Mitosis and Meiosis Video

This video will attempt to highlight the most important features of mitosis and also meiosis. For a quick review, the structure of DNA is composed of two strands of single DNA helices. Together we call it a double helix. And if you take apart the double helix and you look at just one side or one strand, the basic parts are composed of nucleotides, the basic subunits. And each nucleotide consists of a phosphate, a five-carbon sugar, and a nitrogen-containing base. And so, as you see, they repeat over and over and over. These nucleotides repeat over and over and over. And if we if you remember back, the nucleotides, the nitrogen-containing bases may be thymine, adenine, cytosine, or adenine. There are a couple of a few definitions that we have to familiarize ourselves with. One of them being the chromosome. A chromosome consists of a linear DNA double helix bound to proteins. Remember, there is a difference between a chromosome and a chromatin. Once the chromatin is condensed, it's called a chromosome. Uncondensed, it's called, the DNA is called chromatin. And when it's unwound, we generally see it like so. But once it's wound over and over and over and over itself, it's then called a chromosome. And so this diagram shows how it's it's wound over and over itself using histone proteins and coiled several times. The next definition, gene. A gene is just a piece of DNA, a sequence of DNA from hundreds to thousands of nucleotides long, and they provide instructions to build proteins. A gene is just a little segment on a DNA. A locus is a specific place that a gene occupies on a chromosome. So you could say we could find a gene on a particular locus on a particular piece of DNA. Telomeres are at the ends of the chromosomes, but special repeated nucleotide sequences which protect the chromosomes from enzymes, from being consumed by enzymes. They keep the length, they maintain the length of the chromosome. And a centromere is a specialized region of the chromosome which temporarily holds the two daughter strands, the two daughter DNA double helices together after DNA replication. You never see a centromere unless the chromosome has been duplicated. So when the cell is just hanging out and growing and doing its job with no mitosis, no duplication, you'll never see a centromere because the chromosome is, first of all, it's uncondensed in as chromatin, and there's no need for a centromere because there's no need for an attachment point to another sister chromatin because it hasn't yet been duplicated. The kinetochore on the centromere is the attachment site for microtubules that move the chromosomes during salivation. So as you see here in the blue on the chromosome, that's the centromere and just imagine a kinetochore on that. If you take a look at you take a look at figure B or C, these are identical sister chromatids, identical. At first you start with A, then you duplicate it through DNA replication and the sister chromatids are held together by centromere and when they're separated, then you finally see this C figure. Figure C. How can you tell that these that these are identical sister chromatids? Well, first of all, they've been copied. One has been copied and so you have two identical copies. How can you see that just from the diagram? Because the colors of the genes are exactly the same from one sister chromatid to the other. The yellow on the other side is yellow. So we've kind of already covered this. Duplicated chromosome consists of two identical DNA double helices called sister chromatids, which are attached to each other at the centromere. And another term that you need to be familiar with. Not identical chromosomes now, but homologous chromosomes. Homologous chromosomes are two chromosomes. For example, if you go to pair one, we have 46 chromosomes or you could say 23 pairs. And if you go to the first pair, you see that those two chromosomes they look similar. So we would say that those two chromosomes are homologous. They're not identical. They're not attached to each other. Nor are pair two or pair three or pair four or five. None of these are are attached to each other. There's no kinetochore in the middle. They're not attached to each other. There's no duplication seen here in this karyotype. These are homologous pairs, meaning that they are not identical, but they are similar to each other. They hold very similar genetic material. However, if you go back to this diagram in figure B or C, these are identical. One has been copied, has been made, has been copied, and therefore the two are absolutely 100% identical to each other. So when a cell is hanging out, growing, and carrying out its daily functions, what you would see in the DNA, in the nucleus, is what you see in this diagram. Except these are chromosomes and what you would normally find are unwound chromatin. Nothing is duplicated. There are 46 in the human, in each human cell, in the somatic human cells. And you can say 23 pairs because each of the single pairs are homologous to each other or they are similar, but they're not identical. A mutation is a change in nucleotide sequence in DNA, and this can be passed from generation to generation. It can have positive consequences or negative consequences or neutral consequences. One example that comes to mind is the exposure of UV electromagnetic radiation from the sun. Generally, this is negative. This has negative consequences. The UV is so small that it can penetrate the cell membrane and get in penetrate the nuclear membrane and actually mutate, change the nucleotide sequences in the DNA of our chromosomes and generally this is, this has negative consequences. That's an example of a mutation. Autosomes are are pairs 1 through 22, and you can also call these diploid cells, diploid or somatic. And the sex chromosomes are the, are the haploid, are inside of haploid cells. The sex chromosomes are the 23rd pair and those have the XX and the XY. As you see the 23rd pair there, you have one X and one Y. That would be a, this would be an example of a karyotype of a human male. And if the 23rd pair were, had two X chromosomes, then that would make a female human karyotype. Again, diploid cells are somatic cells. Diploid cells have 23 pairs of chromosomes. They have 40, a total of 46. Whereas haploid cells are sex cells or gametes. And these have half the number, you can actually think of, you can actually, it almost sounds the same, haploid like half, they have half the number of chromosomes that a somatic cell would generally have. These have 23, a total of 23 chromosomes, not 23 pairs, but a total of 23. All the cells of the human body are somatic cells, are diploid cells, except for what you would find in the gonads, either in the testes or in the ovaries, you would find gametes, having, containing half the number or 23 chromosomes in each of those cells, each of those sex cells or gametes. The oocytes or the sperm. That way, when two people mate, they, the, those two gametes will combine half chromosome, half the number plus half the number would equal a diploid cell, the full number of chromosomes, 46 chromosomes. It's important to keep track of those numbers. I mean, so far we've only said, we've only had two numbers to memorize, 23 chromosomes and 46 chromosomes. But it's important to to keep track of those numbers as that those can definitely come up on an exam. So the first type of cell division that we'll talk about is mitosis. Mitosis occurs in every day, 24 7 in diploid type cells, somatic cells. In other words, 99% of all the cells in, in, in the body. Mitosis, not meiosis, but mitosis. Let's start with interphase. If you notice the, the yellow area, the cell is, the cell, the interface can be divided into G1 phase, S phase, and G2. And if you count it up, it looks like maybe like 80% of the, of the life of the cell is actually in interphase, all this yellow. And in this, the cell isn't actually going through any mitosis, but it is growing, as you see in G1. Then in the S phase, the synthesis phase, the, the, the chromosomes are then duplicated. And then once they're duplicated, there is the, there's G2 where the cell prepares for cell division, which is called mitosis. It's preparing for the blue region where you have all the steps of mitosis, prophase, metaphase, anaphase, and so on. Now, let's just do a quick review. How many chromosomes does a diploid cell have normally, when the cell is just hanging out, growing, and differentiating, going about its normal functions? A normal somatic cell, for example, the, in the, in the skin of the hand, or muscle cells, or bone cells, or cartilage, or, or, or, or what have you, any kind of connective tissue, bone, blood, these are, or these, each of these have 46 chromosomes. So in the, in G1, how many chromosomes are in a single cell? The answer is 46. However, when the cell begins to enter the S phase synthesis, the chromosomes are duplicated. Well, what's the, what's, what is 46 duplicated? 46 times 2 is 92. So by the end of the S phase, in interphase, the, actually, the cell actually has double the number, not 23 pairs, not 46, but actually 92 chromosomes. And how many does it have in G2? Well, the, the same number as in, as in the S phase, 92. That means that by the time the cell enters prophase, the cell actually has twice as many chromosomes that a normal, regular cell would have, diploid cell would have. By the time the cell enters prophase, the first step or phase of mitosis, the cell actually has 92 chromosomes. So let's go through each of the phases of mitosis. It's your job to, it's your job to know what are the phases of mitosis, the correct order of the phases of mitosis, and also what are the, what, what occurs inside the cell during each of those phases of mitosis. What occurs in prophase, in metaphase, in anaphase, in telophase, and cytokinesis. So in, in prophase, the first phase, notice what bullet A is saying, what letter A is saying. The duplicated chromosomes condense, going from, going from chromatin to chromosomes, but also keyword duplicated. They have already been duplicated. There's, there's 92 here, and they are attached to each other. Each sister chromatid is attached to its identical copy. So now imagine going back to that, that one of those diagrams. At, at this point, they look like this, like figure B, where they're attached with a, with a, with a centrum, because they're, they've, they've already been duplicated. Not like this, but attached with this, with the, with the centrum. Now, so the first important phase, step to know in prophase is that the duplicated chromosomes condense. Also, the nucleolus begins to disappear, if you remember, the nucleolus inside of the nucleus, and spindle microtubules, spindle fibers, begin to form from, from the centrioles. The nuclear envelope also begins to disintegrate. Not the plasma membrane, but just the nuclear envelope. So the nucleolus disappears, the nuclear envelope disappears, and now this, now the chromosomes are free to float around in the cell. And finally, the chromosomes are captured by, by the spindle. And, so let's, let's go forward and take a, take a look at, at, at prophase. So if you see here in this diagram, if you notice in the diagram, the nucleolus has disappeared, the nu, the nuclear membrane is beginning to disappear, and the spindle fibers are beginning to, to appear. Once, so if you see in, in B, that's early prophase, and even in late prophase, um, the chromosomes are, are, are attached to the spindle fibers, attached at the kinetochore, and, um, they haven't yet lined up or anything, but they are, they're, they're getting there. No, and, but you notice that they're sort of floating around all over the cell. Why? Because there's no longer a nuclear membrane that's, that's, that's keeping them bound at, in the middle of the, uh, inside of the nucleolus. Okay, so now we have completed prophase. What ha, what are the, what are the major, um, things to know from, from prophase? First of all, the, there are, the chromosomes are already duplicated. There are 92. The nucleolus disappears, the nuclear membrane disappears, and the, the, um, spindle fibers begin to form from the central, the, the strings. In metaphase, and just look at the word meta coming from middle, the chromosomes line up single file, um, at the center, middle, they line up at the middle. That's, that's the main thing, that's, that's really the main thing, and if you look at the, um, the figure d here, here is metaphase. There are more, the chromosomes are single file, single filed, more or less, at this, at the middle, at the equator of the, of the cell. Now, I'm saying that they are single file. Do they look single file? Well, it looks like they're, they're doubled up. Well, no, that's because they're attached. The identical chromosomes are attached, so we call that single file. You don't, we, we, those, those, remember, they're, we haven't split those, those, those sister chromatids apart yet, so these are called single file at the equator of the cell. This is what occurs in metaphase. Then we have anaphase. The next step is anaphase, where sister chromatids separate, and now they're called back to chromosomes. They're pulled apart, and the chromosomes are drawn to the opposite ends, and so what is the main, um, what is the main, um, thing that occurs in anaphase? If you look at figure e, here's the, here's, um, the diagram. The sister chromatids are pulled apart from each other, the identical copies are pulled apart, and they go to the opposite ends of the, of the cell. How many chromosomes are in this cell? The same as what we started with, 92, because nothing has been split yet. We've, we've made copies, and now, but if you ask yourself, well, how many are going to go to one pole, and how many, well, if there's 92, and half are going to one pole, and the other half are going to the other pole, well, then there's 46 that are going to one side, and 46 that are going to go to the other side. What's the main, what's the main thing that happens during, uh, anaphase? The sister chromatids split apart from each other, and, and they move to opposite ends. And by the way, if one sister chromatid is, is an exact copy of the other sister chromatid, once those, once that cell becomes two cells, what do you think? Are those two cells now, at the end, the final product, are those two cells identical to each other, or are they just close to each other? The answer is, they're, they are absolutely identical to each other. All right, so we have prophase, we have metaphase, we have anaphase, and now we have telophase. In telophase, the spindle microtubules begin to disintegrate. The, the spindle fibers, the little, the little strings, they begin to disappear. The nuclear membrane begins to form again. Do you remember how in, in prophase, it began to disappear, the nuclear membrane, so that the cells, so the chromosomes could freely float around? Well, now the nuclear membrane is beginning to form again. And they wrap, and the nuclear membrane, of course, yeah, the point is, is to contain the chromosomes inside. And also in telophase, the chromosomes then begin to decondense, or they return back to their normal, relaxed state. So they're going to go back to be called, being called chromatin. That's telophase. And finally, in cyto, in cytokinesis, the microfilaments attached to the plasma membrane form a ring around the equator, and the ring contracts and constricts the equator, pinches off the membrane, and forms two daughter cells. So now here we are, we see in cytokinesis, there is a ring, as you see, of microfilaments, and it contracts, pulls, and it splits everything more or less in half. The cytoplasm is more or less, um, you know, split in half, all the water and, and substances inside of the cytoplasm. But of course, inside of the, the nucleus, it is absolutely 100% identical inside the nucleus. But in the plasma, it's more or less the same, about, about half. Half goes to one side and half goes to the other side. So we would say that the product of mitosis starts with one cell and ends with two identical copies. And that is how the cells in our body get, get, uh, get replaced. When cells die out, get worn out, that's why you look the same from Monday to Tuesday and Tuesday to Wednesday. That's why you look pretty much exactly the same. The skin cells, for example, when, when one skin cell, um, as the skin, you know, gets pushed out, more are being, are, are dividing, and there's a layer in the skin that's undergoing high, high level of mitosis over and over and over and over and over to replace the dying out cells. That is mitosis, again, cell division of diploid cells or somatic cells, starting with 46, duplicating to 92, and then cutting each of those again in half into 46 and 46, ending with two cells. In, in meiosis, it's somewhat similar, but there are some significant, some significant differences. And those differences are what account for genetic variability. Where those, there's two things that occur particularly in meiosis, and I want you to know what those two things are that account for genetic variability and where in those steps, in those phases, do they actually occur. Meiosis consists of one round of DNA replication followed by two rounds of nuclear divisions, meiosis one and meiosis two. So if you ever hear prophase one or prophase two or metaphase one or metaphase two, you automatically know what type of replication are we talking about. It's not mitosis, it's meiosis. We have to label it one or two because there are two rounds of, of that that division. So in the first round you have what, what sounds just like mitosis, prophase one, metaphase one, anaphase one, telophase one, and cytokinesis. Well in prophase one homologous chromosomes pair up and remember you notice here we have homologous, we have the the identical copies now pairing up with identical copies. These are homologous, so we're saying homologous pairs line up. So right away if we went, if we went back to seeing what happened during during prophase, you notice in late prophase there we said single file, the identical copies, the single file identical copies. Are lining up. However, in meiosis they line up like this. So we say the homologous chromosomes line up, the homologous pairs line up. I hope that makes sense. And so what occurs then is a phenomenon called crossing over. Here because the homologous chromosomes are, are, are lining up bits of, bits of DNA exchange, get exchanged between the homologous pairs. And so if you see in the, in the, in this figure here this is what's happening during prophase one. You notice that there is some, now how is that possible that, that bits and bits of DNA are being exchanged? Well, it's because the, the part that's being exchanged from one side to the other are very, very similar and so they're able to do that. And this accounts for genetic variability. This is what allows, this is what is the, is the reason why daughters are not exact, don't look exactly the same as mothers or why sons don't look exactly like fathers, but they look similar. One reason is because of the crossing over in prophase one. So this is called crossing over, occurs in prophase one. In metaphase one the replicated homologous chromosomes line up like, like you would like just like in, in mitosis, except it's, they're not singly filed, single filed, they're the homologous pairs line, line up, doubled up. And, and in metaphase one, there's a second phenomena called independent assortment, independent assortment. Let's take a look at what's happening. First of all, in metaphase one, take a look. Do you see the difference of what's happening in metaphase one than what happened in just metaphase of mitosis? Instead of a single file of duplicated chromosomes, we have homologous chromosomes lining up. But there's also the phenomena in metaphase one called, called independent assortment. This means that when the, when the chromosomes line up, they don't necessarily, the, the, the ones from mom don't necessarily have to all be on the same side, and the ones from dad don't always have to be necessarily on one side. If they did, as you see in the first, the, the, the first, in the, in this figure, the first on the left hand side, all the purple are on the top, and all the yellow are on the bottom. If that occurred, and the cells split, then you'd have all of dads on one side, and all of moms in another cell. But they don't necessarily have to do that. And remember, we're only talking three chromosomes here, or six chromosomes. Remember, there's 46 that were, there's 92 here that we're going, we're actually dealing with. So here's an example of four different scenarios that can play out. But in actuality, there's many, many, many more scenarios, because there's so many more chromosomes that, that are in a human cell. If, in the second scenario, there's two purple and one yellow on top, and two yellow and one purple on the bottom. And so when they split apart in anaphase one, the result will be what you see on the bottom, two purple and one yellow in one cell, and two yellow and one purple in the other cell. This also accounts for more genetic variability. It's called independent assortment of meiosis one. In anaphase one, the homologous chromosomes separate. One member of each pair go to each pole of the cell. But the sister chromatids, be careful, the sister chromatids do not yet separate. And in telophase one, as you, as you probably have already guessed, the spindle microtubules disappear. The two clusters of chromosomes have formed, each containing one member of each pair of homologous chromosomes. And the daughter nuclei are therefore haploid. Cytokinesis commonly occurs at this stage, and there is a little, there is little or no interphase between meiosis one and meiosis two. Now, we start in meiosis two, where you have prophase, metaphase, anaphase again. And what do you do? You just split, you just split the cell, the two cells now, once again, each into two. And so, now what it looks like, if you look at the top row or you look at the bottom row of this diagram, it looks more or less like mitosis. You notice in metaphase two, the chromosomes line up just like in mitosis. Just like we, how we described initially in mitosis. And so that's why in this diagram, it, it, it, it shows that in mitosis, there are no stages comparable to meiosis one, from interphase to prophase. But prophase looks kind of like prophase two. Prophase of mitosis looks like prophase two of meiosis. And metaphase of mitosis looks like metaphase two of meiosis. How they line up. Notice the difference between metaphase two and metaphase one. In metaphase two, there's, there are a single file. The chromatids are single file, but, well, actually doesn't really look like it, but it actually is. And in, but in metaphase one of meiosis one, they are, the hom, the hom, the homologous pairs are lined up. Now, the last thing that that's, that I, the last difference I want you to, to realize is that we, the end product of meiosis are four cells, not two cells like in mitosis. Let's start back at the beginning. How many chromosomes are in that initial cell? We said duplicated, so that means 92. In prophase one, 92. In metaphase one, 92. So by the, by, by the time we, we start prophase two of meiosis two, how many chromosomes are in each of those? Well, if we started with 92 and they're going to be cut in half, that means that the top cell has 46 and the bottom one has 46. And if we follow, say, the bottom row, prophase two is 46, 46, anaphase 46. In telophase, the cell begins to split in half. That leaves each of those four haploid cells with 23 chromosomes each. 23 plus 23 plus 23 plus 23 equals the 92 that we initially started with. The end product of meiosis is four cells, four haploid cells, or four cells with 23 each. That way when you, during sexual reproduction, you will make, you will join one haploid cell from an ova, from an oocyte, and one has a sperm making a total of 46. That is the basics of mitosis and meiosis and the differences between mitosis and meiosis. It's important to to remember that in mitosis we are making genetically identical copies, but in meiosis, where we, where it's occurring in the gonads, the testicles, and the ovaries, we are not making genetically identical cells. We are making similar cells, but they are, they have genetic variability. There are slight differences in the DNA, which gives rise to slightly different phenotypically looking offspring, which is why all offspring look different than the parents. And again, remember that in prophase I, we have crossing over, and in metaphase I, we have the second reason for genetic variability, which is called endophenosyroid. And that concludes the main highlights of mitosis and meiosis.