Review of Reproductive Cell Division: Meiosis (Nursing)

00:01 So now let's look at meiosis.

00:04 Recall that there are 46 chromosomes or 23 pairs of chromosomes.

00:10 Think of it like you have 23 pairs of shoes.

00:16 So in meiosis during metaphase I, we will line up these pairs instead of creating one single file line, we will have two lines.

00:27 We also refer to these pairs as homologous pairs.

00:32 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.

00:41 It's important to note that the chromosomes at this point are duplicated and in the form of sister chromatids.

00:51 After anaphase and telophase I, we now have separated the pairs from each other into separate cells.

01:01 The cell is now haploid.

01:04 But they are still in the sister chromatid or duplicated form.

01:11 So in meiosis II, both of these cells are going to contain one set of sister chromatids.

01:19 These will further divide much in the same way that the cells divide in mitosis.

01:25 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.

01:38 So let's take a more in depth dive into this process.

01:43 There are two divisions that occur in meiosis.

01:46 The first is referred to as Meiosis I.

01:50 In meiosis I, 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.

02:04 First, in prophase I we have events that we do not see in mitosis, or meiosis II.

02:15 These events includes synapsis, where the homologous chromosomes are going to pair up forming a tetrad consisting of four chromatids.

02:25 Because remember, these are in a duplicated sister chromatid format.

02:31 Another process that we have in prophase I that we do not see in mitosis, is crossing over or chiasmata.

02:39 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.

02:55 This is going to result in a unique chromosome that's a mixture of both maternal and paternal chromosomes.

03:04 So at the end of prophase I, the chromosomes are already beginning to be genetically distinct from where you started.

03:15 In Metaphase I, the tetrads or the homologous pairs are going to line up randomly at the spindle equator.

03:25 Its important to highlight that the lining up is random.

03:30 Yes, the homologous chromosomes are going to line up next to each other.

03:35 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.

03:45 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.

03:59 The next phase in meiosis I is Anaphase I.

04:03 In Anaphase I, the sister chromatids stay together while the homologous chromosomes get pulled apart, unlike in mitosis, where the sister chromatids get pulled apart and the homologous chromosomes stay together.

04:15 Finally, at the end of meiosis I, each daughter cell is going to contain 2 copies of sister chromatids from each member of a homologous pair is either going to be the maternal chromosome or the paternal chromosome.

04:33 Also, we now have a haploid chromosomal number.

04:38 Even though we are still in the sister chromatid format, there is only one of each type of chromosome in these two daughter cells.

04:50 So after meiosis I, the cells will undergo a second division.

04:56 This is referred to as the Equational Division of Meiosis.

05:02 The events that happen in meiosis II are similar to the events that happen in mitosis.

05:08 The difference is that there's no chromosomal replication before this process begins.

05:16 The sister chromatids from meiosis I are now going to be separated toward opposite poles.

05:23 And we're going to result in one chromosome per cell.

05:30 So recall the random assortment that happens in metaphase I is important for genetic variation.

05:38 In this example, with two pairs of chromosomes, they are 2 equally probable arrangements of chromosomes at metaphase I.

05:49 In Possibility I, dad's chromosomes are in one line, while mom's chromosomes are in the other.

05:57 This was will result in haploid cells that have only dads or only mom's chromosomes at the end of metaphase I.

06:08 However, in Possibility 2 the first line contains a mixture of each.

06:15 So this will result in 2 daughter cells that each has one of dads chromosomes, and one of mom's chromosomes.

06:25 In Meiosis II, in this example, the sister chromatids will separate yielding four different daughter cells.

06:34 From here you have different possibilities of daughter cells.

06:40 For Possibility 1, you will have two daughter cells containing only dad's chromosomes and two daugther cells containing only moms chromosomes, However, in Possibility 2, you will have two daughter cells that contain one combination of mom and dad's chromosomes, and two others with the opposite combination.

07:05 All together, with two pairs of chromosomes there are four different possible combinations that you can get.

07:13 In a human that has 23 pairs of chromosomes, there are two raised to the 23 possible combinations.

07:23 This equals 8,388,608 total possible combinations that you can get from our 23 pairs of chromosomes.

07:37 When you couple this with the crossing over, that happens in prophase I, you realize that each gamete offers many opportunities to pass it on certain traits from the mother or the father through genetic variation.

07:54 So now, let's look at the important tasks that are accomplished by meiosis I.

08:01 First, we're going to reduce the chromosome number by half.

08:05 And secondly, in meiosis I we are introducing genetic variability.

08:11 We do this because of the random alignment of the homologous pairs that happens in metaphase I.

08:18 Also in prophase I, the crossing over, which is going to give us a variability of our gametes.

08:26 In result, we will have no two gametes being exactly alike and all of them will be different from the original cells.