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M.Ed., Stanford University
Winner of multiple teaching awards
Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.
Meiosis is a type of cell division that produces gametes. In meiosis, cells undergo two divisions. The result of meiosis is the production of four haploid cells from a diploid cell with each haploid cell contains half of the number of chromosomes and half of the genetic materials of a diploid cell. Meiosis has many similar mechanisms to mitosis. The stages of meiosis include prophase I, metaphase I, anaphase I and telophase I followed by prophase II metaphase II, anaphase II and telophase II.
There is a form of cell division called meiosis and this is the cell division that is used when an organism is going to do sexual reproduction. Now it is properly pronounced meiosis like I just said but I have found both my own studying of science and when I've been teaching students that a lot of times they have a hard time hearing the difference between mitosis and meiosis. The two basic kinds of eukaryotic cell division. So I've figured out a way for myself and for my students to help hear that slight difference because mitosis m-i-t-o-s-i-s is used for doing cell division to make two identical cells. So that's I emphasised the i there. The mitosis is used for cell division for making two identical nuclei.
Now meiosis, notice that slight missing t? It's spelt m-e-i-o-s-i-s. So I slightly emphasize the e because meiosis can be used for sexual reproduction for making gametes like sperm or eggs which are used for making babies because you reduce the number of chromosomes.You kind of hear the e there, and that allows you to help rememebr the key significant differences between mitosis and meiosis.
So let's dive a little bit more into it. The whole idea here is that you are going from a diploid cell, cells that have pairs of homologous chromosomes one from mummy one from daddy, and you have to create haploid cells. The gametes like sperm or eggs. So in the process of meiosis you go from having one diploid cell. In a human cell that would be a cell that has 46 chromosome to having 4 haploid cells, the sperm or eggs that have 23 chromosomes.
Now there's two parts to meiosis, cleverly named meiosis 1 and meiosis 2. Meiosis 1 is the key part of meiosis because this is where you could have recombination of DNA. That's where you're swapping a piece from dad's chromosome number 17 with mum's chromosome number 17 in a process called crossing over. You also have the separation of the homologous chromosomes. This is what generates the haploid cells that are necessary for the creation of gametes. Meiosis 2 just takes the individual chromatids that make, make up each chsomosome and separates them. Looks a lot like mitosis which I'll discuss towards the end.
Let's take a look at this very simple explanation of meiosis here. And here we can see a normal cell. Let's pretend that this is a cell of some creature that only has two chromosomes unlike the 46 that are in humans or the 110 that I think are in cabbage, whatever. That'd just be too crowded. So let's go with these two.
In interphase right at, towards the end you undergo a process part of the cell cycle called s phase where you copy the DNA and now we have our chromosomes each made out of two molecules or chromatids of DNA. So during meiosis 1, you pair them up and then you separate these homologous chromosomes. Then meiosis 2 you wind up having our individual gametes here with only one copy of each chromosome as opposed to the two that were in our starting cell.
Now let's look more in depth. Here we see our interphase cell. You can see the DNA is still spread out in the chromatin form. Now we wind up going through prophase 1. Now this is one of the things that you'll notice about mitosis and meiosis. The names of the different steps are very similar. Mitosis begins with prophase, meiosis begins with prophase but you have to add in the the 1 or 2. Otherwise bio teachers like me will sit and go haha, you're wrong and mark off your papers if you miss that key one or two.
So in prophase 1 of meiosis 1 we have the nuclear membrane breaks down. We see our chromatin goes into the visible chromosome form and we start to have this process called crossing over where you have parts of the homologous chromosomes exchange.
Now, in this we have a chromosome number of 4. We have 4 chromosomes in this cell and we can see that there is two of the small homologous chromosomes and two of the large homologous chromosomes. And I'll use convention I'll be using blue for daddy's DNA and red for mummy's DNA.
Now after prophase 1 we have metaphase 1. Metaphase remember is when they're in the middle. So metaphase 1 we have the homologous chromosomes line up in the middle of the cell. Now wi- winds up making this very important step is tHAt we'll have the pairs line up in the middle. And we'll see here, again here is our metaphase 1. We see the pairs are lined up and then the homologous chromosomes get separated. So these little guy's separated from the other little guy. This big guy here is separated from that big guy there. That's in anaphase 1 they get separated.
Last in telophase 1,we see the nuclear envelop starts to reform and we'll have cytokinesis occur to separate our two daughter cells from each other. Very commonly you'll undergo a process called meiotic interphase which is when the DNA in our two new cells loosen slightly so that you can rid off the genes needed to control the next step of meiosis, meiosis 2.
So meiosis 2 looks a lot like mitosis and so we'll have prophase 1. Nuclear envelop breaks down again, we tighten up any of the chromosomes that had been loosened up in the chromatin form during meiotic interphase and our spindle fibre start to form and attach themselves to our chromosomes.
In Metaphase 2 part we line up our chromosomes now along the metaphase plate in the middle of the cell and this time we don't need to do any pairing of homologous chromosomes. In anaphase 2 they separate from each other and then in telophase 2 and cytokinesis we wind up reforming our nuclei and we now have our four daughter cells as required by meiosis. And these will form into sperm or eggs or whatever kind of cells are needed.
Now, we're going to go ahead and watch a video on YouTube that'll show all this process and it really will help you see that it's not something where they go jump jump jump from one step to the next. It's a continual process. So let's go ahead and make this full screen and get it started.
So here we see our cell and here we see the nucleus inside the DNA is starting to tighten up, that means we're in prophase 1. And so we start to see our two large and in this. So we have a chromosome number of six. So three pairs of homologous chromosomes. So now during this process of coming together, the DNA will be exchanged. Now in this case, the graph is just kind of erased the membrane as it breaks down and then here'll just kind of make the colors shift. But in reality, physically exchanging parts of their DNA structure. Over here we can see the of the centromere of each of these chromosomes, the attachment point for the spindle fibres.
So now the spindle fibres have attached and they're pulling the chromosome pairs, the homologous pairs to the middle of the cell often called the metaphase plate. Once they're there we'll enter into anaphase 1. We're now in metaphase 1. We're now going into anaphase 1, we separate the homologous chromosomes. Each chromosome is still composed of two chromatids at this point. We're now beginning cytokinesis and we'll start to reform our nuclear membrane during telophase 1. In this case the artiste chose not to do it.
Now we'll begin meiosis 2. We begin with prophase 2. We're starting to attach our spindle fibres to the chromosomes we moving to the middle. That's in metaphase 2. We separate them in anaphase 2 and then once they're at the opposite poles of our new cells we start to reform the nucleus during telophase 2 and the cytokinesis and we now wind up with four cells each with half the number of chromosomes of the starting cell. That's meiosis alright?
So if we go back to the slide I want to show you real quick a comparison between mitosis and meiosis just to help reinforce some of those key diffrences. Because I know that a lot of times we bio teachers we love to ask questions, comparing and contrasting meiosis and mitosis. Also I'll show you one of the other things that is a very common trick question. A lot of times we bio teachers love to show a cell that looks like this or like that and ask, what stage of cell division is this in?
The thing to look for is look for pairs of chromosomes. If you see them paired up a double line here, versus a single line that means it's meiosis. You never have that double pairing in mitosis. Remember mitosis you go from having a particular number of chromosomes 1, 2, 3, 4 chromosomes here, and I have 1, 2, 3, 4 chromosomes here. If they tell you the number of chromosomes, the normal number of chromosomes for a particular cell is say four or six or 22 and then at the end you're going to put that same number for four, six or 22. That's mitosis.
If they tell you the normal number is 4 and then they show you this, where there's only two at the end, that's meiosis. So again mitosis you're making two identical cells, meiosis you're reducing the number of chromosomes so that you can create your gametes like sperm or eggs that have reduced the number of chromosomes.
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