all right welcome back this is the second video for chapter 11 and again we're talking about meiosis 2 this time so this is the second half of that summary figure meiosis 2 and in some cells it's not shown here but between meiosis 1 and meiosis 2 some cells go through a brief interphase called inter Kinesis and this is a brief resting phase before they start the second part of meiosis there is no S phase in interkinesis there's no duplication of chromosomes so after the two cells go through meiosis 1 they go through maybe interkinesis and then the series of events in meiosis 2 so that the chromosome number continues to really stay the same except we're going to separate the sister chromatids by the end of meiosis 2 so what happens in prophase 2 really depends on what kind of cell or species you're looking at if it's if we're looking at human cells it would look a lot like the picture that we see here except instead of the three chromosomes we see here we would have 23 in each of these cells but if the chromosomes did decondense during telophase 1 they're going to recondense in prophase 2 and if the centrosomes were duplicated previously then they're going to move to the opposite poles so the opposite ends of the cell and new spindles form as you can see here similar to prometaphase 1 by the end of prometaphase 2 the nuclear envelopes will completely disappear again the spindles will be fully formed and again each sister chromatid will have or create a kinetic core and the microtubules will attach from the opposite poles remember I drew this earlier we have our cister chromatids we have our Center and then the canetto core proteins will be on the edges will be attached to the spindles or the microtubules remember microtubules are what make up those spindle fibers our book does a nice job showing how the spindle fibers those microtubules attach to our chromosomes differently in meiosis 1 compared to meiosis 2 so let's look at meiosis one this is PR metaphase 1 remember in PR metaphase 1 our homologous chromosomes are still attached at those kayata and I can see the yellow cir and I can see the spindle fibers attached to the kinetic cores right there in anaphase 1 the spindle fibers when they shorten will pull apart the homologous chromosomes so homologous chromosomes are separated during anaphase 1 and you can see those kayata are pulled apart and the tetrads no longer exist because they're pulled apart in meiosis 2 if I look at the difference between prometaphase 1 and prometaphase 2 you can see there are no more homologous chromosomes it's just one chromosome here with its sister chromatid attached at the central mirror this time the spindle fibers are attached to the each side of the cister chromatids and in anaphase 2 we're going to see that the cister chromatids are pulled apart by the shortening of those spindle fibers metaphase 2 and anaphase 2 are really similar to mitosis where the sister chromatids will line up at the metaphase plate but remember we're going to have two cells because we had two cells at the end of meiosis 1 also in metaphase 2 remember these are already haid already haid cells so there are only one set of chromosomes there's no homologue anymore the homologue is in the other cell so this is mom's hair color for example and dad's hair color color would be all the way down here in the other cell what we have is cister chromatids lining up in the equator and in anaphase 2 the cister chromatids are split apart so at the end of telophase 2 the chromosomes have arrived at the opposite poles they're going to decondense and the nuclear envelope will reform around the chromosomes and in cyto canis the cells will be separated into a total of four un unique haid cells remember the four cells are unique because of crossing over that happened in prophase 1 and because of Independent Assortment that happened in metaphase 1 allowing the cells to only carry either mom's version of a chromosome or Dad's version of that chromosome so let's summarize some of the key points that happen in meosis so in meiosis 1 in prophase 1 remember we had interphase happened already so during inphase and S phase specifically we created our sister chromatids when our chromosomes were duplicated so I can see each of these chromosomes has a sister chromatid attached at the Central mirr and in prophase one it's different from mitosis because we have synapsis remember the two homologous chromosomes or the pair of homologous chromosomes come together and they're bound together with that zipper likee structure the synaptonemal complex the kayada are where recombination or crossing over will occur and crossing over occurs during prophase 1 in metaphase 1 is when we had independent assortment of the chromosomes where the maternal and the paternal chromosomes could align randomly on either side of the metaphase plate and each of these assorted independently from the other chromosomes in anaphase homologous chromosomes are pulled apart and by tase and cyto Kinesis by the end of that stage we have two unique appid cells so I should say diploid over here metaphase oh sorry meiosis 2 so meiosis 2 is shown down here this is a lot more similar to mitosis but we start with haid cells and we end with HAPO cells so the poy stays the same kind of like mitosis and something else that's similar is that we have sister chromatids that line up on the metaphase plate during metaphase 2 just like we do in mitosis metaphase cister chromatids are split apart in anaphase 2 just like we had in anaphase of mitosis and this time we end up with four unique cells again they're unique because of crossing over that happened in prophase 1 and independent assortment that happened in metaphase 1 this is the same slide I showed you earlier now that we've gone over the different phases take a look at the outcomes and test yourself see if you can cover up this part and talk about what happens in each of these stages of interphase and meiosis 1 and remember by the end of meiosis 1 we already have uh reduced the chromosome number we go from diploid to haid cells and I recommend doing the same thing for meiosis 2 cover up this part of the figure and see if you can test yourself talk about each of these phases and what happens and again remember in meiosis 2 we start with haid cells and we end up with haid cells we end up with a total of four unique hloy daughter cells and sorry this is capitalized because of the font I use some kind of title font here it doesn't let me do lowercase although mitosis was on a previous exam I do want you to remember the components so that we can compare and contrast myosis and my mitosis so let's look at the two both of them divide the nucleus and eukariotic cells remember procaryotes don't have a nucleus they go through binary fision usually when they're dividing the cell mitosis we're dividing only once we get two cells meiosis divides twice so we end up with four cells in meiosis we have homologous chromosomes pairing up forming those tetrads that will cross over that does not happen in mitosis and in meiosis crossing over happens between those noncy chromatids of the homologous chromosomes we also have random assortment of the chromosomes in metaphase 1 where the maternal and paternal chromosomes line up differently and independently of the other chromosomes and those help to create genetic variation in the daughter cells in contrast mitosis produced genetically identical cells finally meiosis uh reduces the ploy of the cells we start with diploid cells one we end up with four hloy cells and in mitosis we don't change pyy so we have diploid to diploid or in some organisms we have like plants haid to haid this is a nice figure from our book that compares meiosis and mitosis in meiosis I can see there are two divisions right there's meiosis one I get two cells at the end of that and then by the end of meiosis 2 I have four cells in contrast in mitosis I can see I have one division ending up with two cells I can see the chromosomes line up differently in metaphase 1 they line up side by side remember those homologous chromosomes line up side by side and you can't really see it it's too small but there's crossing over during prophase 1 and I can see that meosis 2 looks a lot more like mitosis because the in metaphase metaphase 2 of meiosis and metaphase of mitosis those sister chromatids are lined up and they're going to be split up during anaphase our book also has a really nice chart under that figure I just showed you this is very worthwhile to study for the next exam so let's look at each process DNA synthesis it happens in SASE of interphase for both myosis and mitosis synapsis of homologous chromosomes remember that's when the homologous chromosome line up really really close together through that zipper light complex the synaptonemal complex so what happens during prophase 1 of meiosis it does not happen in mitosis crossing over remember that happens when you form those kayada and that happens during prophase one of meiosis not in mitosis homologous chromosomes lining up side by side at the metaphase plate happens during metaphase 1 but does not happen during mitosis cister chromatids lining up happens during metaphase 2 of myosis and also during metaphase of mitosis and that's why these two are more similar so meiosis 2 is more similar to mitosis let's look at the outcome so at the end of meiosis we have four unique hloy cells unique meaning genetically unique and these are haid again even though we started with a diploid cell in mitosis we end up with two diploid cells at the end of mitosis assuming we started with a diploid cell in humans and other animals as well genetic diversity is increased by a third component random fertilization so in humans we talked about some of these before there's genetic diversity because of crossing over that happens in prophase one independent assortment that happens in metaphase 1 and the new one that we're talking about now random fertilization what this means is that any sperm and there are millions of sperm released canuse with any unfertilized egg and these are called oam This is singular OA is plural so the fusion of the two gametes remember gametes are sperm and egg each of them have about 8.4 million possible chromosome combinations remember we saw that earlier 2 to the 23rd power because of independent assortment each of these combinations uh can produce a zygote a zygote is the first cell that's formed when a sperm and egg come together so that gives us about 70 trillion diploid combinations that is crazy that's a ton crossing over adds even more variation to that random fertilization so each zygote has a unique identity and this is kind of something that you're going to study more in the second course of the series 4B but about 200 to 300 million sperm will enter the female reproductive tract when we're talking about humans um sometimes people ask me about uh identical twins so if you have someone or know someone with identical twins what happened was one sperm fertilized the egg to create a zygote and during develop one cell will rapidly go through mitosis it'll form two cells four cells eight cells 16 cells you'll eventually form a ball of cells if those cells somehow separate during this process you'll get identical twins if you have fraternal twins twins that are not genetically identical usually what happened is you have two eggs that were fertilized by two different sperm all right and I wanted to show you this animation that summarizes the three ways we get genetic variation in sexually reproducing organisms three processes lead to most genetic variation independent orientation of chromosomes in meiosis crossing over of chromosomes in meiosis and random fertilization each pair of homologous chromosomes consists of one chromosome inherited from the father and one from the mother here we have colorcoded them blue and red each pair of chromosomes lines up independently of the other pairs in metaphase 1 of meiosis here you see one of the possible arrangements and outcomes there are two different ways that each chromosome pair can line up that means that in the organism shown here with the diploid number of four independent orientation of chromosomes at metaphase 1 can produce gametes with four different different combinations of maternal and paternal chromosomes in a human being with 46 chromosomes more than 8 million combinations are possible now let's look at how crossing over creates even more genetic variability during prophase 1 of meiosis homologous chromosomes pair up very closely and corresponding parts of two non-c chromatids May trade places this process of crossing over creates variation by produc producing chromosomes that combine the genes inherited from two parents here the process produced a total of four genetically different gamits there are many ways crossing over can occur in humans crossover events happen an average of two or three times per chromosome pair greatly increasing the variation among eggs and sperm note that crossing over produces some parental gametes with chromosomes like those of the parents and some recombinant gametes with a mixture of genes from both sets of chromosomes independent orientation and crossing over occur simultaneously during meiosis multiplying the number of genetic variations among gametes because each pair of chromosomes lines up independently and crossovers can occur almost anywhere along each pair of chromosomes it is possible for a human being to produce an almost infinite variety of gametes a sperm ferti izes an egg producing a zygote the random nature of fertilization adds to the variation arising from meiosis each parent is capable of producing a huge variety of genetically different gametes the number of possible combinations among their offspring is staggering theoretically one human couple is capable of conceiving a number of genetically different offspring that is far greater than the number of humans who have ever lived all right so that summarizes three uh three different processes that increase genetic variation we talked about independent assortment crossing over and random fertilization that takes us to the end of our second video in our third and final video we're going to look at life cycles and that is section 11.2 of the reading