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Main content
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Video transcript

let's now jump into understanding meiosis in some depth so let's start with the germ cell as we mentioned already a germ cell is a cell that it can either go through mitosis to produce other germ cells or it can undergo meiosis in order to produce gametes so this is a germ cell right over here let me draw the nuclear membrane so that's the nuclei let me draw the nucleus a little bit larger just because that's where where we care a lot about the chromosomes in it and let me draw a centrosome which will play a role later on I want to do that in let's see I'll do that in this blue color each centrosome has two centrioles in it but we'll well I just would clarify some of the terminology and in the mitosis videos I focused on a cells of an organism I just kind of made it up that had two chromosomes that had a diploid number of two that had one homologous pair that had one chromosome from each of its parents for this video I'm going to focus on a species not human beings that would have 23 pairs or 46 chromosomes I'm going to focus on a species that has that diploid number is 4 and so let's say it has two chromosomes from the father let me do that I'll do that in this orange color I'll do it in the chromatid of depict the chromatin state it's kind of unwound so maybe has a long one from the father and it has a short one from the father and then it has homologous chromosomes from the mother so it would have the long one from the mother and it would have the short one the short one from the mother and just like that and obviously this is a huge simplification but hopefully this gets the point across so here it has a diploid number of chromosomes so this is let me write this down this is diploid diploid diploid number is equal to we have four chromosomes and then this thing this germ cell let me write this down this is a germ cell right over here it will go through interphase so let me draw that so it will go through interphase and we in which it grows and it can replicate its DNA and its centrosome and so let me draw that so after it goes through interphase I want to use my space carefully because I have a lot of steps to go through after it goes through interphase I am going to have in my nucleus in my nucleus here my DNA will have replicated so this long this long chromosome from my father it will now all the DNA will have replicated so it might look something something like that and it's attached at a it's attached at a centromere all these central words at a centromere right here but I'm still trying to draw it in kind of the chromatin state it's actually all spread out it's not bunched up so you can see it very clearly is these X's and kind of a in a simple microscope so it's just replicated and after replicating it is still one chromosome it has twice the genetic material but it is still one chromosome that one chromosome is now made up of two sister chromatids two sister chromatids we talked a lot about that in the mitosis video but it doesn't hurt to reinforce because it can get a little bit confusing and then you have that shorter that shorter chromosome from the father and then that also replicates into two sister chromatids attached at a centromere so these are still two chromosomes from the father has twice the amount of DNA but it's containing the same information just duplicate versions of that same information and the same thing is going to happen from the from the mother you had that long chromosome from the mother homologous to this one right over here it's going to replicate so it's now going to be two it's now going to be two sister chromatids and then you have a short strand from the mother that was homologous to this one from your father and that's also going to replicate and so it's like that and at the end of interphase it would actually all be it would all be spread out once again it won't be bunched up into these clearly discernible X's I drew them a little that way otherwise because you would have trouble seeing how it replicated and we also have we also have replicated our centrosome as we've gone through interphase now we are ready in fact now we are ready for either mitosis or meiosis but as I said the focus of this video is going to be meiosis so let's let's do some meiosis so the first phase so the first several phases we call meiosis 1 and the beginning of meiosis 1 is prophase 1 so let's see what happens in prophase 1 so prophase 1 so let me draw the cell right over here so prophase prophase 1 a couple of things happen the nuclear membrane begins to dissolve this is very similar to prophase 2 prophase when we were looking at at mitosis so the nuclear envelope begins to dissolve these things start to maybe migrate a little bit so these characters are starting to go it ends at the different ends and the DNA starts to bunch up into kind of its condensed form so now I can draw it so now I can start to draw it as proper so this is this is the one from the father right over here and this is the one from the mother and I'm drawing them overlapping on purpose because something very interesting happens especially in meiosis so this is the mother right over here this is let me see let's all do the centromere in blue now that's a centromere that's the centromere now this is the shorter ones from the father this is these are the shorter ones from the mother and actually let me just do draw them on opposite sides just to show that they don't have to the ones from the father aren't always on the left-hand side so this is the shorter one from the father it could be all on the left-hand side but doesn't necessarily have to be and then this is the shorter one from the mother and I won't draw these overlapping although they could have shorter one from the mother and once again each of these this is a homologous pair that's a homologous pair over there now the DNA has been replicated so in each each of the chromosomes in homologous pair you have two sister chromatids and so in this entire homologous pair you have four chromatids and so this is sometimes called a tetrad so let me just get us give ourselves some terminology so this right over here is called a tetrad or often often called a tetrad now the reason why I drew this overlapping is when we are in prophase one in meiosis one let me label this this is pro prophase one you can get some genetic recombination some homologous recombination once again this is a homologous pair one chromosome from the father that I've gotten from the father of the species or this the cell got from its father cell and one from the mother and they're homologous and that they might contain different base pairs different actual DNA but they code for the same for the same genes so you know oversimplification but in this in a similar place on each of these it might code for eye color or I don't know personality and you know nothing is that simple in or how tall you get and it's not that simple in DNA but just to give you just to give you an idea of how it is and the reason why I overlapped them like this just to show how the recombination can occur so actually let me zoom in so this is the one from the father once again señal in the condensed form this is one chromosome made up of two sister chromatids right over here and I drew the centromere not to be confused with centrosomes that's where they are those two sister chromatids are attached and then I will draw draw the homologous chromosome from the mother so the homologous chromosome from the mother just like just like that Amala guess chromosome from the mother and the recombination can occur at a point right over here so after you're done the recombination this side might look something more like this so let me let me draw it let me draw it like this so they essentially break up and they swap those sections is one way to think about it so this one will now have a little a little piece a little piece from the mother and it might code for similar genes but now contains the mother's genetic information and then and then this one over here this one over here will now have the piece and you could say even the homologous piece from the father from the father and let me do the suit to two centromeres and this is really interesting this isn't you know all the time there there could be recombination and oftentimes it can lead to kind of non optimal things nonsense code and DNA and might lead to a on functional organism but this happens fairly common in meiosis and it's a way once again to get more variation we've talked about sexual reproduction before and sexual reproduction introduces variation into a population and this obviously when different sperms find different eggs that introduces variation but then even amongst homologous pairs you can actually have exchange of exchange between these chromosomes and that's interesting because as we mentioned each of these chromosomes they code for a bunch of different genes and a gene is kind of Wilken code for specific or a set of proteins and so this right over here let's say and this is I'm going to what I'm about to say is going to be a huge oversimplification maybe right over here you coded for eye color or it was related to or it helped code for eye color and that you got that from your dad and here it helped code for eye color and you got that from your mom your mom might have trended you towards a lighter eye color and your dad might have trying to do towards a darker eye color but now the one from your mom is on this chromosome this gene and then the one or they're both the same gene they're just different alleles they're coding for different variants of that gene and then the allele from your dad is over here and once again you know some people get confused with genes and chromosomes and all this each of this these chromosomes contain a bunch of genes these are very long DNA molecules these code for a bunch of different genes so a gene will be will be a little section of here that could get code for a particular protein so that's what happens in prophase one in prophase one you have this content station of your chromosomes of your homologous pairs you have you have you can have this recombination and it's really interesting this recombination doesn't tend to happen at just random points that would kind of break the Jetta get genetic genetic information it tends to happen at fairly clean points and the places where this breakup is happening these are called the plural if you just talk about one point it's a key asthma or if you're talking about the plural is chiasmata sounds like it could be a horror movie so key Ozma key Ozma and the fact that they happen they tend to happen fairly cleanly this is you know once again kind of the beauty of the universe or at least of biology is that through billions of years of evolution these things have have kind of optimized for more variation to happen in fairly clean ways so I'm going to leave this video right there I know I just got to prophase one but this was a really really important idea of this homologous recombination or this chromosomal crossover that we see right over here and then from there we can continue through the rest of meiosis 1 and then meiosis 2
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