hello everybody my name is Iman welcome back to my YouTube channel today we are covering chapter 13 of biology and this chapter is about meiosis and sexual life cycles now as the title suggests meiosis is a really big part of this chapter in short meiosis is a special type of cell division that produces cells with half the chromosomes of the parent cell it only occurs in specialized cells cells like the those found in the testes and ovaries in humans but of course it's more than just understanding that there are several objectives that we want to understand and cover here that will really help us conceptualize and begin to understand meiosis and sexual life cycles and how genetic variability arises from these processes so these are the following points that we're really going to try to hone in on in this video first objective offsprings acquired genes from parents by inheriting chromosomes second objective that we're going to elaborate on is that fertilization and meiosis alternate in sexual life cycles objective three is that meiosis reduces the number of chromosomes set from diploid to haploid and four will end this chapter with a conversation about genetic variation produced in sexual life cycles and how they contribute to Evolution now let now let's start by really motivating this chapter's objectives that we're gonna you know spend some time covering one by one you know friends might tell you that you have your mother's nose or your father's eyes although they don't mean that literally of course and that's because the transmission of traits from one generation to the next is called inheritance or hereditary at the same time Sons and Daughters they're not identical copies of either parents or their siblings along with inherited similarity there's also variation so while you might inherit your mother's freckles or your father's eye colors you are not an exact copy of either of them and neither are your siblings and this is because of variation the study of both hereditary and inherited variation is called genetics and that's kind of what we're starting a little bit about um talking about in this chapter and even a couple of following chapters we're really focusing on the genetics now with that said let's start let's let's take a quick overview of of this infograph here that shows us um quickly meiosis it defines a defines it it asks what biological mechanisms account for the resemblance between offsprings and their parents and while we stated a little bit of that the point of covering these objects is so that this infographic makes a lot more sense by the end of the chapter we're going to see something similar at the end after we've covered the details and then we'll be able to retouch bass and see how this makes more sense with that being said first objective all right offspring's acquired genes from parents by inheriting chromosomes parents endow their offsprings with coded information in the form of hereditary units called genes all right hereditary units called genes the genes we inherit from our mothers and our fathers are our genetic link to our parents and they account for family resemblances like shared eye color or freckles or specific shape of nose Etc our genes program those specific traits that emerge as we developed from fertilized egg into adults and the genetic program is written in the language of DNA which we're gonna talk about here we're going to cover in great detail in the next few chapters but we also have been introduced to when we've talked about new nucleotides and nucleic acids previously but what we should know is this all right what we should know is that the transmission of hereditary traits has its molecular basis in the replication of DNA which is going to produce copies of genes that then can be passed from parent to offspring and animals and plants reproductive cells are going to be called gametes and they are vehicles that transmit genes from one generation into the next now during fertilization male and female gametes the sperm and the Egg unite all right and they unite and in that process they are passing on genes from both of the parents from that sperm and egg to their offspring all right every species has a characteristic number of chromosomes so let's take let's take us as an example as human beings right humans have 46 chromosomes in their somatic cells and what somatic cells means is all the cells of the body except the gametes and their precursors each chromosome is going to consist of a single long DNA molecule that's elaborately coiled in associate in association with various proteins like histines like we've covered previously now one chromosome just one chromosome all right is going to include all right several hundreds to a few thousands G to a few thousand genes each gene being a precise sequence of nucleotides along the DNA molecule and a gene specific location along the chromosome is called the Gene's Locus now something else that's worth mentioning and talking about now that we've set up how offspring's acquired genes from parents by inheriting chromosomes right we have the you know the sperm and egg when they unite they pass on genes of both parents to their offspring all right well something else that's worth mentioning and talking about is the difference between sexual and asexual reproduction I made mention about how you might have is you know the same nose as your mother or the same eye call color as your father but you are not an identical replication of either of them and that's because humans partition participate in sexual reproduction on the opposite end of that is asexual reproduction only organisms that reproduce asexually have offsprings that are exact copies of themselves in asexual reproduction a single individual is the sole parent all right and they will pass copies of all their genes to The Offspring without the fuss and the fusion of gametes on in sexual reproduction on the other hand right so that was asexual reproduction all right it's pretty much like a cloning right you can think of it like cloning all right sexual reproduction on the other hand right you have two parents that give rise to offsprings that have unique combinations of genes inherited from both of them all right so in contrast to this cloning of asexual reproduction offsprings of sexual reproduction are gonna vary genetically all right from their parents from their siblings all right they are variations all right of common themes of family resemblance but they are not exact replicas right now sexual or asexual reproduction all right a life cycle is the generation to generation sequence of stages in the reproductive history of an organism and that's kind of what we want to slowly begin to transition into but very quickly takeaways from our first objective offsprings acquired genes from parents by inheriting chromosomes the takeaway here is that each gene in an organism's DNA exists at a specific Locus on a certain chromosome in asexual reproduction single parent produces genetically identical Offspring by mitosis but in sexual reproduction you're going to combine genes from two parents and that's going to lead to genetically diverse offsprings all right let's talk about our second objective now all right fertilization and meiosis alternate in sexual life cycles right a life cycle is like we said the generation to generation sequence of stages in the reproductive history of an organism this is from conception to production all right now for this objective let's start by using humans as an example to track the behavior of chromosomes through the sexual life cycle right we're going to begin by considering the chromosome count in human somatic cells and human gamete cells all right in humans each somatic cell all right so that's all the cells besides your gametes besides you know the the cells in your ovaries and testes all right those are somatic cells they all have 46 chromosomes before mitosis Begins the chromosomes are duplicated then during mitosis those chromosomes become condensed enough to be visible all right under under a light microscope or right not to the naked eye and then at this point okay they can be distinguished under the microscope from one another by their size the position of their centromeres the patterns of colored bands produced by certain chromatin binding stains Etc all right now careful examination right of the 46 human cells from say a single cell in mitosis shows you that there are two chromosomes of each 23 types and this becomes really clear all right this becomes clear when images of the chromosomes are arranged in pairs starting with the longest chromosomes all right and this resulting order of the 23 types of chromosomes all right of which there are two copies of each all right in this kind of display that we see here is called a karyotype all right so notice we have 23 pairs or this is 22 this is 23 pairs all right of chromosomes all right in each pair there's two right so total of 46 all right 23 pairs that means these are copies of each other essentially all right there are two copies of a type so the resulting ordered display here is called a keratype the two chromosomes of a pair all right they're going to have the same length the same centromere position and the same staining pattern and so what we call this pair all right of chromosomes this pair of chromosomes that have these same property same length centromere position Etc these are called homologs all right they are homologous chromosomes all right so either of this terminology being used to discuss this means the same thing homologues homologous chromosomes all right both chromosomes of each pair carry genes controlling the same inherited character all right the same inherited characters for example if a gene for eye color is situated at a particular Locus all right on a certain chromosome right there all right then it's homologous chromosome it's homo it's homolog is going to have the same it will have a version of the eye color Gene at the equivalent Locus all right that doesn't mean they both have necessarily the gene for blue eyes but the gene for eye color is in the same location for these two homologous chromosomes all right so that's what that means that there's a very important distinction here to be made between those two statements all right now what you noticed also is here at the end we have this instead of numbering at 23 this X Y pair all right the two chromosomes referred to as X and Y are going to be important exceptions to the general pattern of homologous chromosomes in human somatic cells now typically human females will have a homologous pair of X chromosomes so they'll have XX while males have One X and Y one chromosome of course there's exceptions to this variation as well that's probably going to be a topic for later chapters now most of the genes carried on the X chromosome they do not have counterparts on the tiny Y chromosome all right and the Y chromosome has genes lacking on the X and so due to their role in sex determination the X and Y chromosomes they're called sex chromosomes all right so these have a specific name this X Y this 23rd pair of chromosomes they're called sex chromosomes every other pair of chromosomes 1 through 22 those are called autosomes all right that's spelled like this autosomes all right that's going to be pair one through 23 of this parent all right fantastic so homologs homologous chromosomes Define as chromosomes with the same length same centromere and Gene location all right fantastic see how these have the same length all right they have the same gene location Let's Pretend that's the gene for eye color right look at them they're at the same location they could both be for the genes for blue eyes one could be blue one can be brown it doesn't matter it's just that it's the same gene identification location all right um and so that's just I had to repeat it it is an important distinction to be able to make now the occurrence of pairs of homologous chromosomes in each human somatic cell it's a consequence of our sexual origin we inherit one chromosome of a pair from each parent so that means the 20 the 46 chromosomes in our somatic cells are actually two sets of 23 chromosomes all right a maternal set of 23 chromosomes and a paternal set of 23 chromosomes all right so let me repeat that 46 chromosomes and our somatic cells are actually two sets of 23 chromosomes a maternal set from our mother and a paternal set from our father all right the number of chromosomes in a single set is then represented by the letter n all right any cell with two chromosome sets is called a diploid cell and it has a diploid number of chromosomes and so it can be abbreviated as 2N now we said our somatic cells all right so every cell in our body except our gametes they're going to have two n number of chromosomes they're going to have two sets of 23 chromosomes for a total of 46 chromosomes in those somatic cells all right and so what do we call somatic cells well how do we describe them we use the word diploid cell right because it has a diploid number of chromosomes all right unlike somatic cells are gametes they can they they consist all right of they contain a single set of chromosomes all right they only contain one set of chromosomes so they only have n chromosomes all right so our gametes have 20 organic cells have 23 chromosomes we call that haploid all right a haploid cell a haploid set all right because these gametes they only have 23 chromosomes they have n whereas our diploid has two n chromosomes so that's another way to distinguish that fantastic all right so that's how we can Define diploid cell has it has two complete sets of chromosomes most cells in humans are diploid comprising of 23 chromosome pairs so 46 chromosomes in total haploid refers to the presence of a single set of chromosomes in an organism cell in humans only the egg and sperm cells are haploid now the human cycle the human cycle begins when a haploid sperm from the father fuses with a haploid egg from the mother and this Union of gametes cultivating in in the fusion of their nuclei all right when they when they combine when they come together it's called fertilization all right the resulting fertilized egg is also called a zygote zygote by the way it's diploid now because it contains two haploid cells of chromosomes bearing genes that represent them Eternal family line and the paternal family line right they have chromosomes from both the mom and the dad now as the human develops into a sexually maturing adult mitosis of the zygote and its descendant cells um generates all the somatic cells of the body right both chromosome sets in the zygote and all the genes they carry are passed with Precision to the somatic cells the only cells of the human body that are not produced by mitosis are again the gametes which develop from specialized cells called germ cells in the gonads that's going to be ovaries in females and test these in males all right so the fertilization of egg and sperm creates a diploid zygote all right which from there um produces and generates all the somatic cells of your body and of course from your germ cells in the gonads all right you develop those those um gamete cells that are haploid now gamete formation specifically involves a type of cell division called meiosis all right this type of cell division reduces the number of sets of chromosomes from two in the parent cell to one in each gamete all right and that kind of counterbalances the doubling that occurs at fertilization all right so as a result of meiosis each human sperm and egg is then a haploid all right fertilization restores the diploid condition right so you have this woman who's all of her cells are somatic they have 46 chromosomes except for her her ovaries her gametes all right same thing for the men all right and then his sperm and her egg unite all right and they now form a diploid zygote all right so this fertilization of gametes you know the egg from the women the sperm from the man restores that diploid condition by combining two sets of chromosomes and the human life cycle is repeated that way generation after generation all right now just as a little bit of a side note we're not going to get into this too much but although the alternation of meiosis and fertilization is common to all organisms that reproduce sexually the timing of these two events in the life cycle varies depending on the species well now we're not going to worry too much about the life cycle of Plants algae or fungi fungi but what we do need to know is the following the common feature of all three Cycles is that alternation of meiosis and fertilization key events that contribute to genetic variation among offsprings and the cycle differs in the timing of these two events depending on whether you're talking about Plants algae or fungi all right there is great variety of sexual life cycles that's the takeaway and I guess a bit of a summary of all the objectives that of all the points that we've covered in objective two now all right we've discussed that normal human somatic cells are diploid they have 46 chromosomes all right that are made out of two sets of 23 chromosomes one set from each parent human diploid cells have 22 pairs of homolog homologs that are autosomes and one pair of sex chromosomes all right the later typically determines whether the person is female or male in humans ovaries intestines they produce haploid gametes by my by meiosis each gamete containing a single set of 23 chromosomes now during fertilization an egg and a sperm unite ignore that forming a multicellular forming a diploid single cell zygote which develops into a multicellular organism by mitosis all right and of course the last point that we've made mention of is that sexual life cycles differ in the timing of meiosis relative to fertilization and in the points of the cycle at which multicellular organism is produced by mitosis all right with that we move into our third objective meiosis all right meiosis reduces the number of chromosome sets from diploid to haploid here we're going to cover and go over a lot of the details of meiosis and we're even going to cut compare it to mitosis which was the topic of last chapter now several steps of meiosis closely resemble corresponding steps in mitosis meiosis very much like mitosis is preceded by interphase which includes a S phase where the duplication of chromosomes happen however the difference is that this is not followed by one but two consecutive cell divisions in meiosis called meiosis 1 and meiosis II and these two divisions result in four daughter cells all right as opposed to mitosis which resulted in two daughter cells all right and in meiosis those four daughter cells each have only half as many chromosomes as the parent cell so they have one set rather than two all right now some quick reminders of some very important definitions right I just want to make sure that we remind ourselves of some of the vocabulary used here I know we covered it mitosis still important to cover hair because understanding that vocabulary will make understanding the steps of meiosis a lot easier all right so if you recall um one definition we want to go over is sister chromatids right sister chromatids are copies of one chromosome all right so notice here here you have two chromosomes and they're a pair of homologous chromosomes in a diploid parent cell so this cell has 46 chromosomes total all right two sets of 23. all right these are homologous chromosomes in that diploid cell remember what that means they're going to have the same length position of Centrum years same staining they're going to have the same Locus for the same gene all right they're going to duplicate all right these chromosomes are going to duplicate so now this guy duplicates himself all right and this this guy duplicates himself now what we have is two identical copies of this one chromosome all right and we call them sister chromatids so this is these two these two chromosomes are sister chromatids all right now these right here were homologous pairs they duplicated this is a sister chromatid they are still homologous chromosomes all right they've just duplicated themselves and attached themselves to their sister chromatids all right so that's one important definition to remember sister chromatids are two copies of one chromosome closely associated all along their length this Association is called a sister chromatid cohesion together the sister chromatids make up one duplicated chromosome so this is a chromosome this is a duplicated chromosome all right the duplicated chromosomes are still homologous chromosomes all right now in contrast right the two chromosomes of a homologous pair are individual chromosomes that were inherited from each parent so this may be Mom's chromosome and this may be dads all right they're not duplicated copies of each other they're just the homologous pair all right that means they have similarities in things again like length centromere position all right location of Gene all right on that length so those are two distinctions I will continue to repeat because it is important to make that contrast all right homologous pillars of individual homologous pair are individual chromosomes that were inherited from each parent homologs do appear like in the microscope but they can have different versions of genes at the corresponding Locus right each version is called an allele of that Gene right so you can have these two homologous chromosomes one from Mom one from Dad and they both have the same location and that homologous chromosome for Gene for for eye color but one chromosome can have the blue color Gene the other one the brown color Gene right so each version of is called an allele of that Gene fantastic now that that was just two definitions I wanted to make sure we understood before we move into all the steps of meiosis all right meiosis like we said is a specialized form of cell division that occurs in sexually reproducing organisms to produce gametes to produce sex cells with half the number of chromosomes as the parent cell it involves two consecutive rounds of division called meiosis one and meiosis II and we're gonna go um we're going to read a summary of the key steps involved in meiosis all right now they have we're going to do a quick summary and then we're going to read the details in this infographic all right and then we're going to repeat that again right repetition is key here all right now just like mitosis there is an interface step first right the cell undergoes a period of growth and DNA replication that's going to result in the formation of identical sister chromatids all right then we start with prophase one all right let's read the bullet points here and then we're going to summarize it all right so prophase one the centrosome movement spindle formation and nuclear envelope breakdown occurs as it does in mitosis all right and the chromosomes are going to condense progressively throughout prophase one all right also during early prophase one each chromosome pairs with its homolog all right aligned Gene by Gene and sometimes crossing over can happen so parts of each homolog chromosome can just swap out places we're going to talk about that even more in just a second all right so that means the DNA molecules of non-sister chromatids are broken by proteins and are rejoined to other all right now at the stage shown above each homologous pair has one or more x-shaped region called um chiasma where crossovers have occurred and then later in prophase one microtubules from one pole or the other are going to attach to the kinetochores one at the centromere of each homolog all right and microtubules are going to move the homologous pairs towards the metaphase plate all right so the summary here of prophase one is that your chromosomes are going to condense and your homologous chromosomes are going to pair up to form tetrads all right because they've already duplicated in the interface right what you have now are these duplicated chromosomes and the the duplicated uh homologous chromosome as well all right they're gonna pair up to form tetrads this pairing process known as synapsis sometimes allows for the exchange of genetic material between non-sisterchromatids all right so between homologous chromosomes all right and this is called crossing crossing over then the nuclear envelope breaks down and the spindle apparatus begins to form all right and then we move into metaphase one pairs of homologous chromosomes look at that are now arranged atom at the metaphase plate so here's a duplicated chromosome and its homologous duplicated chromosome pair all right and look at all of them they're aligned on that metaphase plate in the center all right with one chromosome of each pair facing each pole each pair has lined up independently of other pairs all right we're going to discuss the consequences of that later in independent assortment all right both chromatids of one homolog are attached to the kinetochore microtubules from one pole all right the chromatids of each homolog are attached to microtubules look at that they're attached to these microtubules one over here one over here all right they're attached to each other from opposite poles all right so in metaphase one these tetrads align along the cell's equator with each homologous chromosome attached to microtubules from opposite poles of the cell then we move into anaphase one all right breakdown of proteins that are responsible for sister chromatid cohesion along chromatid arms allows homologs to separate all right so now each duplicated chromosome all right and it's homologous pair each one gets moved to the opposite side of the cell all right so homologous chromosomes are now separated all right the the homologs are going to move toward the opposite poles Guided by those spindle apparatus sister chromatid cohesion persists at the centromere causing the two chromatids of each chromosome to move as a unit along the same pole notice sister chromatids are not broken they stay together but homologous pairs are separated now in anaphase one all right so you had these homologous chromosomes pair up at the metaphase all right but each one of the homologous chromosomes gets taken to opposite sides during anaphase one so the two homologous chromosomes of each pair separate all right then there's telophase one and cytokinesis essentially two haploid cells form each chromosome still consists of two sister chromatids so when telophase one begins each half of the cell has a complete haploid said set of duplicated chromosomes each chromosome is Con composed of two sister chromatids one or both chromatids include include regions of non-sister chromatid DNA because of that crossing over that happened in prophase one and then cytokinesis happens um which is the division of the cytoplasm to form two haploid daughter cells all right fantastic so that is meiosis one you had duplicated homologous chromosomes pair up and exchange some segments of of each other to one another all right then the chromosomes the homologous the chromosomes line up by homologous pairs that's metaphase one in anaphase one we had the two homologous chromosomes of each pair separate and then in telophase one and cytokinesis two haploid cells form each chromosome still consists of two sister chromatids all right so that is meiosis one then we can move into meiosis too all right here we're separating homologous chromosomes now we're going to be separating sister chromatids all right in prophase two you're gonna have spindle apparatus form and in late prophase two chromosomes each still composed of two chromatids are moved by microtubules towards the metaphase plate all right then in metaphase 2 your chromosomes are going to align at the equator of each daughter cell all right the chromosomes are positioned at the metaphase plate because of crossing over in meiosis one the two sister chromatids of each chromosome are not genetically identical anymore and the kinetochores of sister chromatids are attached to microtubules that are extending from opposite poles then in anaphase two sister chromatids are gonna separate and move towards opposite poles of each daughter cell all right and then in telophase two chromosomes reach the opposite poles nuclear envelope forms around them and then cytokinesis follows resulting in the formation at the end of this a four haploid daughter cells so the end result of meiosis is the production of four genetically unique haploid cells gametes in other words from what we started off with which was one diploid parent cell these haploid cells can then fuse with other haploid cells during fertilization to restore the diploid chromosome number in the resulting Offspring all right meiosis introduces genetic a lot of genetic diversity as you saw in prophase one there's crossing over that happens um in anaphase one there's the possibility of what is called independent assortment of chromosomes which we'll talk about later and even the random Fusion of gametes during fertilization all that introduces genetic diversity all right but that is summary of meiosis all right fantastic let me actually before we even move on let me restate this one more time in meiosis one we start off in the interface everything has been duplicated already then starting off with prophase one chromosomes condense they pair up crossing over can happen between homologous chromosomes this is where the first major source of genetic variation comes in then we have metaphase one this guy right here the tetrads are going to line up on the metaphase plate chromosomes line up by homologous pairs then we have anaphase one all right the homologous chromosomes separate and move towards opposite poles and then telophase one and cytokinesis chromosomes are going to reach the poles nuclear membrane reforms and cell divides notice the two cells produced have half the number of chromosome as the parent cell and with that we move into meiosis II we start off with prophase two the nuclear envelope breaks down chromosomes condense and then in metaphase two chromosomes line up again along the Equator of each cell and a phase two sister chromatids separate and they move towards opposite poles telophase two and cytokinesis the chromosomes reach the poles the nuclear membrane reforms and the cells divide again at the end we now have four haploid daughter cells containing half the number of chromosomes as the original parent cell each with a unique combination of genes all right fantastic now something else we want to discuss is kind of what makes genetic variability possible crossing over is one of those things and we briefly mentioned that it happens in prophase one but let's let's talk about it right crossing over is a process in which homologous chromosomes are gonna exchange genetic material all right it occurs between non-sister chromatids of homologous chromosomes that have paired up during synapsis and that exchange of genetic material is gonna result in the recombination of alleles between those homologous chromosomes so in the in meiosis one right we have these homologous pairs that have duplicated all right so this is one homologous pair all right I'm going to draw another one in a different color all right another homologous pair right here all right we're going to highlight them in a different color too all right they've duplicated all right one more I just want to draw this out okay cool so we have these duplicated now we have these homologous chromosomes all right these are homologous chromosomes all right now in in one of these all right this was a duplicated chromosome that's now attached to its sister chromatid through sister chromatid cohesion all right what happens is that it's going to exchange some parts of this let me make that highlighter smaller all right some part of this sister chromatid is gonna exchange with its homologous chromosome all right these are gonna swap out all right when that happens all right these these sister chromatids now all right let's let's color in let's erase they've swapped so now they have different parts all right they they are not what they started off with when they were duplicating all right now what what's happening is that between the sister chromatids themselves they are no longer identical even though that when they were formed when the sister chromatid pair was formed this duplicated and attached itself to its sister chromatid through sister chromatid cohesion but because of recombination between its homologous chromosome the sister chromatids are no longer identical this introduces genetic variation the exchange of genetic material results in the recombination of alleles between the homologous chromosome all right the sister chromatids are no longer identical duplicated version of each other you know cohese together and this process contributes to genetic diversity by shuffling and combining genetic information from both parents so crossing over introduces genetic diversity it's facilitated by the formation of protein structures called chiasma chiasmas or chias Mata for plural which physically connect the homologous chromosomes at the points of chromosome of the points of of crossing over so the homologous pairs actually get really close all right and these two parts right here are so close that they can swap out now all right and so that's how crossing over happens now synapsis is the pairing of those homologous chromosomes during prophase one of meiosis all right so together oh let me let me elaborate on that synapsis right we said is the pairing of homologous chromosomes during prophase one of meiosis homologous chromosomes one inherited from each parent they align closely become connected along their length all right the paired chromosome form a structure called the tetrad which we mentioned right over here when we have homologous chromosomes aligned next to each other they form what is called the tetrad all right within this tetrad they are Health these homologous chromosomes are held together by protein complexes and these complexes facilitate the alignment and stabilization of the paired chromosomes during crossing over so the synapsis allows for Accurate Alignment of homologous chromosomes ensures that crossing over occurs between the correct chromosomes so that's how crossing over becomes possible it's through the proper alignment and can and closely close connection between the homologous chromosomes all right and that ultimately plays a role in genetic diversity so here's another way that you can you can see that right sister chromatids there's crossing over that happens in between and so now um different parts of the homologous chromosomes are exchanged fantastic one more thing that is important is let us compare and contrast mitosis and meiosis now there are three events all right this is a really great visual representation to help you see the differences between mitosis and meiosis but there are three events that are unique to meiosis um that are gonna occur during meiosis one actually all right that that creates one of the biggest differences between mitosis and meiosis all right first thing is synapsis and crossing over so during meiosis specifically prophase one duplicated homologs are going to pair up crossing over occurs all right and what you notice is that synapsis and crossing over do not occur during prophase of mitosis so that's our first different difference between mitosis and meiosis second difference between mitosis and meiosis is the alignment of homologous pairs at the metaphase plate so at metaphase one of meiosis pairs of homologs are positioned at the metaphase plate rather than individual chromosomes like you see in mitosis so that's a second difference and the third big difference is the separation of homologs so at anaphase one of meiosis the duplicated chromosomes of each homologous pair are going to move towards opposite poles all right but the sister chromatids of each duplicated chromosome they remain attached in anaphase of mitosis in contrast those sister chromatids are what are separating so that is the third big difference between mitosis and meiosis and this summary is really awesome I highly recommend that you know if you're watching this video and but time passes and you want to review this is this this graphic right here is the best way to re-familiarize yourself with this content it just summarizes everything that we have discussed up to this point about meiosis beautifully and then contrasts it to mitosis now the last and final objective um that we want to discuss here is genetic variation produced in sexual life cycles and how they contribute to Evolution so we've talked about crossing over already how crossing over occurs during prophase one of meiosis and how it's a process where homologous chromosomes they pair up and they exchange genetic material at specific points called the chiasmata all right crossing over results in the recombination of alleles between non-sister chromatids of those homologous chromosomes and this exchange of genetic material between chromosomes it generates a new combination of alleles and that contributes to genetic diversity in The Offspring but there's other things that also contribute to to genetic variability and variety right in species that reproduce sexually the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation and on top of crossing over things mechanisms that contribute to genetic variation that arise during sexual reproduction are things like independent assortment of chromosomes or random fertilization so I want to quickly cover a few of those things so an independent assortment of chromosomes all right I'm going to explain it give you like a definition and then we're going to break it down all right so during meiosis homologous chromosomes they line up randomly at that cell's equator during metaphase one now as a result the arrangement and the separation of chromosomes during anaphase one will then be random this random alignment and separation leads to different combinations of maternal and paternal chromosomes in those resulting gametes so independent assortment then generates genetic variation by pretty much shuffling and distributing different combinations of genes from two parents into The Offspring all right so let's think about this imagine you have a pair of shoes all right you have one blue shoe and one red shoe all right one blue shoe one red shoe and then you have two pairs of salt you have one pair of socks one sock is striped all right let's draw Stripes hook and your other sock is dotted all right so you have blue shoe red shoe striped sock dotted sock all right now if you were to pack one shoe and one sock into a box without carrying which shoe or sock you choose there's going to be a total of four possibilities you can have blue shoe with striped socks or a blue shoe with dotted socks or you can have red shoe with a striped sock or red shoe with dotted sock this is essentially what happens when chromosomes pair when this is essentially what happens with chromosomes during the formation of gametes each pair of chromosomes like a pair of shoes or socks are going to separate independently from the other pair and that's going to lead to different combinations in the gametes all right so in the end just like how you can end up with a red shoe and a dotted sock and new cell can end up with any combination of chromosomes from its parent cells and you can see how that gives rise to genetic variation all right we've covered crossing over but one more thing is random fertilization random fertilization refers to the chance encounter between gametes during fertilization so each gamete carries a unique combination of genes resulting from that independent assortment and crossing over we've talked about so far then when two gametes fuse during fertilization that specific combination of genetic material from each parent contributes to the genetic diversity of The Offspring and the random Fusion of gametes pretty much ensures that each individual individual has a unique genetic makeup different from both parents all right so the takeaway points from our last objective all right is there's three events that lead to genetic variation crossing over independent assortment of chromosomes and random fertilization I hope this was helpful that's all I have for you for chapter 13. let me know if you have any questions comments concerns down below other than that good luck happy studying and have a beautiful beautiful day