Introduction and Reduce of Chromosomes – Meiosis

00:01 Previously, we looked at cell division and considered mitosis. But now, let's take a look at cell division and meiosis. Meiosis is for sexual reproduction in eukaryotes. By the end of this lecture, you should be able to recall the language of chromosomes. It's particularly important when we consider meiosis and comparing it to mitosis. We will also be able to discuss the distinguishing features of meiosis. What makes it specifically different from mitosis? In addition to summarizing two means of acquiring genetic variation. One of the important things of sexual reproduction is precisely that.

00:43 That we can obtain some genetic variation upon which evolutionary mechanisms can act.

00:48 Next we'll be able to relate meiosis to gamete formation. Understand how it relates to the production of sperm and eggs. And in addition, you will be able to distinguish between mitosis, meiosis I and meiosis II.

01:05 And define some of those distinguishing factors. So, let's begin by thinking about why it is that we need meiosis at all. We have 46 chromosomes as humans right. 23 pairs and each of those pairs of chromosomes need to separate in order to form haploid gametes and sperms. So we are diploid. Our gametes, sperm and eggs are haploid. So each of them has to have 23 chromosomes. One of each copy.

01:42 So, diploid organisms have a pair of each chromosomes. Haploid organisms have a haploid, one set of each of those chromosomes. So, germ line cells are the cells that are going to become sperm and egg.

02:01 So germ line cells themselves are actually diploid. And sperm and eggs become haploid after the process of meiosis. And then sexual reproduction leads the sperm to the egg and they fertilize each other.

02:15 And once again restore the diploid condition in the zygote. So once we have a diploid zygote, it will develop and cell division will occur. And this is when we see many of those cell cycle controls coming into place. So now let's look at the language of chromosomes. It's particularly important for us to review this before we move into understanding what goes on in meiosis. More important than knowing what happens during prophase, metaphase, anaphase, so on and so forth, is that we understand what's happening with the chromosomes. So, important to be able to diagram it rather than just to be able to describe the events of each phase. So, let's step into that for just a quick moment.

03:03 First of all recall that we each have two copies of each chromosome. During S phase, those chromosomes are replicated or synthesis of a new chromosome happens. So now each chromosome has an exact copy of itself. One of them originally came from your father and one of them originally came from your mother. And then during the process of meiosis, we're going to separate the homologous chromosomes that are replicated and we are going to separate the sister chromatids in meiosis II.

03:41 So, reviewing the language homologous chromosomes are two of the same kind. Once they undergo replication during S phase, these homologous chromosomes can be called replicated homologous chromosomes cause they each have an identical copy. Now, recall that there is a small indentation or tightly packed DNA region which we call the centromere. And the centromere has some kinetochore handles on them.

04:13 Those kinetochore handles are again what we're going to use or what the cell uses in order to pull the sister chromatids or homologous pairs apart.

04:26 So, moving on from here we will look into how this cell division happens. When do we actually reduce chromosome number. That's a question to keep in your mind. So meiosis, has two nuclear divisions.

04:43 and only one round of DNA replication. So, the germ line cells are undergoing the cell cycle producing more and more germ line cells. They know they are going to eventually undergo meiosis and become sperm and eggs. But they undergo mitosis until they're ready. At some point in time, there is going to be some sort of cell signalling that tells these germ line cells that are diploid, that it's time for them to divide into four gamete cells. And that's the point that we are going to look into with more depth right now.

05:22 So, meiosis I is where we are going to have the reduction division. I'll repeat that a number of times because it's really important to recognize that that is when we go from diploid to haploid.

05:35 After meiosis I we have one copy of each of our chromosomes in each resulting cell.

05:44 So either the mothers chromosome or your fathers chromosome end up in one of the resulting cells from meiosis I. During meiosis II, we take this separated homologous chromosomes and split the sister chromatids so that each of the resulting cells has one copy of each chromosome.

06:06 So meiosis I, we had our homologous chromosomes. They separate. And in meiosis II, sister chromatids separate.

06:16 So that's where the language becomes especially important.