Okay. So we know that meiosis involves a lot of genetic variation. This is because DNA is reorganized and shuffled around to produce genetically distinct offspring. I'm going to talk about a few of the terms and the processes that happen during meiosis to achieve this genetic shuffling. One of them is called homologous recombination. Homologous recombination allows for chromosomes to exchange similar DNA sequences. We've talked about this before in DNA repair, but this is a little different in meiosis. In DNA repair, we talked about using identical DNA sequences to repair the DNA. But homologous recombination in meiosis actually uses non-identical, but similar sequences from non-sister chromatids in the bivalent. So, what do I mean by bivalent? Remember, a bivalent is going to be the four sister chromatids: 1, 2, 3, 4. So, the identical sequence would be from 2, but if I wanted a non-identical, but similar sequence, I could choose either from 4 or from 3. So DNA repair would always use 2, whereas meiosis is going to use the non-identical sequence from 3 or 4. That's homologous recombination, the shuffling between similar DNA sequences.

Then the second form is through crossing over. Crossing over is when maternal and paternal homologs physically swap chromosomal segments. So, what we get here is we have a maternal and a paternal chromosome. If there's a segment right here, it can come over here, and that one will replace, and they'll swap the segments. When these are connected, they're called chiasmata or chiasmata, and this holds bivalents together because in meiosis, we're dealing with a lot of genetic information. Chiasmata would form, like, here and here that would connect the bivalents together where crossing over has occurred. This is super important because it actually keeps the bivalents together and prevents them from separating early during meiosis, and we'll talk about that more in just a minute.

And then finally, we have reassortment, and this is just the random division of chromosomes in the cells. So remember, during metaphase, the chromosomes line up in the middle, and then they separate from each other during anaphase. But which side they actually line up on is kind of random. So, which chromosomes the cell gets is entirely random; that's called reassortment. Now, this doesn't necessarily have to be successful, and so nondisjunction actually describes when the homologues fail to separate. So, remember we have these chromosomes that line up in meiosis. And so, the order in which these line up and which ones get separated out to the two cells is completely random. If nondisjunction occurs and say, both of these chromosomes go here, this one goes here, and this one goes here, what we get is one pair of chromosomes here and three here. That's nondisjunction. And so this causes what we say is, aneuploidy, and that's where there are, typically eggs, which are the sex cells, with the wrong number of chromosomes. We see this in Down syndrome, also called trisomy, because there is an extra chromosome there. This is what it looks like during crossing over. You can see these maternal and paternal chromosomes. They eventually cross over; the points where they cross over are called the chiasmata. And then after the end, you can see that now they have a mixing of genetic information. So, that is a bunch of different ways that genetic shuffling happens during meiosis. So, with that, let's now move on.