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9. Mitosis and Meiosis

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Meiosis Overview

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Hi in this video we're gonna be talking about mitosis. So first let's do an overview of mitosis mitosis is this is the type of cell division and it creates daughter cells with half the genetic material of the starting cell. Because there are two divisions in mitosis. The first division is called reduction. All division. And this is where the genetic material is reduced by half. And the reason that it doesn't stop here and needs an extra division is because even though the genetic material is reduced by half there's still two copies of every chromosome. And so you have to have a second division called equatorial division. And this is when each of those sister chroma tips get their own cell. So my Asus will start with a deployed cell has two copies of everything and end with a half Lloyd cell with one copy. Now there are many different chromosomal forms that happened during mitosis and people get really confused on what we're talking about. So at the beginning of my Asus you start with hom*ologous chromosomes and we've talked about this a lot. But essentially this is what you start with. So these are hom*ologous chromosomes right? There's two of them but they have the same genes on them. So if I were to label them say T. And T. They have the same genes. They may have different alleles but it's the same gene right after D. N. A. Is replicated. So you get another copy have each one. So you get two big tees and two little tease. These are sister committed. So the two big tease. Our sister committed and the two little teas are now sister promenades now it's called by violent when you have the pair of hom*ologous chromosome. So here is by violent. You have a tetrapod referring to all four chroma tit* that are formed after replication and you have a diet. And this refers to two sisters promoted. So these two either the black or the red pair. So um, those are the two, the three or the four really different terms that you'll see and it's important to understand the difference. So here we have my Asus let me back up and if it'll let me there it goes. Um and so you start off with hom*ologous chromosomes which these are drawn kind of odd. But these two are the hom*ologous chromosomes and the bigger ones of the hom*ologous chromosomes, they get replicated. And when they do that ends up with these um structures were most more commonly used to seeing. Right. But the hom*ologous chromosomes stay the same. So here we have the hom*ologous chromosomes, these two and these two but now there's an extra copy of each side. So the first thing that happens is you divide the cell and what you notice is you only get one copy of each hom*ologous chromosome. And so because you only have one copy you have to sister chroma tides on each one, right? One sister committed into but you still only have one big and one little this means that this is half Lloyd. And if because you have essentially T. T. And Z. Z. And uh T. T. And C. Z. Essentially that's what we're saying. So because you have the same chromosome, it's just multiple copies of it. That makes it happen Lloyd. And then after mitosis, what happens is um you get one copy. So this has TZ TZ TZ TZ. So these are also half Lloyd and they just have one chromosome. Now people get really confused so I want to make sure that this is clear. So when you start off, you start off off with hom*ologous chromosomes. So you start off with Big T. And Little Tea and we'll just label these Malaga's chromosomes. These aren't representing jeans, these are representing chromosomes after replication you get two big tease and too little tease because each hom*ologous chromosome was replicated after my Asus one. What you get is you get two cells, one with two big tease and one with two little tease. This is half Lloyd because you have the same type of chromosome. Yeah, you have two copies of it. You could have 40 billion copies of it. Right? If you have 40 billion big tease, it's still half Lloyd because all you have is a big T. You don't have a big T. And little Tea. Which is what you started with. People get very confused. I get a lot of questions. You know why is after my Asus one, why the cells have Floyd? It's because you only have one, you only have one type of chromosome, You just have multiple copies of it. Then after my Asus to what happens is you get four cells you get big t big t little t little T. And so these are also half Lloyd but now you just have one copy of it instead of two. So this is super important understand how the chromosomes change and why after my Asus one it's half Lloyd and not deployed. Because you will probably be asked that question at some point, professors like to use it to trip students up. So after bios this one the cells are half Lloyd. So with that let's now turn the page and get to my Asus steps.

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Meiosis Steps

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Okay, so now let's talk about the steps of mitosis. So mitosis is completed in two parts. And luckily for us, each one of these parts is followed by either one or two. So we know what's going on. So the first step is pro phase one. So this is happening in my Asus one. And there are five steps in pro phase one. And here they are. I usually bunch of the names. Um but there's the five steps and there's different things that happen in them. So in the first one, the chroma tin begins condensing in the second one. What happens is this complex called the synaptic synaptic momentum complex forms because the chromosomes are aligning and the Houma logs are actually coming together. So we always are given this picture of chromosomes and they look perfectly lined up together. But normally in the cell they're actually like not condemn they don't look like our typical representation of chromosomes. They're just kind of long streams that float around. They're not paired or anything. So in pro phase one. And especially in the Z. Go name a stage of pro phase one. This is when they start pairing together in forming what's called the synaptic mental complex. Then you have the third step. This is where crossing over occurs. So crossing over is the really important part that we talked about a lot that happens with genetic recombination. And so crossing over happens in this step of aosis. This will likely be a question. So know that crossing over happens here. And so what happens is this guy's mata form and this is the structure that forms between chromosomes that are exchanging information through crossing over and you can actually see these connections between the chromosomes at this point, then you have the chromosomes that are just sort of staying here. And then in the very last step the chromosomes begin to pull apart. But they can't really yet because the guy's mata that helped them crossing over are still there. So it's keeping them together and that's really important because if they pulled apart now you wouldn't have proper diagnosis. But because those kids matter keeping keeping the chromosomes together, it's it's good news. So here we have the beginning of May aosis you've had DNA replications, you have a green and a green. So these are replicated, read and read replication has happened. And then you have these exchange of materials here where crossing over has occurred and this occurs in this stage. So then we go to meta phase one. So the chromosomes are now or the tetrapods now, right because you have four because they've been replicated are lining up at the meta face plate and the hom*ologous chromosomes lie on either side. So what happened is we started off with hom*ologous chromosomes right then they got replicated now. When they got replicated, they're still hom*ologous chromosomes. Right? So if we were to label these chromosomes, it would be like this. So they have the same genes on them. They may have different alleles, but they're hom*ologous chromosomes. So what happens is when they line up in the middle, this is the middle. So the hom*ologous chromosomes line up on either side of the metal faceplate while the sister chroma tides are on the same side, supposed to say saying same side of the metal faceplate. So then so you can see this here in this image. So here's the metal faceplate. So these chromosomes go to this cell and these go to this cell. Now when that happens you get one cell that has T. T. On one cell that has T. T. And so these are happily remember remember this. So but that hasn't happened yet. We're still there still lined up in the middle. So then we have an A. Phase one. The diocese begin to separate into one pole. This is called disjunction. When the hom*ologous chromosomes separate properly. Non disjunction happens when the separation doesn't occur properly. So here's the meta phase play and we have two chromosome here and that each one separates like this. That would be disjunction. But if we had chromosomes here or like this and these three separated this way and this one separated that way, that would be non disjunction. And it does happen. It causes a lot of different genetic disorder. So it definitely can happen. Now separation is beginning and anNA phase um tele phase, they're pretty much at opposite poles now and the nuclear membrane is beginning to reform around them and then you have psychokinesis which separates the cytoplasm and actually forms to cells. So here is what is pretty much a mix between anna phase and tell a phase. And you can see that the chromosomes have now separated to the opposite end of the pole inside of genesis is happening here and we'll pinch these into two cells. So now we're starting with the two cells here and we're on to mayo sis two. So what happens is you have pro phase two and that lets us know we're in my Asus two. This is really a short phase. Um pretty much the cells look like this here. Um Nothing super big happens here. It's usually very short, sometimes almost non existent. Then the chromosomes go to meta phase. So now the sister chrome it'd. So before we had the chromosomes like this lined up on the meta face plate right now because this has gone into its own cell. So for so for mitosis, to what happens is the sister chromosomes line up on the metal faceplate. So this is my Asus one and this is my Asus two. And this is the difference between the two. So you can see here that now the chromosome the sister chroma tides are lined up in the middle. So what will happen is here's the meta phase plate. So these two sister chroma tides will go to this cell and these two sister chromosomes will go to this cell and the same happens for the other sell these two chroma tides go to this, sell these two chroma tides go to this cell go this way. So I don't have a great image for this. It's a little confusing. You can if you compare the actual sister chromosomes here you'll see that their their colors are exactly the same. But essentially and a face to the sister chromosomes begin separating tele face to the chromosomes go to each pole and then you have side of kinesis again and this creates for half fluid cells. So I've drawn to here but there would be four with the same amount of chromosomes in them. And you can say that you know like I said before these two sister chroma tides go to one cell. So that would be this cell here. Whereas these two sister chroma kids on this side of the metal faceplate go into this cell and the same for the other cell as well. So these are happy. So if I were to draw this this would be T T T. T. So just to review it's been a second reviewing start off with two hom*ologous chromosomes. I'll write it like this and I'll draw it like this. Oh I wanted to do a different color. Hold on. So here we go. Then replication occurs. So then you get two copies right Then you get my Asus one Right and that's that that creates four cells. So the meta phase play is here or here the face plate. So once I get this one sells get fat or this this however you like it. Then you have my Asus two. These are hap Lloyd. Remember Because you have one type of chromosome even though you have two copies you can have 40 copies could still be one type. Um so it's happily then you have my Asus to where you get four cells. So that would be big t big t little t little T. Or if you like the chromosome drawing two black and two red which is also hap Lloyd because you either have black or red not both together. These are deployed. So hopefully you understand the difference between deployed and hap Lloyd and why um this is how May Asus works. So this is these are the steps of mitosis so hopefully that makes sense. Um So with that let's not move on.

Problem

In which step of prophase I does crossing-over occur?

Leptonema

Zygonema

Pachynema

Diplonema

Diakinesis

Problem

Are the two daughter cells produced from meiosis I diploid or haploid?

Diploid

Haploid

Problem

Are the four daughter cells produced from meiosis II diploid or haploid?

Diploid

Haploid

Problem

Which of the following are sources of genetic variation during meiosis?

Crossing over, independent assortment

Crossing over, DNA replication

DNA replication, independent assortment

Disjunction, crossing over

Problem

During which stage of meiosis do sister chromatids separate?

Prophase I

Anaphase I

Prophase II

Anaphase II

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PRACTICE PROBLEMS AND ACTIVITIES (20)

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The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains ...
The diploid number of the hypothetical animal Geneticus introductus is 2n = 36. Each diploid nucleus contains ...
Explain how the behavior of hom*ologous chromosomes in meiosis parallels Mendel's law of segregation for autoso...
Suppose crossover occurs between the hom*ologous chromosomes in the previous problem. At what stage of M phase ...
Define and discuss these terms: (a) synapsis, (b) bivalents, (c) chiasmata, (d) crossing over, (e) chromomeres...
Contrast the genetic content and the origin of sister versus nonsister chromatids during their earliest appear...
Given the end results of the two types of division, why is it necessary for hom*ologs to pair during meiosis an...
Explain why meiosis leads to significant genetic variation while mitosis does not.
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Considering Problem 15, predict the number of different haploid cells that could be produced by meiosis if a f...
A woman who sought genetic counseling is found to be heterozygous for a chromosomal rearrangement between the ...
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For Problems 25–30, consider a diploid cell that contains three pairs of chromosomes designated AA, BB, and CC...
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