27. The Reproductive System
Meiosis
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In this video, we're going to introduce the first part of meiosis, which is meiosis 1. And so recall from our previous lesson videos that before meiosis can even begin, the process of interphase needs to occur first. And so notice over here in our image down below on the left hand side, we're first showing you that interphase needs to occur before meiosis can even begin. And recall that interphase consists of the g one, the s, and the g two phase. And during the S phase of interphase that is when the DNA gets synthesized or replicated. And so before meiosis can even begin interphase takes place and the DNA must be replicated. And then at that point after the G2 phase of interphase, the cell can enter into meiosis 1. And what you'll notice about meiosis 1 is that it actually has very similar steps to mitosis in terms of having a prophase, a metaphase, an anaphase, a telophase followed by cytokinesis. And so one thing that you'll note is different is that the phases of meiosis 1 are all followed by a 1. And so notice here we have prophase 1, metaphase 1, anaphase 1, telophase 1, indicating that these are the phases of meiosis 1. Now recall from our previous lesson videos when we were talking about my mitosis that sometimes the phase prometaphase, which would come between prophase and metaphase, is sometimes batched with the previous phase, prophase 1. And so even though we're not showing you a prometaphase here in this image, the events of prometaphase are still taking place in prophase 1. And so, really the events of prophase 1 and telophase 1 are going to be very very similar to the events that occurred in mitosis. And so, really, meiosis 1 is gonna be similar to mitosis, but meiosis 1 is going to differ significantly in both metaphase 1 as well as anaphase 1. And so because really it's just metaphase 1 and anaphase 1 that differ significantly, really this is where we're going to put most of our focus. And so notice that our image, down below has these two phases in blue here to help emphasize that these are really the two phases that differ the most from mitosis. And so, in metaphase 1, of course the m in metaphase represents the m of the middle. And so, in metaphase 1, what's going to happen is that homologous chromosomes are going to pair up, and they're going to align themselves in the middle of the cell, but they'll align themselves in the middle of the cell in 2 rows. And really this is the big difference here that these chromosomes are aligning themselves in 2 rows instead of aligning themselves in one row like what we saw in mitosis. And so if we take a look at our image down below at metaphase 1, what you'll notice is that in metaphase 1 the homologous chromosomes are pairing up with each other to create 2 rows. And so notice we have one row of chromosomes right here and the second row of chromosomes is over here. And so they're not all lining up in a single file row, they're lining up in 2 rows. And that is a big difference between meiosis 1, metaphase 1 of meiosis 1, and metaphase of mitosis. Now during anaphase 1 of meiosis 1, it's actually the homologous chromosomes that are going to separate from each other, while the sister chromatids themselves remain connected. And this is another big difference between my, meiosis 1 and mitosis because during mitosis, the sister chromatids would separate, but during meiosis 1, the sister chromatids remain connected and it's actually the homologous chromosomes themselves that are going to be separated. And so when we take a look at anaphase 1 down below right here, notice that the homologous chromosomes they line up in these pairs here in metaphase 1 and then it's the homologous chromosomes that are separated. So notice that this homologous chromosome is separating in this direction and so is this one here going towards this end of the cell while this homologous chromosome here is going towards this end of the cell and this one here is going towards this end of the cell. And so the homologous chromosomes are separating but the sister chromatids themselves still remain intact and so that means that we still have replicated chromosomes here. And so, that is going to be a big difference between, again, meiosis and, mitosis. And later in our course, we're going to do a side by side comparison further differentiate between these two and their similarities as well as their differences. But again, the main phases that are going to be significantly different in meiosis are going to be metaphase 1 and anaphase 1. And again prophase 1 and telophase 1 are going to be pretty much the same exact events that occurred during mitosis in those same phases. So after telophase 1, of course you can see here, there is going to be the process of cytokinesis and cytokinesis is the division of the cytoplasm splitting the cell into 2. And so cytokinesis during meiosis 1 is actually going to produce 2 haploid daughter cells. And then these 2 haploid daughter cells that result right over here can then begin the second half of meiosis which is meiosis 2. And so, meiosis 2 is going to begin with these 2 cells that are now haploid. And so you can see they have half the number of chromosomes in comparison to the very very beginning. And so notice that there were originally 4 chromosomes here in this image in the beginning, that would be the diploid number for this example, and at the end each cell only has 2 chromosomes, which is half of 4, meaning that these are going to be haploid. And so this here really concludes our introduction to meiosis 1 and how it differs most significantly from mitosis. And we'll be able to get some practice applying these concepts and then talk about meiosis too as we move forward in our course. So I'll see you all in our next video.
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