Meiosis (Cellular Reproduction Series)

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Meiosis. We're gonna see a lot of similar activity to what we saw in mitosis here. So let's do a quick recap of mitosis before we move on. Remember, after the G2 phase, we entered mitosis, where the DNA condensed into chromosomes. Mitosis occurs in all the somatic cells, but meiosis is only in the sex cells, okay?

We start meiosis with a germ cell. That's a diploid cell capable of undergoing mitosis or meiosis. It undergoes mitosis so that it can reproduce itself and just stay functioning, generation after generation. When it undergoes meiosis, it is producing cells whose descendants will be sperm or ova.

Now, sperm and eggs don't have 46 chromosomes, they're haploid, and they have to be. Because when fertilization happens and a zygote is formed, it needs to inherit DNA from both parents. And that's a bit down the road a bit. In meiosis, we aren't yet talking about fertilization.

We're talking about the precursors to that, the process by which a diploid germ cell gives rise to four haploid cells. Meiosis happens in stages, many stages, so does mitosis. But meiosis has kind of like what I call two chapters, each of which has multiple phases. And we're gonna look at meiosis I first.

What happens in meiosis I is you have the same two chromosomes that have gone through S phase. And so they've doubled their genetic material. And the chromosomes make their way through the phases of meiosis. Prophase in meiosis very much resembles prophase in mitosis, with the exception that during prophase I of meiosis, crossing over occurs.

Crossing over is the exchange of genetic material between homologous chromosomes. Homologous chromosomes, these are not identical. Regions of one chromosome will exchange with the region of this homologous chromosome. After crossing over has occurred, the homologous chromosomes move to the center of the cell in metaphase I.

And metaphase I looks very much like metaphase looked in mitosis. But now in anaphase, instead of the sister chromatids splitting apart as they did in anaphase of mitosis, sister chromatids actually stay together. So our division is actually just separating the homologous chromosomes, not the chromatids, but separating the pairs. And now in telephase I, the cell divides for the first time, and we have two cells.

But we need to get four, so we move into meiosis II. So let's take a look at prophase II, and I do want you to look at the diagram. The regions of red and blue, and hopefully one of the first things you notice is that these are not identical cells. Remember, because they have been pulled apart by homologous chromosomes, those chromosomes contain different material.

In each of these cells now, a new spindle forms around the chromosomes. In metaphase II, the chromosomes line up at that equator or the metaphase plate, you can call it. This is very much like in mitosis. And then in anaphase II, the centromeres divide, chromatids move to the opposite poles of the cells.

And this is more like mitosis than was anaphase I. And then telephace II, very much like telephase in mitosis, a nuclear envelope forms around each new set of chromosomes and the cytoplasm divides. So here's a comparison of mitosis and meiosis to help you kind of pinpoint the main differences.

One of the most important is that mitosis, you have 2N chromosomes, 46, going to 2N, 46. Whereas in meiosis, you have the halving of genetic material, you wind up with four cells that have only 23 chromosomes. Mitosis occurs in all somatic cells, meiosis occurs only in sex cells. In mitosis, there is no genetic material being exchanged, there is nothing parallel to the crossover.

Whereas in meiosis, there most definitely is. And here's a summary so you can kind of eyeball it all at once.

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