Review of Reproductive Cell Division: Meiosis – Physiology | Lecturio Nursing

So now let's look at meiosis. Recall that there are 46 chromosomes, or 23 pairs of chromosomes. Think of it like you have 23 pairs of shoes. So in meiosis during metaphase 1, we will line up these pairs. Instead of creating one single file line, we will have two lines. We also refer to these pairs as homologous pairs. And these pairs will line up at the midline, much like if you were to line up your right shoe and your left shoe to create two lines. It's important to note that the chromosomes at this point are duplicated and in the form of sister chromatids. After anaphase and telophase I, we now have separated the pairs from each other into separate cells. So this would be like separating the left shoes from the right shoes of each pair of shoes that you have and putting them into separate closets. Because we now only have half of the pair. The cell is now haploid, but they are still in the sister chromatid or duplicated form. So in meiosis 2, both of these cells are going to contain one set of sister chromatids. These will further divide much in the same way that the cells divide in mitosis. The difference is the final daughter cells will each contain one set of 23 instead of one set of 46 that you find in mitosis. So let's take a more in depth dive into this process. There are two divisions that occur in meiosis. The first is referred to as Meiosis1. In Meiosis1, this is going to be the reduction division of meiosis, where we're going to reduce the chromosome number from diploid or 2n to haploid or n. First, in prophase 1, we have events that we do not see in mitosis or in meiosis. Ii, these events include synapsis, where the homologous chromosomes are going to pair up forming a tetrad consisting of four chromatids. Because remember, these are in a duplicated sister chromatid format. Another process that we have in prophase I that we do not see in mitosis is crossing over our chiasmata. This involves the exchange of genetic material between the male and the female chromatids. So that you're going to get a little piece of the male on the female chromatid and a little piece of the female on the male chromatid. This is going to result in a unique chromosome that that's a mixture of both maternal and paternal chromosomes. So at the end of prophase One, the chromosomes are already beginning to be genetically distinct from where you started. In metaphase I, the tetrads or the homologous pairs are going to line up randomly at the spindle equator. It's important to highlight that the lining up is random. Yes, the homologous chromosomes are going to line up next to each other. But if we were to look at each line of 23, what we would find is that it's going to be a mixture of chromosomes from dad and mom. So, using the shoe example, it would be like if you made a line of 23 pairs of shoes, but you did not put all the left shoes on one side and the right shoes on the other side. The next phase in meiosis I is anaphase I. Here, the sister chromatids of each homologous pair are going to be separated from each other toward opposite poles. Finally, at the end of meiosis 1, each daughter cell is going to contain two copies of Sister chromatids. From each member of of a homologous pair. It's either going to be the maternal chromosome or the paternal chromosome. Also, we now have a haploid chromosomal number, even though we are still in the sister chromatid format. There is only one of each type of chromosome in these two daughter cells. So after meiosis 1, the cells will undergo a second division. This is referred to as the equational division of meiosis. The events that happen in meiosis 2 are similar to the events that happen in mitosis. The difference is that there's no chromosomal replication. Before this process begins, the sister chromatids from meiosis 1 are now going to be separated toward opposite poles, and we're going to result in one chromosome per cell. So recall, the random assortment that happens in metaphase 1 is important for genetic variation. In this example, with two pairs of chromosomes, there are two equally probable arrangements of chromosomes at metaphase 1. In possibility 1, dad's chromosomes are in one line, while mom's chromosomes are in the other. This will result in haploid cells that have only dad's or only mom's chromosomes. At the end of metaphase I. However, in possibility two, the first line contains a mixture of each. So this will result in two daughter cells that each has one of Dad's chromosomes and one of Mom's chromosomes. In meiosis 2, in this example, the sister chromosids will separate, yielding four different daughter cells. From here, you have different possibilities of daughter cells. For possibility one, you will have two daughter cells containing only dad's chromosomes and two daughter cells containing only mom's chromosomes. However, in possibility two, you will have two daughter cells that contain one combination of mom and dad's chromosomes and two others with the opposite combination. Altogether, with two pairs of chromosomes, there are four different possible combinations that you can get. In a human that has 23 pairs of chromosomes, there are two raised to the 23 possible combinations. This equals 8,388,608 total possible combinations that you can get from our 23 pairs of chromosomes. When you couple this with the crossing over that happens in prophase one, you you realize that each gamete offers many opportunities to pass along certain traits from the mother or the father through genetic variation. So now let's look at the important tasks that are accomplished by meiosis I. First, we're going to reduce the chromosome number by half. And secondly, in Meiosis 1, we are introducing genetic variability. We do this because of the random alignment of the homologous pairs that happens in metaphase 1. Also in prophase 1, the crossing over, which is going to give us a variability of our gametes. In result, we will have no two gametes being exactly alike and all of them will be different from the original cells.