hey everyone Dr D here and in this video we are going to be covering chapter 13 from our Campbell's 12th edition biology textbook uh this chapter covers meiosis so let's go ahead and get started D Dr D Dr D Dr D D D Dr D Dr D explain stuff all right welcome back so again chapter 13 meiosis so we're going to be talking about the formation of the gamt cells the sperm and the Egg so recall that genes are present on chromosomes genes typically code for proteins and these genes are the units of heredity that are made up of segments of DNA heredity is the transmission of traits from one generation to the next this is called inheritance or heredity we are studying how our genes are passed On to the Next Generation the the first step of that being meiosis the formation of what are called the gtes the gtes are the reproductive cells these include the sperm and the eggs and your non reproductive cells are called your somatic cells these are the rest of the cells in your body so your body you can think of it as your body is made up of two main types of cells your somatic cells which make up pretty much your entire body and then your gamt cells these are only the sperm or the eggs does that makes sense so somatic cells that's your skin cells your brain cells your heart cells your liver cells and then the gamt cells which are only referred to the sperm and the eggs and in this chapter we are learning about meiosis and that is the cell division that only occurs in the reproductive organs the ovaries or the testes to form the gamt cells the sperm and the eggs so again the only purpose of meiosis is the formation of the sperm and the eggs so the only Place meiosis occurs is in your reproductive organs now do you remember from chapter 12 when we were talking about the cell cycle and mitosis I told you that humans have 46 total chromosomes and this is because you inherit 23 from the egg and 23 from the sperm so you essentially inherit a set of 23 from the egg and a set of 23 from the sperm so take a look here this is how it works the the sperm cell contains only 23 chromosomes so it contains a set of chromosomes one through 23 same with the egg the egg contains a set of chromosomes 1 through 23 and when the sperm fertilizes the egg now you have your two sets of chromos omes you have 46 total chromosomes you have a set of 23 from your paternal lineage your father and you have a set of 23 chromosomes from your maternal lineage your mother and remember the these are known as homologues or homologous chromosomes and so we have 46 chromosomes now any cell with homologous chromosomes inside any cell that has the you know the set from Mom and the set from Dad um any cell that has both sets from both parents is called a diploid cell so the fertilized egg is called a zygote and because it has two sets two homologous sets of chromosomes inside this is known as a diploid zygote diploid meaning two homologous sets of chromosomes inside of the cell now let me ask you this can you beat Wicked um are the sperm and the Egg diploid do they have two homologous sets of chromosomes inside a set from Mom and a set from Dad oh that's right Wicked uh you can see here that the sperm only has 23 chromosomes and the Egg only has 23 chromosomes and I'm going to show you how the sperm and the Egg got those chromosomes and why they only have 23 chromosomes that's the process of meiosis that's what we're going to be discussing today is how the sperm and the Egg formed but you should know that these are not called diploid uh cells these are called haid cells so this is the haid sperm this is the haid egg and please remember that the egg and the sperm are called the gam cells these are the sex cells or when the sperm fertilizes the egg this is known as the zygote and it's a diploid cell now because you have two homologous sets of chromosomes so your body your somatic cells that make up your body those skin cells brain cells heart cells liver cells these cells are all diploid cells and the haid cells in your body are simply the sperm and the Egg again each pair of homologous chromosomes includes one chromosome from each parent the 46 chromosomes in a human somatic cell are two sets of 23 one from the mother one from the father remember this is what makes up a diploid cell now I also want you to know that instead of writing diploid cell you can shorten that to 2 N 2 N means a diploid cell you have two homologous sets of chromosomes so if you just see n by itself that's a haid cell it has one set of chromosomes but if you see 2 N that refers to a diploid cell with the two homologous sets from each parent or or one set from each parent so for humans the diploid number is 46 total chromosomes and that's 2 N you have two sets which makes up your 46 chromosomes and again it's only the reproductive organs the ovaries and the testes that produce the haid gametes the gametes are the only type of human cells produced by meiosis rather than mitosis we learned about mitosis in chapter 12 the previous chapter we learned all the phases of mitosis and everything uh when the cells of your body divide those somatic cells that make up the major of your body divide they they divide by mitosis but specifically in the reproductive organs this is where meiosis occurs and that's only to form the sperm and the Egg the sex cells and as you saw with the sperm and the egg in that previous figure meiosis results in one set of chromosomes in each gamt that means that the sperm and the Egg contain a total of 23 chromosomes one set all right now one thing I want you to understand is that sperm cells them cells do not divide so one sperm cell does not become two sperm cells and egg cells do not divide either one egg does not become two eggs so where do these sperm and these eggs come from well they come from special diploid cells diploid cells that are in the reproductive organs diploid meaning they have 46 chromosomes they have two homologous sets of chromosomes but they're special these are the cells that divide to give rise to the sperm and the Egg does that make sense so these cells have a special name they are known as The Germ cells and they are the cells that undergo meosis and result in the sperm and the Egg and you can only find these germ cells in the reproductive organs in the reproductive organs you have those germ cells with 46 chromosomes those germ cells undergo interphase do you remember interphase we talked about that in the previous chapter if you don't recall interphase make sure to go back to chapter 12 uh because we discussed that during interphase this is uh this includes the subphases G1 where the cell grows S phase where the DNA is replicated so if you started with 46 chromosomes and then during interphase SASE those chromosomes are replicated how many chromosomes would the germ cell have now that's right Wicked 92 chromosomes right your 46 chromosomes get replicated so now that germ cell possesses 92 chromos and then remember following S phase is G2 phase this is the phase where the cell continues to grow but it also uh replicates the centrioles and the centrosomes and now you're all good to go you're ready for the process of meiosis now what's really interesting about meiosis is that there isn't just one round of cell division um like mitosis you remember what happened in mitosis you had interphase and then you had mitosis which was you know that one cell became two well here there's actually two rounds of cell division does that make sense so your your diploid um germ cell divides One Time by a process known as meiosis one which has its own prophase its own metaphase its own anaphase and its own telophase these should look familiar right from mitosis but then those that results in how many cells that results in two cells right well those two cells divide again okay those two cells divide Again by a process known as meiosis 2 which has its own prophase called prophase 2 and its own metaphase anaphase and tase so let me ask you this if you started with one cell one germ cell in the reproductive organs and the cell divided twice how many cells would you end up with that's right Wicket four cells you'd have four cells and those are known as the gamt cells okay those are your gamt cells the sperm and the Egg okay now let me ask you this if you had if you started with a uh germ cell with 46 chromosomes that germ cell copied all 46 chromosomes during S phase of interface and that resulted in a germ cell with 92 chromosomes okay well then remember a cell with 92 chromosomes divides once by meiosis 1 to give two cells with 46 chromosomes and then each of those cells divides again to give four cells with how many chromosomes 23 now does it make sense why the sperm and the Egg have 23 chromosomes and not 46 like a typical sematic cell well it's because the DNA was only copied once but the cell uh the cell divided twice does that make sense so what I want you to know is that there's only one interphase where the DNA is replicated and that occurs before meiosis 1 there is no interface between meiosis 1 and meiosis 2 and because the DNA is only replicated once but the cell divides twice you're going to have gamt cells the SE the sex cells the sperm and the egg with only 23 chromosomes per cell I hope that makes sense we're going to go into more detail now but then once once the DNA has been copied and the cell has a copy of all the chromosomes we are ready for meiosis 1 prophase 1 so let's go into see what really occurs during this profase one step of meiosis 1 so here is prophase one I want you to look inside and see do you spot any difference here between prophase one of meiosis and prophase of mitosis from the previous chapter I just want you to take a quick look here the it looks like it looks like the cental have already been replicated from G2 that's great uh they start moving to opposite ends of the cell it looks like the nucleus is intact and that's great that's what you would expect right during prophase the nucleus should be intact and inside we see our chromosomes but there's something strange about these chromosomes I just want you to take a look at these strange chromosomes it looks like do you guys remember how the difference between homologous chromosomes and sister chromed pairs remember in chapter 12 I said it's very important that you remember the difference between the two because that makes these chapters so easy if you can just keep that clear in your mind so what I'm going to do is I'm going to hop to the board real quick and I'm going to show you again quickly the difference between sister chromatid Pairs and homologous chromosomes and then I'll meet you back here so we can make better sense of what's happening during prophase 1 um don't forget that before meiosis uh special germline cells in your testes or ovaries your reproductive organs undergo interphase uh if you don't remember interphase uh quick review uh during interphase your diploid cell with 46 chromosomes under goes a G1 or cellular growth the cell grows in size notice that you have homologous chromosomes inside you have one pair of centrioles you then have subphase S where the chromosomes get replicated now you have two of every chromosome you have 92 total chromosomes at this point you have what are known as sister chromatids remember homologous chromosomes are the maternal and paternal set right or maternal and paternal version of every chromosome sister chromatids on the other hand are identical copies of the same chromosome so at the end of S phase you have sister chromatid pairs forming and then in G2 which is the final subphase of interphase the centrioles replicate the centrioles replicate and they start moving to opposite poles of the cell okay the nucleus is still there the DNA starts to condense but it's not quite condensed all right so that's where we leave interphase behind so don't forget interphase has just happened we are now ready for what's called meiosis one during Pro one tetrads form and crossing over happens what does that mean if you look inside the nucleus the sister chromatids pair up with homologous sister chromatids as a tetrad what that means is both of Mom's chromosome one attached to both of Dad's chromosome one with a synaptonemal complex holding them together as a tetrad so this would be the tetrad for chromosome 1 for example this would be the tetrad for chromosome 2 and notice what's happened here it's almost like the tips have exchanged between one of the two sister chromatids here and one of the two sister chromatids there they've swapped genetic information that's called crossing over crossing over has happened okay so now you have 1 2 3 four chromosomes that are no longer at all identical uh crossing over increases genetic variability crossing over results in more genetic variability okay none of these four chromosomes are the same after crossing over so again what's happening during during this prophase one don't forget each cell has 92 total chromosomes uh tetrads formed crossing over happened okay and crossing over increased genetic variability we're now ready for metaphase 1 so welcome back from the board I hope you noticed that at profase phe one of meiosis some strange stuff is happening with the chromosomes if you remember during mitosis it was those sister chromatids that formed right and so during prophase of mitosis sister chromatid pairs formed however if you noticed from my board uh presentation just a second ago something more complex is going on during prophase 1 of meiosis and that is this these structures are forming they actually consist of four chromosomes and these are known as tetrad structures and I'm going to show you exactly what a tetrad structure is because it's these tetrads that form during prophase 1 so here is a tetrad structure a tetrad is when a pair of sister chromatids joins up with the hom homologous pair of sister chromatids to form a tetrad structure tet means four and this is a fitting name because this is four chromosomes all hanging out together so for example let me give you an example if these were two exact copies of Dad's chromosome one this would be the two exact copies of mom's chromosome one underneath does that make sense that's what homologous chromosomes mean the two blue ones for example are the paternal homologue and the two red ones are the maternal homologue the two blue ones are identical to each other the two red ones are identical to each other so the two blue ones are sister chromatids the two red ones are sister chromatids but they're binding together and it's this synaptonemal complex that holds the sisters together the as as homologous uh pairs now here this is known as a tetrad structure so what you need to know is that during prophase one of meiosis tetrads form and the reason they form is really interesting at this point uh one of these Blue Sisters is going to exchange genetic information with one of these Red Sisters so there's going to be a break imagine if I have scissors imagine if I could cut this blue tail right here and then cut the red tail at the same spot okay imagine if if I could cut this tail and cut the tail right below and swap these out swap the swap the two out so that I have the the tip of the red tail up here and the tip of the blue tail down there this is a process known as crossing over so this is what happens first tetrads form and then the there is this crossing over event that happens you see this crossing over happens and look what look at the result result of crossing over is look at this after crossing over has happened and remember Crossing over only happens in one of the blue tails with one of the Red Tails the other sister does not undergo crossing over okay so if that's true let me ask you this after crossing over are any of the four chromosomes identical anymore cuz before look the two blue ones were identical and the two red ones were identical right but after crossing over are any of these chromos identical no that's right Wicket no they're not right because of crossing over none of these four chromosomes are identical anymore and that's great because that is exactly what you know the the purpose of meiosis is the purpose of meiosis is to increase what's known as genetic variability having more genetic shuffling going on because this is part and parcel with sexual reproduction uh during sexual reproduction the genes are shoveled around the chromosomes are shuffled around as much as possible because this gives The Offspring the best chance at surviving right this is this leads to what's known as adaptation so the purpose of crossing over is to increase genetic variability isn't that neat so this is exactly what's going on during prophase one of meiosis so what do you need to know during prophase one of meiosis tetrads form and crossing over happens so let's move on to metaphase 1 take a look at metaphase 1 during metaphase 1 it's those tetrads that line up at the center of the cell remember that uh imaginary plate called the metaphase plate so during mitosis it was the sister chromatid pairs that lined up at the center of the cell but during metaphase one of meiosis it's the tetrads that line up at the center of the cell and remember those tetrads have undergone crossing over so the little tips have been swapped over um and it's these tetrads that line up down the center of the cell but there's a little more to it than that so let me give you a little more detail about what's going on and exactly how they're lining up at the center of the cell cuz that's important to understand all right so at this point a human cell in metaphase 1 would have 23 tetrads that's how many tetrads would be found in a human uh cell which is in metaphase 1 however just for illustration this here shows a cell with three tetrads okay only three tetrads so let's call this chromosome 1 chromosome 2 and chromosome 3 so again remember the tetrads are you know all they all share the same chromosome number what does that mean so for example if this is chromosome one from Dad that's an exact copy of chromosome one from Dad and then if this is chromosome one from Mom that's an exact copy of chromosome one from Dad and that's a tetrad and by the way I don't know why but they have not drawn the crossing over but remember this has undergone crossing over and if this was chromosome 2 this would be two copies of chromosome two from Dad and this would be two copies of chromosome 2 from Mom two copies of chromosome 3 uh from dad two copies of chromosome 3 from Mom so again there's three tetrads in this cell and uh this is a cell in metaphase 1 now what's really interesting is that there's a reason why they drew this cell in eight different ways right and if you notice if you take a close look you'll notice that the reason they drew eight different versions of this cell in metaphase 1 is because the tetrads uh are lined up in different orientations so what do I mean by that look at this for example take a look at this here for chromosome 1 the blue ones the paternal sister chromatid pairs are on the left and the maternal sister chromatid pairs are on the right however did you know that it whoops however did you know that it's equally likely that they would have lined up the other way with the maternal homologues on the left with the paternal homologues on the right so what I'm trying to say is there's an equal there's an equal probability of the chromosomes lining up with this way with the paternal homologues on the left as they are lining up the other way with the paternal homologues on the right and that's not just true for chromosome one that's also true for chromosome 2 see here with chromosome 2 you have the paternal homologues on the left whereas here you can see the paternal homologues on the right and chromosome 3 here the paternal homologues on the the left but in this cell the paternal homologues on the right so what that means is that not only do the tetrads line up at the center of the cell during metaphase 1 they also line up completely randomly with each tetrad having an even chance an equal chance of lining up with the paternal homologue on the left or the paternal homologue on the right so this means that there are many different combos there's many different combinations of chromosomes that can line up this way in fact to figure out exactly how many you would take the the uh two you would take two and Rise it to the power of the number of tetrads you have so for example here we have three tetrads so to understand how many different combos of Arrangements we could form during metaphase 1 you would do 2 to the power of three which is eight and that's why eight different pict pictures are drawn here humans have 2 to the power of 23 right so that's a huge number that's over 8 million okay so there are many different ways that your tetrads can line up down the center of the cell isn't that really neat and that does what remember that increases genetic variability and this ability for the tetrads to line up randomly giving you such genetic variability is known as independent assortment why because these tetrads assort independently of one another each has a 5050 chance of having the paternal homologues on one side or the other all right that about sums up what happens during metaphase 1 but now we're ready for anaphase 1 remember anaphase when we talked about mitosis it was the sister chromatids that's separated from one another right but here during anaphase one of meiosis we have tetrads and it's the maternal homologues that separate from the paternal homologues so what does that mean you remember that synaptonemal complex that was holding these uh the the this whole complex and play the tetrads together well those that synaptonemal complex breaks down and the chromosomes separate and the two red ones go in One Direction the two blue ones go in another Direction let me show you how that looks again during anaphase 1 what's happening it looks like the tetrads have broken apart such that the paternal homologues that means both copies of Dad's chromosome 1 for instance separates from the maternal homologues which is both copies of of Mom's chromosome one these would be known as sister chromatids but because of this little blue tail due to crossing over these would technically be called nonidentical sister chromatids because they're not fully identical anymore though they once were and then on the other side these are also known as non-identical sister chromatid pairs right so so again the same thing happens with chromosome 2 let's say this is chromosome 2 both of Dad's chromosome 2 go to this new cell both of Mom's chromosome 2 go to this new cell so what's happening is you could say it this way during anaphase 1 the homologues separate that means blue separates from Red paternal separates from maternal another way of saying the exact same thing is that this pair of non-identical sister chromatids or the paternal pair of non-identical sister chromatids separates from the maternal pair of non-identical sister chromatids that's a fancy way of saying the blue ones separate from the red ones at this point those chromosomes move towards opposite ends of the cell and you're ready for tilo Phase 1 during tilo phase 1 you have the daughter nuclei starting to form so a nucleus starts to form form around this DNA and a nucleus starts to form around this DNA now I want you to I want you to ask you a question and that is if you take a close look at this would you say that these two are genetically identical cells you know kind of like remember at the end of mitosis after cell division you had two genetically identical cells we'll take a close look at these two cells and and let's see if you can beat Wicket and answer the question are these two cells genetically identical that's right as always no these cells are not genetically identical there is two red ones in this cell you know like for example look at this cell on the bottom here this cell has two copies of Mom's chromosome one but zero copies of Dad's chromosome one both copies of Dad's chromosome one went to this cell so and and also this cell has two copies of Dad's chromosome 2 you see that so there's no way these cells are identical because this cell has both copies of Mom's chromosome one be it that one is a non-identical these are non-identical sister chromatids but they're still technically sister chromatids so there is two copies of Mom's chromosome one zero copies of dad's chromosome one so guess what that that means that these cells are anything but identical this one has both copies of Mom's chromosome one this one has both copies of Dad's chromosome one this one has both copies of Dad's chromosome 2 and this one has both copies of Mom's chromosome 2 so what I'm trying to get across is that by the end of tilo phase 1 by the end of meiosis one this results in two cells that are not genetically identical and this is something else that really confuses students and that is that even though both of these cells would have 46 chromosomes at the end of meiosis 1 each of these cells has 46 chromosomes so they technically have two sets of chromosomes right are would this qualify as two diploid cells and let's see if you can beat Wicket yes right this does not qualify as two genetically identical cells and it does not qualify as two diploid cells and look what it says here in my notes Here by the end of meiosis 1 two genetically unique that means different hloy cells have formed and I know this is very confusing for students because the students say well there's 46 chromosomes in this cell and there's 46 chromosomes in that cell so that's two sets so that's diploid right well that's not quite right and I'll explain why here do do we have two copies of chromosome one yes but do we have a a mom and a dad chromosome one like for examp so essentially do we have homologous chromosome ones do we have a mom chromosome one and a dad chromosome one or just two copies of Mom's chromosome one that's right we only have two copies of Mom's chromosome one okay that that would be cheating to call this a diploid cell you can't just have two copies of almost the exact same thing and call yourself diploid does that make sense you guys follow me on this so you can't just have twice the number of the same exact chromosomes and and call that two sets of chromosomes and call yourself diploid a diploid cell not only does it have two sets of chromosomes it has two sets of homologous chromosomes which means one set is from your mom one set is from your dad this cell obviously does not have that it does not have one set from Mom and one set from Dad this is two copies of Mom's chromosome one there's zero copies of Dad's chromosome 1 both copies of Dad's chromosome 1 are over here right and then this cell has two copies of Dad's chromosome too with zero copies of Mom's chromosome 1 both copies of Mom's chromosome 1 went to the other cell so yes technically these two cells have 46 chromosomes 46 chromosomes two sets of chromosomes but because there aren't technically homologous chromosomes in each of these cells just two copies of the same information that's cheating that's not that doesn't qualify as two diploid cells so by the end of meiosis 1 this results in two genetically unique hloy cells and because because of this meiosis 1 is often referred to as what's called reduction division because the ploy of the cells reduced from diploid 2N to haid 1 n okay now we are entering meiosis 2 and this begins with prophase 2 during prophase 2 remember where we left off we had two cells but I'm just going to pick up with one of the two cells but remember remember at the end of meiosis 1 we had two cells but I'm just going to show you what's happening in one of those two cells but obviously you know know that there's double everything in your mind right so remember what we had we had uh a cell that's hloy even though it has 46 chromosomes it's appid because it has two of Mom's chromosome 1 two of Dad's chromosome 2 etc etc and remember these are technically nonidentical because crossing over had happened all right so what do I need you to know these are still technically sister chromatid pairs so sister chromatid pairs form that's what I need you to know during during uh prophase 2 sister chromatid pairs form and if you want to be extra specific yes you could say that nonidentical sister chromatid pairs form okay that's what's happening during prophase 2 now in metaphase 2 those sisters those nonidentical sisters line up okay the non-identical sister chroma titits line up down the metaphase plate anaphase 2 you guessed the sisters separate from one another the non-identical sister chromatids separate from one another so for example one copy of Mom's chromosome one is headed to this cell one copy of Mom's chromosome one is headed to the other cell and now for tilo Phase 2 now during tilo phase 2 daughter nuclei form and let me ask you this how many total cells would be present at the end of tilo phase 2 that's right it would be four cells because remember during meiosis one one germ cell became two haid cells by the end of tase 2 those two cells divide again to form four gamet cells right so at this point you have four genetically nonidentical gamt cells cuz look at this let me ask you this is this cell identical to this cell take a close look no because look this mom's chromosome one has a part of the paternal homologue uh it has a part of Dad's uh information on it whereas this mom's chromosome one is just mom's chromosome 1 does that make sense so because of crossing over these two cells are not identical and if you're curious because of Independent Assortment these two are not going to be identical to the other two so what that means is at the end of tilo phase 2 at the end of meiosis 2 this results in four genetically nonidentical gamt cells these are the sperm and the Egg okay does that make sense none of these four sperm none of these eggs have the same genetic material they are all genetically different they are all genetically distinct and that's wonderful because that's exactly what the purpose of meiosis is the more the genes are shuffled around and the better The Offspring have at a chance to adapt and to survive right isn't that neat and let me ask you this how many chromosomes total are in each of these gamt cells at this point can you beat wicket that's right 23 chromosomes per gamt so each sperm cell will contain 23 chromosomes each egg would contain 23 chromosomes uh so yes you have one set of chromosomes in each of the gamt cells and because let me tell you this because during meiosis 2 the cell goes from having 46 total chromosomes to only 23 chromosomes per cell meiosis 2 is termed equational Division equational division so now let me ask you this this is important and I may ask it on the exam for my class um which meiosis is it meiosis 1 or meiosis 2 that most resembles mitosis what do you think that's right Wicket it's actually meiosis 2 and I'll tell you why let me let me explain why first of all let's look at myosis 1 remember during prophase 1 what did the DNA form remember it Formed tetrads and crossing over happened that's all new Concepts isn't it did any of that stuff happen during mitosis the answer is no and then remember during metaphase one those tetrads lined up randomly by independent assortment that's also a no that none of that happened during uh mitosis anaphase the homologue separated that didn't happen in mitosis and so however look at meiosis 2 during meiosis 2 prophase 2 sister chromatid pairs form uh I yes they're nonidentical but sister chromatid pairs form isn't that what happened during mitosis yes uh then in metaphase 2 it was those sister chromatid pairs that line up down the center of the cell isn't that what happened during mitosis yes anaphase sister chromatids separate from one another that's the same as mitosis telophase those sister chromatids move to opposite ends and form New Daughter nuclei so what I want you to understand is meiosis 2 is much more similar to mitosis than meiosis one is and and why well because you know tetrads never formed during mitosis crossing over never happened independent assortment meiosis one is its own Beast it's its own thing so if I were to ask you which one's more similar to mitosis it would have to be meiosis 2 again mitosis which was covered in chapter 12 mitosis occurs in the somatic cells of your body the purpose of mitosis was to conserve ve the number of chromosome sets producing two cells that are genetically identical to the parent cell however in meiosis which was the focus of this chapter chapter 13 involves reduction in the number of chromosome sets from two diploid or 2 N to one haid or just simply n producing four cells that differ genetically from each other and from the parent cell and these are known as the haid gamet cells the sperm and the Egg there were three events unique to meiosis and all three occurred in meiosis 1 now this include synapsis and crossing over synapsis being when the tetrads formed and crossing over which increased genetic variability remember that then the alignment of the homologue pairs at the metaphase plate with independent assortment which also increase genetic variability and then the separation of those homologues remember the homologues are what separates during anaphase 1 which uh is also unique to meiosis as well now let's take a quick break time with Gizmo and Wicket and see what these two little guys are up to and we'll be right back to finish off this [Music] chapter welcome back from break time with Gizmo and Wicket let's carry on I have some interesting discussion here about meiosis and it has to do with the genetic variability that occurs during meiosis remember that the during meiosis this is responsible for the variation that arises in each generation three mechanisms contribute to genetic variation remember crossing over which occurred during uh prophase one of meiosis independent assortment which occurred during metaphase 1 of meiosis and then lastly random fertilization right because it's a random sperm that fertilizes a random egg so all three of these actually contribute to genetic variability and genetic variation remember what I said earlier during metaphase 1 that you can figure out how many variations there could be during metaphase 1 by taking two to the power of the number of different chromosomes there are so for humans we have 23 different chromosomes so 2 to the^ of 23 is over 8 million different combinations of chromosomes that means that each person can form 8 million different combinations of chromosomes in their gametes uh in the sperm or the eggs and that's true not just for the you know the father for instance but the mother as well so the father can form 8 million different types of sperm based on their chromosomes and the mother can form 8 million different types of egg uh based on their chromosomes and so for for a child to you know come to be a one in8 million sperm would have to fertilize a 1 in8 million egg right so isn't that rare wouldn't that result in an Ultra ultra rare combination Ultra unique combination in The Offspring isn't that really interesting to think about and so what they say is this random fertili fertilization of a random 1 and8 million sperm variation with a one in8 million egg variation adds to the genetic variation because any sperm could fuse with any ovam or unfertilized egg the fusion of two gametes each with 8.4 million possible chromosome combinations from independent assortment produces get this everyone produces a zygote with any of about 70 trillion diploid combinations so think about that you you were born right and you were a you know combination of your genetic material from both parents right that was a 8 1 in 8.4 million sperm uh fertilizing a 1 in 8.4 million uh combination egg so the odds of having you were 1 in 70 trillion isn't that fascinating to think about and I just want to point out that this is just this 1 in 70 trillion has to do with just independent assortment and random fertilization does it count does it factor in uh crossing over does this 70 trillion figure count crossing over no crossing over adds even more variation if you added the crossing over variation it would be in the quadrillions in in you know in the amount of different types of sperm and egg and combinations that are possible it it would be just astronomically High uh amount of variation if you think about it that way because crossing over can you know doesn't necessarily happen the same way twice you know you can have single crossover events double crossover events triple crossovers there's so many different ways that crossing over could have happened so you know this 70 trillion just refers to independent assortment and random fertilization so what am I trying to get at I just want to do a thought experiment for you like imagine like so your parents had you right but what would be the odds of your parents having you again right so imagine you were born with your genetics so it's possible for your parents to have a child with your genetics cuz you were born right but what would be the odds of your parents I don't know like it in two years down the road having another child and that child just randomly having your exact genetics right it's is it possible yes is it probable no way not really right it's like a really really really really not probable it's probably so unprobable that it's probably never ever happened in the history of humanity isn't that interesting so you know the you know for for someone for a couple to have a child with particular genetics and to later on have a child with that exact same genetics it would be so astronomically rare that it would be like winning the mega jackpot Lottery several times in a row I'm betting you know uh so so it would it would just not happen now invariably students would ask me well what about identical twins what about identical twins you know yes it's true that identical twins have the exact same DNA but do you know why it's because identical twins started out as one total sperm and one total egg so it really was one sperm fertilizing one egg and that led to a diploid zygote that zygote started to divide and divide and divide and at some point a little part of that zygote a little part of that blast Tois popped off and divided as though nothing happened and that resulted in two people isn't that interesting so the reason why twins have the exact same genetics is because they didn't come from two sperm and two eggs those twins started out as one sperm and one egg so they were the ultimate roommates at one point if you think about it they literally were roommates and the room was a cell called the zygote isn't that interesting so if you know identical twins or if you are an identical twin just think about that for a second you and your twin were in the same exact cell at one point and you came from one sperm and one egg isn't that interesting and that's why you have identical DNA to one another but the odds of you guys having identical DNA but you were born at different times with with different sperm and different eggs would be so astronomically small that it's probably never ever happened and so you know what they say this is a funny one everyone they say that you are incredibly incredibly unique uh just like everyone else right so let's let's finish off so with that I I just wanted to get back to the board and I want to do the board talk with you one more time because it just you know it just Nails all the most important steps all the most important aspects of every phase of meiosis so let's get back to the board and then we'll call it a chapter so don't forget interphase has just happened we are now ready for what's called meiosis 1 during prophase 1 tetrads form and crossing over happens what does that mean if you look inside the nucleus the sister chromatids pair up with homologous sister chromatids as a tetrad what that means is both of Mom's chromosome one attached to both of Dad's chromosome one with a synaptonemal complex holding them together as a tetrad so this would be the tetrad for chromosome 1 for example this would be the tetrad for chromosome 2 and notice what's happened here it's almost like the tips have exchanged between one of the two sister chromatids here and one of the two sister it's there they've swapped genetic information that's called crossing over crossing over has happened okay so now you have 1 2 3 four chromosomes that are no longer at all identical uh crossing over increases genetic variability crossing over results in more genetic variability okay none of these four chromosomes are the same after crossing over so again what's happening during during during this prophase 1 don't forget each cell has 92 total chromosomes uh tetrads formed crossing over happened okay and crossing over increased genetic variability we're now ready for metaphase 1 where tetrads line up by independent assortment okay notice that the tetrads have lined up down the center of the cell along this imaginary plate called the metaphase plate okay tetrads line up and what does that mean independent assortment independent assortment means each tetrad has a 50/50 chance of lining up this way or the other way see the the way the way the first tetrad lined up it lined up with the blue chromosomes on the left or the paternal homologues on the left well that one could have very well you know that tetrad could have lined up the other way with the blue ones on the right and so could the second one the blue ones are on the right but it could have very well 50/50 50 chance lined up the other way that's what independent assortment means and and that leads to a lot of genetic variability okay that leads to what over 8 million different ways that our tetrads could line up because we have 23 of these tetrads during this phase not just two like this simple picture here all right so then at the next phase here anaphase one the DNA separates but what's separating from what okay we say that the homologues separ what does that mean this pair of sister chromatids separates from the homologous pair of sister chromatids in layman's terms these two dad's chromosome one separate from these two mom's chromosome one so all of Dad's information for chromosome one is headed to the cell on the left all of Mom's information for chromosome 1 is headed to the cell on the right and if we're looking at chromosome 2 it's the opposite story all of Mom's information for chromosome 2 is headed to the left all of Dad's information for chromosome 2 is headed to the right so this is going to result in two genetically different cells right so tilo phase uh one you see you form two cells and those two are considered hloy uh let me get to this in a second first of all notice that daughter nuclei form daughter nuclei form each cell now has 46 chromosomes okay but realize this um look in this cell you have two dad's chromosome ones and no Mom's chromosome ones in the other cell you have two mom's chromosome ones and no dad's chromosome one this means that even though you have 46 chromosomes which means two sets of chromosomes it's not quite diploid anymore it's not quite diploid anymore these cells are technically half Ploy because you are missing in this cell a mom's chromosome one you only have two of Dad's chromosome one if you do not have homologous chromosomes inside of a cell that cell is not technically deployed does that make sense so even though yes technically this cell has two sets of chromosomes and this cell has two sets of chromosomes because they're not homologous sets of chromosomes that's not diploid so for uh in layman's terms because you don't have a dad's chromosome one and a mom's chromosome one to back it up you just have two of Dad's chromosome one that's not diploid that's hloy and because of that meiosis 1 meiosis one is considered reduction division which means what did you do you went from 2 N uh diploid cell 2 N diploid cell which has homologous chromosomes Mom and Dad information for every chromosome too and 46 chromosomes and hloy because you no longer have Mom and Dad homologous information in each cell okay so moving on I'm just going to show you uh meiosis 2 I'm just going to show you what's happening in this cell I'm not going to worry about that cell but just know that whatever's happening in this cell is happening in that cell I'll show you what's happening in this cell here in my meiosis 2 time all right this is meiosis 2 prophase 2 and notice what's going on here this cell has 46 chromosomes even though it's n it's hloy because it it's lacking homologous chromosomes uh during this step sister chromatids form and condense you see this sister chromed pair forms for example this would be two of Dad's chromosome one this sister chrom pair forms for example that would be two of Mom's chromosome 2 okay and you're done with prophase 2 next you have metaphase 2 what's lining up down the center of the cell it looks like the sister chromatid pairs line up at the center of the cell and I wrote that here the sister chromatid pairs line up anaphase 2 what's separating from what it looks like those cohes proteins holding the sisters together broke and the sister chroma separate from one another so the sister chromatids separate from one another all right sister chromatids separate from one another now lastly I I I'm showing you here what happened to all you know all of the cells so not just what happened to the left cell here but also what's happening to the right so I'm showing you all four of the resultant cells each cell now has 23 chromosomes these are now called the gamt cells daughter nuclei form okay and what you should realize is none of these four cells is genetically identical anymore these are all four genetically non-identical cells so for example this cell and this cell are not genetically identical and you know why because crossing over you see this this uh dad's chromosome one is just dad's chromosome one this one has a bit of Mom's chromosome one in it because of crossing over so this cell and this cell are not the same due to crossing over neither are this cell and this cell you see these cells are not the same due to crossing over this is Mom's chromosome one this is Mom's chromosome one with the blue tip so these are not the same these are not the same and then these two are not the same as these two you see these two up here are not the same as these two down here because independent assortment remember during metaphase 1 independent assortment happened uh so that's why these these two and these two are not the same okay so none of these four cells are the same genetically all of these cells are now um hloy they have one set of chromosomes and these are called the gamt cells and meiosis 2 is considered what's known as equational division okay this process of equational division why because you've simply gone from 46 chromosomes to 23 whereas remember meiosis one was called reduction division why because you've gone from a diploid cell to to haid cells you've reduce the ploy of the cells and one last thing you should understand about meiosis and that is that it looks like meiosis 2 is a lot more similar to mitosis than meiosis 1 okay and look meiosis 2 sister chromatid pairs form sister chromatid pairs line up and sister chromatids separate from one another well that's that's pretty much what was happening during uh mitosis over there whereas in meiosis 1 tetrads form tetrads line up oh tetrads formed and did crossing over which is different tetrads lined up with independent assortment and then homog separated that's completely different than what happened during mitosis over there so I hope this helps this is my rundown of meiosis okay welcome back hopefully this was helpful please let me know in the comments below if you have any questions or if it made sense I appreciate you tuning in and I'll catch you guys next time Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D Dr D A Dr D Dr D Dr D Dr D Dr D Dr D D Dr D Dr D Dr D Dr D Dr D Dr D