't know because we have no means of comparison by looking at this but I want to also make it known that the DNA in the form of Chromatin right now the uncondensed DNA in the nucleus has already duplicated now I'm showing two different colors here in the chromatin and there's a reason for that that you'll see as we continue on in the in the lesson okay so in prophase one you see that the nuclear membrane is starting to disintegrate while the chromatin condenses into chromosomes now these chromosomes are in the form now of the duplicated chromosomes or sister chromatids but this looks a little bit different from mitosis because of the fact that there's two pairs of sister chromatids that are forming this Association together and that is called a tetrad those chromosomes within the tetrad are homologous and what that means is that they code for the same genetic traits like let's say hair color or height for a human and the reason now I'm using the different colors red and blue is to represent the maternal and the paternal genetic material every animal has a mother and a father and there cells have DNA from both parents each of those maternal and paternal chromosomes duplicated during s of interphase and so this is why we have a pair of blue and a pair of red sister chromatids and together again they form a structure called a tetrad I'm going to zoom in and on the next slide to show what's happening in that tetrad and what this crossing over might be but before I do that let me finish talking about prophase one including the formation of the spindle as shown at the top left so the pair of duplicated centrosomes which with pairs of centrioles inside now have started to migrate and the spindle fibers are forming in between them and this the picture I drew would be more of like an early prophase but this is going to continue to move the spindle fibers are going to start to gather the um the chromosomes by attaching to their centromere so that's what's described in the last bullet point even though that's not reflected in the drawing here you might see early prophase something like what I drew or it could be a later stage of prophase so now we're going to zoom in to a tetrad and try to understand what that is and what this crossing over could be okay so let's take a look at that tetrad this would be the condensed chromatin forming a chromosome but they've been duplicated and they're attached at their centromeres and then so those are the maternal duplicated um pair of sister chromatids and then we have the paternal pair each of these has a kinetochore at their centromere just like again we saw in mitosis these are special motor proteins that are going to be helping those um sister chromatids separate and then also become daughter chromosomes ultimately in the final daughter cells so again these are centromeres which are the central part that didn't come out too great let's see here okay that's a little bit better so now you can see I've labeled the centromere which is the central region where the pairs of sister chromatids are attached and at each centromere is a kinetochore protein and I've also added a note up at the top that this whole thing this Association of two pairs of sister chromatids is called a tetrad of homologous chromosomes okay so the next thing I want to show you here is something that could happen sometimes where you have the the overlap of the um maternal and the paternal chromosomes there could be uh this phenomenon called crossing over where basically they swap places a little bit of the blue will switch places with the red piece and this is a very important phenomenon that can actually lead to an increase in genetic diversity of the daughter cells so I'm going to be showing this crossing over for the rest of the steps so I'm just gonna show here that it it happened imagine that they've already switched places and so in the remaining steps of the process we're going to show that that crossing over has already taken place okay so I'm just going to make a note here that crossing over [Music] leads to an increase of genetic diversity of the daughter cells also crossing over can only take place in prophase one of meiosis so you're not going to see that happen again even though we'll see the results of that carry through to the later stages all right so here we have metaphase one notice that the tetrads still together have lined up on the metaphase plate which is that imaginary equator of the cell and it runs perpendicular to the direction of the spindle fibers coming from the centrosomes so at this point the spindle microtubules have attached to the center centromeres of the sister chromatids and line them up I also want to point out that on the long set of chromosomes on the left I have shown I've tried to shown crossing over there so that already happened remember in prophase one but I'm just going to continue to carry that through till the end of the process moving along we have anaphase one where the homologous pairs within the tetrad have separated now into the sister chromatids so we have for example on the left the long pair of red sister chromatids where we do have a little bit of Blue from the crossing over has separated from the blue pair of sister chromatids with a little bit of red and then on the right side we have a shorter pair of blue sister chromatids that separated from the shorter pair of red sister chromatids and they're being pulled to opposite sides of the cell as the kinetochore microtubules shorten [Music] those are the ones that are attached to those kinetochore proteins remember there's actually I'm not going to show them for everything but there's proteins there that these spindle fibers of this of the spindle are attached to while that's happening the non-kinetochore microtubules lengthen and this is what's causing the cell to stretch notice how it's looking more like an egg shape than perfectly round in telophase one you will no longer see the spindle fibers because the spindle has disintegrated as both of the pairs of sister chromatids have fully made it to the opposite poles of the cell and while that's happening cytokinesis occurred where the cytoplasm divided and that resulted in two independent daughter cells that we have here each daughter cell right now has two pairs of sister chromatids a long pair and a short pair and see how the long pair has that little bit of crossing over that took place in prophase one so we did see it follow through all the way to the end so we're going to take a pause here for this video because we've just completed all the stages of meiosis one and we'll pick up in our next lesson with meiosis II so recall where we left off and be ready to go through a second round which ultimately ends with the daughter cells that are going to be haploid and will become the eggs or sperm for the animal that they're in