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Current time:0:00Total duration:6:12

Video transcript

let's take a look at the nucleus of a cell that's just starting to undergo meiosis so we have these 46 chromosomes and beforehand each chromosome probably looks something like that that's the centromere in middle and then as meiosis is beginning they will duplicate into each chromosome will duplicate into something like that that's again that sensor made middle so we have these 23 purple chromosomes we're going to say that these are the maternal chromosomes and then we're going to say that the blue ones are the paternal chromosomes they come in homologous pairs and the nucleus that we're looking at must belong to the cell of a male because right over here you can see that is the Y chromosome it's a bit smaller than most of the other chromosomes I'm going to digress just for a moment to clarify a very common point of confusion and that is that if you look over here at these two chromosomes well this is considered one chromosome but so is this called one chromosome and this can be confusing because this is only one chromatid and this is two chromatids so why are we calling them each one chromosome and the answer that question is because we count chromosomes by the number of centromeres so this single chromatid has one centromere but these two chromatids are attached in the middle by one centromere so we also call that one chromosome just keep that in mind it's just a technical point to know that even though they're different there they're still both considered one chromosome and in this chromosome the two chromatids are duplicates of each other so it's just a copy of itself anyway back to our nucleus so we have these 46 chromosomes 23 homologous pairs and they're not just the chromosomes were not necessarily arranged in the way that I drew them I just drew it that way for the sake of organization but we have these piers and we're going to focus on one pair of homologous chromosomes but I want you to keep in mind throughout this video that whatever we're describing that's happening to this pair of homologous chromosomes is also happening to the other 22 pairs of homologous chromosomes just that it would be too hard to depict in a video how that's happening but keep them keep in mind it's not just happening to the pair that we're talking about but what we're just going to be describing is happening to all of the pairs of chromosomes in the nucleus so here we have our pair of homologous chromosomes and during prophase 1 of meiosis the homologous chromosomes pair up with each other and form a unit called a tetrad and it's called a tetrad because well tetra means four and this unit has four chromatids right 1 2 3 and 4 and the process during which the homologous chromosomes pair up with each other is called synapses so during synapses the amalgams chromosomes will get a little bit closer to each other something like that and at a certain spot they might actually cross over or overlap so I'm going to circle that spot and that's called the chiasma and in some cases another thing happens this protein complex that resembles something like a railroad track forms we'll see in a minute why and this is called the synaptic amyl complex you can actually see the word synapse in there because this happens during synapses so we form the synaptic complex and with the help of the synaptic complex these two chromatids the ones that are crossing over will actually swap material downward of that point so we're going to get something that looks like that look at how the purple chromosome now have some blue over there and look at how the blue chromosome now has some purple over there and the way that happened was that the DNA in the chromosome actually some bonds and that DNA broke and the DNA's just kind of swapped places so what we just described this process by which the two chromosomes swap information is called crossing over or another way to say this is genetic recombination and let's see why this is called genetic recombination so we're going to fast forward to the end of meiosis 2 where the chromosomes get split into two and all the chromatids get separated into different gametes and I want to pause and remind you that everything we're describing that's happening to this pair of chromosomes is also happening to all the other 22 pairs of homologous chromosomes but anyway so now let's put each one of the chromatids in a different gamete and look at how we get four different gametes and we can call these two gametes recombinant and we're calling them recombinant because they have a combination of alleles that's new we haven't had this combination of alleles even at a parent and just to clarify things let's see what the gametes would look like if crossing over did not happen so let's go back to our original chromosomes and let's split them and let's put them into four different gametes and we are going to get that and you can see that in this case we only have two different types of gametes so we can see how genetic recombination increases genetic variability which is usually a good thing