this is the video for D 2.1 on cell and nuclear division we'll focus on standard level content about meosis before we get started on meosis I want to do a quick review of homologous chromosomes we'll be talking about them a lot so homologous chromosomes can be found in diploid cells and we use 2 N to abbreviate diploid and homologous chromosomes come from each parent so you might have one from the biological mother one from the biological father this is only showing one pair of chromos in humans we have 23 pairs of chromosomes okay so a chromosome that is homologous um will always come in pairs they'll have the same genes in the same location but not necessarily the same alals and we'll find these homologous chromosomes again in diploid cells so like our somatic or body cells and you'll notice that they're going to be in pairs there's two ways we can get these diploid cells one is mitosis so cells produced by mitosis are going to be diploid or in fertilization the fusion of two haid cells holid cells look like this their chromosomes do not come in pairs and we abbreviate them just as n so these are going to be our gametes or our sex cells and they are produced by not mitosis but meosis so let's think about human cells for just a moment normal diploid human cells are going to have 46 Chrome romes when they undergo mitosis the daughter cells will also have 46 chromosomes if mitosis was used to produce gametes well then we're in a little bit of a pickle here because gametes fuse together during fertilization to form the zygote and that would mean that the zygote would have 92 chromosomes which is way too many this is why we don't use mitosis to make gametes we we use meosis instead meosis is going to reduce the chromosome number by half so that means for humans the gametes the product of my of meosis will have 23 chromosomes not 46 when that gamt fuses with another gamet during fertilization that would give the zygote or that fertilized egg the 46 chromosomes that it needs okay so ensuring that this zote gets the correct number of chromosomes is why we need meosis instead of mitosis to make those gamt cells the way that this is accomplished is by replicating the DNA but dividing the cell twice so one round of replication followed by two rounds of division is going to reduce that chromosome number by half a cell that is undergoing meosis is going to start out being diploid how does that result in in haid cells well it has to deal with the division of chromosomes and chromosome pairs we'll talk more about the specifics of each stage in a couple of moments but normally in meosis those chromosomes line up in the center and the chromatids are pulled apart in metaphase one okay of meosis it's not individual chromosomes that line up but they line up as homologous pairs so in anaphase 1 it is not the sister chromatids that separate but the homologous pairs that separate after this first division of meosis because the chromosomes are no longer in pairs they are holid cells then we go through another round of meosis at this point the chromosomes have lined up on the center and it is the sister chromatids that separate in Ana Phase 2 and again normal meosis is going to result in four hoid daughter cells understanding the difference between anaphase 1 and anaphase 2 is definitely something to put on your remembrance list okay now you'll notice also that all four of these cells are genetically unique there are a couple of things that are going to lead to that that we'll talk to later on so don't forget to Circle back around to that idea but for a moment I want to stand this concept of anaphase and separation and we're going to get to one of the weirdest words in bi ology which is non-disjunction so junction means to join together dis means not so not joined together okay and this means not not joined together so what does that actually mean well chromosomes and anaphase are supposed to separate whether it's anaphase 1 or anaphase 2 there is supposed to be a separation of either the chromosomes or the chromatid that is supposed to happen nondisjunction is failure of those chromosomes to separate and that can happen either in meosis 1 where the homologous Chrome pairs are separating or in meosis 2 okay but either way it result in gametes that have either too many or too few chromosomes in those gamt cells so either the homologus paars can stick together or the cister chromatids can stick together now usually this is going to result in cell death so cells with um incorrect numbers of chromosomes will usually self-destruct um either on their own or after they've been fertilized and are trying to develop into an embryo but non-disjunction in the 21st chromosome does not result in self-destruction it results in a condition known as Down syndrome so down syndrome results from triem 21 so these are not interchangeable terms I hear a lot of students using them as if they are synonyms triem 21 refers to the fact that there are three copies of chromosome 21 instead of the normal 2 Down syndrome is the set of symptoms or the actual physical um characteristics that result from this triom it is highly correlated with maternal age so if I look at the age of the mother at the time of birth and I look at the percentage of triem 21 or down syndrome there's an almost exponential increase after age 35 that is because of um gam Genesis or oogenesis or production of the eggs in females and how long that process takes and how well those spindle fibers are forming and you don't need to know those mechanisms but you do need to understand that it is highly correlated with the mother's age it can happen with the father too it's just not as um common so typically an egg would have 23 chromosomes right and that would mean it has one copy of every uh pair or one half of every pair I guess you could think of it in nondisjunction um especially for tric 21 there's an extra copy and the the egg actually carries um two copies of chromosome 21 so in this case two copies would come from the mother now when that egg is fertilized by a sperm even if the sperm has a normal chromosome number of 23 that is going to lead to a zygote with 47 chromos mosomes and you can see here that even if the father only donates one copy or one set from chromosome 21 um we still end up with three total chromosomes here so what do we need to know about all of this well we need to understand the terminology we need to be able to recognize this on cararo uh cographs like this one and we need to understand that it's highly correlated with the age of the mother at the time of conception and birth if you've already studied natural selection and evolution then you already know how important variation is to that process and if you haven't that's okay because you will um diversity and especially sexually reproducing organisms is vast if you think about your own siblings even if you share the same parents you're very different sexually reproducing organisms have three main sources of variation mutations which can also occur in asexually reproducing organisms um meosis producing very genetically unique gametes and then fertilization so out of all of the gametes this one and this one happen to fuse together we're going to focus in on how meosis creates um variation we'll talk about two main events in meosis that causes variation and the first one is called crossing over crossing over occurs in prophase one that's prophase of the first Division and meosis and it occurs between homologous chromosomes so here I have a chromosome from the dad and a chromosome from the mom you can tell that each one has been replicated and again this is just one of the pairs so you can pretend this is let's say chromosome number five okay I've got chromosome number five from the dad and chromosome number five from the mom so first we're going to see the formation of what's called a balent and that just refers to this structure that I get when I have these homologous chromosomes attached to one another so they kind of snuggle up to one another some texts May refer to this as a tetrad which is confusing because by means to so by refers to the pair one from the mom or sorry one from the mom one from the dad tetrad refers to one two three four sister chromatids but they're talking about the same structure okay synapsis is the formation of the balent so the balent is the structure synapsis is the event that results in the balent now you're going to notice where they are attached this is um a specific point called a kayma so the kayma is the point where this event called crossing over is going to occur and that is going to happen where those chromosomes attach I have shown one kayma here it's possible that there there may be several attachment points okay that they hold on to each other in several different places but this is is the limitation of my drawing to only show one if you have to draw chromosomes on an exam it's more likely that you're just going to have to draw one kayma but do pay attention to that so now for the main event this event called crossing over this is the exchange of alals between nonsister chromatids of H olus chromosomes that is a mouthful so nonsister chromatids means that I'm going to take just this one chromatid from the dad and just this one chromatid from the mom and we're going to exchange some alals it would not make sense to exchange alals between sister chromatids because they're identical so nonsister chromatids of a homologous pair let me show you what this looks like after crossing over so after they have exchanged information and it's easier to see after they've separated of course um you'll see that they have exchanged this bit of their chromatids now I've again shown one kayma the number of kayma the number of attachment points and the amount that is exchanged seems to be totally random and this is going to result in again different mixtures or different combinations of alals so when these chromosomes split up some of these gametes are going to have this combination some will have this combination some will have this and some will have this so this is a way to reshuffle those alals between the maternal and paternal parts of those homologous pairs the other feature of meosis that leads to a lot of variation is called random orientation random orientation occurs both in metaphase 1 and in metaphase 2 so when chromosomes are aligning in the center the way in which they Orient themselves is completely random and so you can see this alignment is equally possible as this alignment and depending on the alignment that you have that is going to give a very different set of alals um distributed in those gametes okay so this produces a lot of variation when those chroma are finally separated and we can actually mathematically quantify how many um orientations or how many combinations are possible due to random orientation using this equation 2 to the N where n is the diploid sorry n is the hloy number so let's say I have an organism where n equal 6 so that's their hloy number their diploid number would be 12 well to find the number of different combinations in their gametes I would take 2 to the 6th power so that's 2 * 2 * 2 * 2 time are you getting what I'm saying here 1 2 3 4 five six so not 12 but rather 128 so just if something has six chromosomes in its hpid cells there's 128 different combinations just due to random orientation and that doesn't even account for the amount of variation produced by crossing over so imagine if this where a human cell humans have a haid number of 23 so to find the number of different gametes possible you would take 2 to the 23rd power which is a lot okay so there's a lot of variation here produced by random orientation we're almost ready to to dive in into the specifics of what happens in each stage but before we do that I just want to give you an overall view of what to expect so in meosis the first thing that should be happening is the replication of genetic material so we got to copy that DNA then we're going to undergo meosis one okay so this is all meosis one everything in here at the end of meosis one I have two cells okay so there is a cyto canis I have the separation of these two cells then we're going to undergo meosis 2 okay and after cyto canes following meosis 2 I should have four genetically unique hpid cells let's focus in on meosis 1 for a moment okay so we already know that we should have DNA replication happening and then we're going to go through some phases in the same order as mitosis but with a little twist so when we move out of interphase and into to prophase some things are the same you're going to get the nuclear membrane dissolving you're going to have the condensation of chromosomes this uh spindle fibers are forming what's going to be different and unique about meosis is that in this first uh meosis in profase one you're going to have that crossing over between nonsister chromatids of homologous pairs that does not happen in mitosis that is a very special thing for meosis and it happens in prophase one metaphase is also weird Okay in metaphase one again the chromosomes aren't lining up um you know on the equator just as single chromosomes they're lining up still in their homologous pairs so they are still attached they are still by veence and they line up as homologous pairs which means that in anaphase 1 I don't separate the sister chromatids it is this homologous pairs that are going to separate in anaphase one one we go through telophase um as normal cytokinesis as normal and we end with two haid cells meosis 2 is going to look very similar to mitosis now one of the differences is that we don't go through another round of um DNA replication so we're just going to go right in to um meosis 2 without replicating the DNA prophase 2 is normal all the same things that normally happen in prophase there's no crossing over it's just the normal condensation of chromosomes nuclear membranes dissolve spindle microtubules form metaphase 2 is exactly like mitosis the chromosomes align in the center and anaphase 2 is exactly like mitosis the cister chromatids are pulled apart due to the shortening of the spindle microtubules telophase 2 is the same okay decondensing new nuclear membranes and cyto canes is the same so at this point okay I'm undergoing this very familiar cell division with the two daughter cells that result from meosis one and that means my final product are four hloy genetically unique cells and again this is a great example of why meosis represents the change part of the continuity and change theme Here In theme D um because each of these is unique and could offer a source of variation to any Offspring um created from them