Meiosis: Formation of Gametes

Video Transcription

Welcome to this lesson today on meiosis. Today, we are going to be discussing how meiosis forms gametes. So meiosis is a type of cell division that happens in sex cells. So it's specific to sex cells. So in this type of cell division, germ cells are basically going through a division in order to form gametes, which are sperm cells or egg cells. So the result of meiosis is that you end up with haploid daughter cells. So haploid cells contain half as many chromosomes as the original parent cell.

So we'll start out with a germ cell of 46 chromosomes and end up with our haploid daughter cells that each have 23 chromosomes. So in meiosis, there are actually two rounds of cell division. We call them meiosis I and meiosis II. And it goes through these two rounds of division in order to cut those number of chromosomes in half. And so each round of meiosis is actually pretty similar to mitosis, set if you're familiar with that. So it's almost like mitosis is happening two times in a row, in order to cut this number of chromosomes in half.

So meiosis leads to, as I mentioned, the formation of gametes. So spermatogenesis is meiosis in sperm cells. So it's the formation of sperm cells. So in that process, we end up with four sperm cells. And oogenesis is the formation of egg cells. So in that process, we actually end up with one egg cell, and actually a polar body. But basically, today we are just going to be looking at meiosis as a whole. And this process, the main goal of it is to cut the number of chromosomes in half, produce haploid daughter cells.

So let's take a look at the diagram right here that explains this process. So we start with our germ cell in prophase I. In this phase, kind of similar to prophase of mitosis, homologous chromosomes will condense and pair up, and then also at this point, they will swap segments. So the swapping of segments between these chromosomes just basically creates genetic variability. And then also at this point, the nuclear envelope will break down and spindle microtubules will attach to the sister chromatids.

Then we move to metaphase I. And in metaphase I, the homologous chromosome pairs will align at the metaphase plates in the middle of the cells. Our next step then is anaphase I. I hope this writing isn't too small. So in anaphase I, homologous chromosomes will then start to separate. And they start moving towards the opposite ends of the cell, the opposite poles. OK. Then anaphase I is finished. And we end up with telophase I.

And during telophase I, the nuclear envelope then starts to reform. And we end up with two haploid nuclei. So we have half as many chromosomes in this cell as we started with here. But we still need to now go through another round of cell division in order to make sure we only end up with 23 chromosomes in each cell. And I'm going to talk about why that's important in just a few moments. So I'm going to draw a dashed line here and separate this into meiosis I and meiosis II. So over here, we have meiosis I. And over here, we have meiosis II.

OK, so after telophase I is done, we began meiosis II. So now we have prophase II. So during prophase II, similar to prophase I, it's like we're going through this process again, the spinal microtubules will attach to the sister chromatids and the nuclear envelope will start to break down. Then we have metaphase II. And during metaphase II, the chromosomes well then again line up at the middle of the cell. We have those spindle fibres attached to them. And then similar over here. I didn't draw that. There we go.

And then we move into anaphase II. And then again in anaphase II, those sister chromatids separate and start moving towards opposite poles. And then our final step is telophase II. And then in telophase II, the nuclear envelope will reform. And we end up with four haploid nuclei, all that have 23 chromosomes. So by going through these two different stages of cell division here, we end up with these haploid nuclei, all that have 23 chromosomes. So we've cut a number of chromosomes in half. So we started with one cell that had 46 chromosomes. And we ended up with these four cells that each have 23 chromosomes. So these are haploid cells.

Now the reason why this is important in sex cells is because if an egg cell has 23 chromosomes, and a sperm cell has 23 chromosomes, when they fertilize, when the sperm fertilizes the egg and the nuclei combine, we end up with a total of 46 chromosomes. Now 46 chromosomes is the normal number of chromosomes in a human cell. So it's important that the sperm cell and egg cell each only have 23 chromosomes, so that when they combine, we have a total of 46 chromosomes. So of the 46 chromosomes each of your cells, half were from your mom and half were from your dad.

So it's important that they are haploid at the end of the process of meiosis for this reason here. If we had 46 chromosomes in the egg cells, and 46 chromosomes in the sperm cells, when they combined, that would be 92 chromosomes in the resulting zygote. So with each generation, you would be doubling the number of chromosomes that are in each cell. So we need to make sure that the egg and sperm only have 23 each, so when combined, we end up with the normal 46.

So this lesson has been an overview on the process of meiosis, and how it forms gametes.