Understanding Cell Division Processes

Okay, chapter five, and this is going to be the start of the material that you're going to review for exam two. This one's going to cover cell division. Now, cell division is what we need to have growth and reproduction in any life form. Remember from the earlier chapters, a cell has to come from a cell. This is the chapter where we're going to see how that works. And then, of course, give you a lot of insight into reproduction and growth. and how the body actually helps to protect itself. So cell division allows a single-celled fertilized egg to grow into an organism with trillions upon trillions of cells. And again, this starts with one cell. And then we get something as complex as our cells, a redwood tree, a whale, your pet dog. It's all the same thing. Somatic cells. Somatic cells are often called whole body cells. And When we do the first part of this, when we do mitosis, we're talking about these somatic cells. This is every cell that you can see on the outside of your body. The skin, the cells that make up your eyes, your lungs, even a lot of the internal stuff are all somatic cells. Now, a lot of these cells actually continue to divide constantly as we live, and then some do not. For example, there are portions of your nervous system that once those cells mature, they will not divide again. That's why damage to the nervous system is so severe in some cases. But blood cells, blood cells are constantly replicating themselves. Skin cells and cells that line the digestive tract and your respiratory tracts constantly have to replace themselves because they're constantly being abraded. Blood cells, red blood cells might last 120 days, but you've got to replace those. And so the cell cycle is responsible for that. Apoptosis. Now apoptosis is programmed cell death. Now this might be a timed event where an internal timer is going to go off and then once that cell has reached that time it will simply disintegrate and essentially die. But also it's also a way for the body to check itself. If you have a mutation in a cell, a cell that might become cancerous, apoptosis is one way the body can actually deal with those. Some examples of programmed cell death, if you've ever had the fortune to have a pond or been by a lake or stream and seen tadpoles. Tadpoles are the young of frogs and toads and a tadpole very young in age has a tail so it can swim through the water. Frogs and toads don't have that and so this is one way that helps the transformation of this organism. And believe it or not, when you're young, when you're in the uterus, you do have a little bit of skin between all of your fingers and toes. And normally that goes away again by apoptosis. Now the cell cycle is how we're going to organize what a cell goes through on its journey to division. And just like any cycle, we have to do one thing first, then we're going to do the next and then the next. And then eventually we're going to get two copies. And so the result are daughter cells. Now, gender is not implied here. They're not all female cells. But what these are are the offspring of the cells that came before them, literally two new cells. So the cell cycle, we're going to break it down into two primary stages. We have interphase and we have mitosis or the mitotic phase. Interphase is in between the actual division, and it can be divided into three stages. We have G1. G1 is the first stage after the cell has divided from a previous mitotic event. And so the cell is going to grow, it's going to mature itself, it's going to make sure it has all of its organelles. Now once a cell is on its way to division, it's going to enter the S phase. First thing that cell is going to do is replicate its DNA. It's going to replicate its coding information. It needs to make two copies. Because once this cell divides, you have to put a copy in each one of those cells. Right before division, that cell is going to enter a complete G2. This is the stage that's going to prepare the cell for division. We're going to get some more proteins. We're going to make some more organelles if necessary. Now after the G2 stage, we have the mitotic stage. This is broken down into two sub-stages called mitosis. This is the active division of the nucleus. And that's the whole key point is we want to divide the nucleus. We want to have copies of the DNA so that when we do get two cells, they're essentially a carbon copy of each other. They're going to be identical. Now cytokinesis is going to happen right at the very end before we have two cells, after the nucleus has divided, and this is going to be the final division of the cytoplasm and the organelles that are contained in that cytoplasm. So here's a graphic explaining all of those details. So first stage is the G1 phase. This is growth. This is what the cell is going to do right after division. And I mentioned some cells that don't go any further. I mentioned the nerve cells, parts of the nerve cells. They will stop right here. They're not going to go any further. This is how you're going to spend the rest of their lives, literally your life. They're going to stay suspended in this G1 phase. If the cell is going to divide, it's next going to go into this S phase. This is where it's going to continue to grow, and we're going to replicate the DNA. We're going to make copies of that. G2 is getting ready for division or mitosis. This is where we're going to mix the proteins. We're going to get things all in order so the cell can divide. Now mitosis we're going to get to, and this is where we're going to spend a bunch of time on the first part of this chapter, but the first phase is going to be prophase. We're going to move into metaphase, anaphase, telophase, and then finally before we separate the cells, we're going to go into cytokinesis, and we're going to get two identical cells. So again, G1, this is where it's going to double its organelles. It's going to grow. It's going to become a mature cell. And then, as I mentioned before, there's an additional step in here called the G0, such as some nerve cells, some muscle cells. They're not going to divide again. They're going to just stay the way they are. In fact, some of the nerve cells in your body, if you live to be 80 years old, you're going to have 80-year-old nerve cells. If the cell is going to continue on, it's going to enter into the S phase. It's essentially going to make a copy of itself and those copies are going to be called sister chromatids. And again gender is not implied. This just indicates that these are exact copies of the DNA that gets copied in the S phase. And these are now called chromatids. Chromatids are two copies joined together. G2 the cell is getting ready for Mitosis is going to finalize all the proteins in accessory organelles to help it divide. Back to apoptosis, again this is timed cell death, but also it's a way for the body to help kind of clean itself up. It does need chemical signals and most cells are held in check by what are called inhibitors that don't allow the cell to degenerate or kill itself. unless there's a signal for it to do so. Severe mutation, like it was mentioned before, during growth, the body needs to reorganize itself. So it's going to absorb some of that tissue, and it does do that by apoptosis. Now, after we've been in interphase, we're going to enter the mitotic phases. Essentially, by definition, mitosis is the division of the nucleus. That's the whole punchline. We're trying to take those two copies of those sister chromatids, and we want to distribute them into two daughter cells, two identical cells. Cytokinesis, this again is the division of the cytoplasm. Any organelles get divided up here too. And then at the end of this whole cell cycle, you have two daughter cells that are identical to the another cell. and they can go on to divide again if necessary. So very important with mitosis is what we start with is what we get, meaning the number of chromosomes in that cell is going to be the exact same number that we're going to get after mitosis. If you remember, eukaryotic chromosomes are composed of chromatin. Chromatin is the long nucleic acid informational fibers that once it starts to coil up, are going to be the chromosomes and this is DNA, essentially the coding region for everything that your body is going to do. Also to help keep it together there's a protein called histones and that's going to give DNA its structure. Now in a cell that's not dividing, part of that cell's job is to make copies. It needs information to make proteins or substances and so it's important that it is dispersed or not condensed into those chromosomes while it's working. And so it can be read. Essentially you can take the information from it. But when it gets ready for division, it's going to be condensed and then these are going to be chromosomes. So each species, and again, it doesn't matter what it is. It can be the tree in your backyard, it could be your cat, your dog, the fly on the wall, yourself. Each species has a unique chromosome number and composition and mitosis has to maintain that. Because once we divide a cell, we want to make sure it's exactly the same cell. So some more definitions. When you hear the word diploid, this means that that cell has a complete set of chromosomes. It is also known as 2N, so two pairs of chromosomes. In the human body, this would equal 46 total or 23 pairs. And important to note, and we'll get back to it in meiosis, 23 pairs, each of these 23, you get one set for mom, you get one set for dad. When you add 23 up, you get a total of 46. Now, haploid number is only one set of chromosomes. Again, in humans, this is going to be 23. And we're only going to find these types of cells when the body's making human eggs in the females or sperm in the males for reproduction. Again, the whole punchline of mitosis is to divide the nucleus. So we start with a 2N cell, and then we're going to end with two identical 2N cells. During DNA replication, we're going to duplicate the chromosomes. Remember, that happens in the S phase. Each duplicated chromosome is composed of two sister chromatids. So these are twins. These are identical copies. They are held together in the middle by a structure called a centromere. And that centromere is going to hold them together until about the middle of mitosis. So remember, sister chromatids are genetically identical. They have exactly the same information. And then after mitosis, each one of those sister chromatids is going to end up in different daughter cells. But those daughter cells and the daughter chromosomes are identical. to the cell that it just came from. So here's one chromosome. This is one chromatid. So this would be in our G1 phase. G2, or I'm sorry, the S phase would be the replication. And then the result of that replication, we're going to have two sister chromatids held together by a centromere, kind of forming this X structure. And again, to note, this side this side exactly the same information. So in our example, as we go through the rest of mitosis, we're just going to follow four chromosomes. So this is a very simplistic cell. We also remember that the centriole is present only in animal cells, and so it is important to mitosis in animal cells. So we're going to follow four chromosomes. This is a somatic cell. It hasn't divided. And so it is a diploid cell. with a total of four chromosomes. If we assign mom the blue color, or I'm sorry let's assign dad the blue color, I guess it just doesn't matter. So we have the blue chromosomes, we have the red chromosomes. Remember red comes from one parent, blue comes from the other parent. Again in that S phase during interphase we're going to make identical chromatids. We still have four chromosomes just like we have here it's just now that we have copies attached to each other still have the 2n equals 4 2n equals 4. we enter mitosis and again the whole punchline is mitosis is to divide the nucleus and during that division we get exactly the same four chromosomes in each cell that we started with up here at the top So now let's just kind of go through and highlight the important things in each step of mitosis. So again, mitosis, the whole point is to form two daughter nuclei, exactly the same number of chromosomes and the same kind of chromosomes. The daughter cell, the result of mitosis, is going to be genetically identical to the parental cell. And a structure called the spindle is going to help us move those chromosomes around. Until we have division. So we're going to start with prophase. To move around the chromosomes, the nuclear membrane is going to disappear. This is going to allow those chromosomes to move through the cytoplasm and eventually into the two different cells. Proteins called spindle fibers are going to appear. These are going to help move the chromosomes in the cell. The chromatome that was in the nucleus is going to condense and now we're going to have chromosomes. And with a good microscope, these are actually visible through that microscope. When DNA is in chromatin, you can't see it. It just looks kind of hazy inside of the nucleus. So each of those chromosomes has two sister chromatids. Remember, those are carbon copies. Sister mirrors are going to attach to those spindle fibers. And then eventually, we're going to move those chromosomes throughout the cell. Here's a micrograph of an animal cell. This is early prophase. We can start to see the condensation of the chromosomes. And then the asters, these are the spindle fibers. This is the complexes fibers along with the centrioles that are going to help move those chromosomes around. We get into prophase, the middle of prophase. We see that we have chromosomes. You can really distinguish each one of these apart if you look close. The other noted thing that happens in prophase is this nuclear envelope disappears. We have to get rid of the nuclear envelope because the chromosomes are much too large to go through those nuclear pores. And so in prophase we'll get rid of that. Now towards the end of prophase, before we go into the next one, those spindle fibers are starting to attach to the central mirrors and they're going to start to push and pull the chromosomes. within the cell. Metaphase. When you hear metaphase, think middle because that's what's going to happen to the chromosome. They're going to line up at a structure called the metaphase plate which is essentially the middle of the cell. Most animal cells are roundish and so we can actually assign two different poles. Kind of like the poles on the earth, you have a north pole, you have a south pole. That's where eventually the chromosomes are going to be pulled toward. But right now in metaphase, we're going to put them on the metaphase plate or the equator, if you will, of that cell. Like we have the equator of the Earth right in the middle. After metaphase, we have anaphase. The centromeres are going to divide and then the sister chromosomes are going to separate and then move to the opposite poles or the opposite sides of the cell. Canita core spindles. polar spindle fibers. These are going to shorten. They're going to pull and push until those chromosomes are at the opposite ends. So metaphase, everything lines up in the middle. Anaphase, we're going to pull those sister chromatids apart and start to move them to the opposite poles. Now telophase, separation of the sister chromatids is complete. And we're going to start making the individual nuclear envelopes around those chromosomes. Spindle fibers are going to disappear. And then after that, we're going to enter cytokinesis, where the cell eventually will divide. In telophase, the chromosomes are going to go back down into chromatin. They're going to unwind. And then the nucleolus, which also disappeared in prophase, is going to reappear. in each one of the daughter cells. So let's look at that again. We have animal cells in a micrograph and then we have our cartoons and we're still in our example just following these four chromosomes. Metaphase, everything's on the middle and then we have a pull at each end. Now these spindle fibers are gonna start to pull and push until those sister Chromatids are now being pulled apart and now they're going to be dragged towards each end of the pole. So this is anaphase, separation of those sister chromatids. Telophase, all of the chromosomes are now at the poles. We're going to have reformation of the nuclear envelope and also the nucleolus is going to reform as well. And then in telophase, we're going to start cytokinesis. Now mitosis in plant cells is very very similar except for cytokinesis. What's the difference again between plant cells and animal cells? Somebody evidently just said it. It is a cell wall. Cell wall we have to get around that especially during cytokinesis and also remember plant cells do not have any centrioles. They don't have those structures. That was one of the definitions that we saw in a previous chapter. So here's a series of micrographs. Here's a plant cell in prophase. I have chromosomes. They're still in the nuclear region, but we don't see a nuclear envelope. Metaphase, they've more or less ended up on the middle of that cell. Antiphase, the sister chromatids are now being pulled apart, going to the opposite poles. And then in telophase, the nucleus is beginning to reform. And then something called a cell plate is now forming between those two new nuclear regions. This is eventually going to become the new cell wall that we find in plant cells. So this is the start of cytokinesis in plant cells. Now in animal cells there's no cell wall and so after the nuclei are reformed something called a cleavage furrow is going to form. And essentially what this is, is there's a little fiber that wraps around the cell that becomes tighter and tighter and tighter. It's kind of like closing a bag that has a drawstring on it until it's fully closed. So here's actually a scanning electron micrograph showing the outside of two animal cells. Cleavatero is right here. It's a nice division. This one's actually almost done. There's that little protein. You can almost see it as the little line in between them. And it's going to keep contracting until literally this cell pinches itself apart. So cytokinesis in plant cells, again, we have to first have a new cell wall. It starts by originating from a flattened small disc that appears between the daughter cells. This is known as that cell plate that I showed you on a previous slide. Vesicles coming from the Golgi are going to bring new cell wall material in and add it until that cell wall reaches each side. And then other vesicles are going to bring across plasma membrane material and put it on each side of the cell wall. Here's a transmission micrograph. new cell membrane this is the cell plate that's forming between the new cells now meiosis so meiosis is for reproduction and again this happens in humans it happens in flowers crickets flies anything that sexually reproduces is going to go through this process now the difference between mitosis amiosis is at the end we're going to have a reduction in the chromosome number in half and again this is important because when a male and a female if anything come together each one is going to contribute an egg the other is going to contribute the sperm and when those join in fertilization you want to come back to the exact same number of chromosomes that you started with because remember we're trying to make more flowers we're trying to make more humans we're trying to make more crickets if you will and so we can't change the number and so for meiosis this is the important outcome for that so it all begins with a diploid parental cell so we have 46 chromosomes in the human so in your reproductive system you have diploid cells that are going to do this and eventually become reproductive cells To do that, we're actually going to go through two different cell divisions. And then the punchline is we're going to end with four haploid cells. Remember, haploid is half the number of chromosomes. And again, daughter cells, this doesn't imply gender, but these are just offspring of that original parental cell. It involves pairs of chromosomes called homologs. Now, homologue... is a chromosome that you got from dad, one from mom, and they're going to pair up during various stages of meiosis. So again, homologs, this is a pair of chromosomes, one from mom, one from dad. During meiosis one, they're going to line up at the middle. This is called synapsis. When homologs separate, each daughter cell receives one member of the pair. So either mom's chromosome or dad's chromosomes. And then at the end of meiosis one, we have two haploid cells. Meiosis two... No replication of DNAK is going to occur between meiosis I and meiosis II. So we're not making copies anymore. We're just going to separate the sister chromatids that were formed in the S phase. So the centromeres are going to divide, and sister chromatids are going to migrate to the opposite poles to become individual chromosomes. And so at the end of meiosis II, we're going to have four daughter cells. each with the haploid number of chromosomes again. So let's look at an overview and then we'll go through the steps. So we start with a diploid cell. Remember 2N is diploid and we're only going to follow again four chromosomes. Believe me, you don't want to follow 46. One pair from mom, one pair from dad. meiosis one they're going to form homologs meaning moms and dads chromosomes that are identical are going to line up they're going to form that homolog again mom and dad's joining together exactly the same chromatone lining up at the end of meiosis one we're going to separate the homologs and so you're going to get one chromosome you from dad and then one chromosome from mom in each one of these cells. They're not going to travel together, but notice we still have the sister chromatids. We still have the carbon copies of these chromosomes. Meiosis 2, we're going to separate the sister chromatids and then we still end up with two chromosomes in each of the four cells. So we start with a diploid number. And as we enter meiosis one, we're going to go and exit with a haploid number. Four chromosomes here, four here, four here. Now we're at two, so this is haploid, this one's haploid. We divide the sisters, and we still have two chromosomes in each one of these. And so, fertilization. These are daughter cells as a result of meiosis are going to mature into gametes. Again, sperm for the males and eggs for the females. And the act of fertilization is fertilization or the fusing of their two nuclei. This is going to restore the diploid number. And again, this is important because if we breed a couple dogs, we want to get dogs back. And so we don't want to increase the number of chromosomes. We don't want to reduce it. We have to get exactly the number we started with to get dogs out of that development. And so again, meiosis requires two nuclear divisions. It's going to result in four daughter nuclei, and then each of those nuclei are going to have half the number of chromosomes as the parental cell. Meiosis I is divided into four phases. prophase 1, metaphase, anaphase, telophase. And just like we saw in mitosis, the chromosomes are going to be doing very similar things, except with a couple exceptions that I'm going to point out as we go through this. One thing that meiosis does is it helps ensure genetic variation. And so you're a result of your parents, but you don't look exactly like either one of your parents. Probably you can see traits. from both your parents in yourself and just when you have kids your kids are going to be a result of a union with your with your spouse and so you're going to see genetic variation in your children too and through meosis this is how this is accomplished first way something called crossing over that i'll illustrate with a graphic but this is the sharing of genetic information between two chromosomes and then they divide so you get something unique and then independent assortment is something that happens during cell division because there's no specific way that the homologs have to line up and so we can get a lot of variation out of that as well So let's look at that and we'll come back to those two other points in a minute. So first thing that happens, DNA replication is going to take place. We're going to have our sister chromatids form. We're entering to prophase 1. Homologs are going to line up. In metaphase 1, anaphase, the homologs are going to be separated. We're going to join into telophase 1 where we're going to rebuild the nuclear envelope and then cytokinesis is going to happen and two chromosomes here two chromosomes here again we started with four right here we're going to separate those homologs and eventually we're going to have a single set of chromosomes in each one we're going to have a haploid cell and then two haploid soils are the result of the first division so again prophase one synapses occurs these are where the homologs are going to come together membrane is going to break down around the nucleus spindle fibers are going to appear and then we're going to start moving the homologs to the center now it's in pro phase one when these homologs are lining up that we're going to have crossing over and I'm going to show you a graphic in a second how that works. But after crossing over the chromatids held together are no longer identical. We've actually made a brand new combination. One of them is going to have the original genetic combination. The other one is going to have something called recombine where we've changed the genetic material. So again this happens in Pro Phase 1. Synapse. are when the chromosomes again come together. Again one from mom, one from dad. They're going to line up and pair up. Now sometimes the individual chromatids will cross over when they come into close contact. If that happens and if these two pairs stick, there can be an exchange between moms and dads resulting in a brand new combination on each one of these chromosomes. This is what we started with. This is all blue. This is all that reddish orange color. But after crossing over, this is different. The genetic information is going to be different on each one of these chromosomes. So that's going to lead to genetic variation. And then that's going to be carried through right to the end of meiosis all the way to the end. Metaphase 1, anaphase 1, homolog pairs are going to line up the metaphase plate, again the equator. And this is where we're going to have independent assortment. It doesn't matter what side of the cell that mom's chromosome or dad's chromosome is facing. Moms could all face left. Dads could all face right. But it's random. So maybe 50% and 50%, maybe 25 and 25. And so that independent assortment is also going to add toward diversity. So let's talk about that just for a second. Let's see, these are our homologues. And in this example, what they have is all of moms lining up, going to the left. All of dads are going to go to the right. And this is our result. But there's nothing to say that all the blues can be here, all the reds can be here. And then there's nothing to say that they all have to be on the same side. And so we could have this combination where we actually get two. different combinations out of that. That's independent assortment. There's no rule saying how these have to line up. This is going to be differently genetically because mom's chromosomes, dad's chromosomes, they're different. They look different. And so it's going to have different information and then we're going to mix it up here too. You add in crossing over, we can really have some different combinations. So again, we're going to replicate the DNA. We've got the sister chromatids. Prophase 1, remember we're going to lose the nuclear envelope. All this material is going to condense into chromosomes. Metaphase 1, we're going to have everybody lining up in the center. And then in this example, you can see that there's going to be some swapping. We also have some examples of crossing over. And so that when we get to the end of this, each four of these cells is going to be genetically different from one another. So the first division, metaphase, anaphase one, homologs. align at the metaphase plates. And again, it doesn't matter how they align. Independent assortment tells us that moms and dads don't have to be on the same side. Independent assortment is going to generate cells with different combinations of material and parental chromosomes. Now, for example, in humans with 23 pairs of chromosomes, the number of possible combinations In that is over 8 million different combinations just by the way mom's chromosome and dad's chromosome lines up on that plate. It doesn't even include what can happen in crossing over. This number could go up another couple numbers out here. We could go to nine numbers. We go to billions of different combinations when we include crossing over. When we enter into telophase 1, the nuclear envelope is going to reform. Nuclei are going to reappear. Cytokinesis might occur or it might just go into the second division, getting ready for those four eventual cells. Here's anaphase 1. Homologs have separated. Remember, we still have the two sister chromatids. Telophase 1. nuclear envelope appears cytokinesis is starting and again at the end of the first division we have two cells with different genetic combinations. Now interkinesis this is the time or the gap between the period of time between meiosis 1 and meiosis 2 and when we go into the second division we're not going to do anything with the DNA again we're not going to do We're not going to duplicate it again. Now the phases are going to be similar, but this time we're separating out the sister chromatids. Prophase 2. Cells have one central chromosome from each homologous pair. A spindle appears and the nuclear envelope disassembles again. Each duplicated chromatid attaches to the spindle and that nucleolus is going to disappear again. Metaphase 2, sister chromosomes are, sister chromatids, I'm sorry, are going to line up. Anaphase, those sister chromatids are going to separate. Now they're going to be called daughter chromosomes, and they're going to go to each end of the cell to the poles. Telophase 2, the spindle is going to disappear. Nuclear envelope is going to reform. And then cytokinesis is going to occur, and then we're going to have cell division at that point. point. And so this is meiosis two. Remember we had two cells at the end of that. We have different combinations of genetic information in here. Metaphase two, those chromosomes are going to line up. Antiphase two, the sister chromatids separate. telophase two we're going to reform the nucleus and then at the end we get four daughter cells each one haploid and each one with a different genetic combination so again meiosis produces haploid cells from the diploid ones this is a way we have to do it otherwise fertilization would not work we wouldn't get the same thing In this process, genetic variation produces cells no longer identical to either parental cell. And again, this happens in two ways. We have that crossing over, which happened in prophase I, and then we have the independent assortment during anaphase I of the homologs. Now when fertilization happens, the combination of the chromosomes from genetically different gametes help ensure the offspring are not identical to the parents. And so that's again why we all look a little bit like our parents, or maybe somewhat quite a bit, but we're not identical. The genetic variability is the main advantage of sexual reproduction, and for that long-term genetic variation actually is beneficial. It helps some Species survive different situations. Now meiosis, mitosis. DNA replication occurs only once prior to either one. Meiosis requires two divisions, mitosis one. Meiosis produces four daughter cells, mitosis produces two, which are identical. Those four daughter cells are going to be different. Again, the four daughter cells for meiosis are haploid, and then the two from mitosis are diploid or have a full set of chromosomes. Daughter cells from meiosis are genetically variable, meaning they're going to have different chromosomes, different genes in them, and then the ones from mitosis are identical. They're going to be carbon copies. Again, we're going to start with prophase, metaphase, anaphase, telophase, essentially the same thing is happening. We're condensing the chromosomes in both. We're getting rid of the nuclear envelope in both. Metaphase, we're aligning the chromosomes. Anaphase, we're pulling apart the pairs. It's just the difference in meiosis. We're separating homologs. In mitosis, we're separating the sister chromatids. And then the end result of mitosis again are two identical diploid cells. And then after meiosis two, we get four haploid cells that are genetically different. So meiosis occurs at only certain times of the life cycle in reproduction organisms. This happens in humans right around puberty. In other organisms, it also happens around when the organism is ready to reproduce. They have to be a certain size. They have to be a certain point of maturity. Mammatosis happens from conception, literally the point of fertilization, all the way through the end of your life. You're constantly, again, replacing your skin cells. You're replacing the cells. of your digestive tract and this won't stop until you die and then meiosis only happens when that organism is reproductively ready so homologous chromosomes pair and cross over during prophase 1 of meiosis 1 but not during mitosis this is why we get carbon copies all the time in mitosis here at homolog Homologous chromosomes align at the metaphase plate during metaphase 1. Sister chromatids align at the metaphase plate during mitosis. Homologous chromosomes separate and move to the opposite poles during anaphase 1. And it's the sister chromatids that are going to move to the opposite poles in anaphase of mitosis. Now here's a chart just basically doing the same thing. Meiosis 1 pairing of homologs, mitosis, prophase. We're not going to pair the chromosomes. And I'll let you use this as kind of a way to kind of condense and study for that material that we just covered. Same thing, moving into meiosis 2 and comparing it to mitosis. And then that is going to complete chapter 5.

And then, of course, give you a lot of insight into reproduction and growth. and how the body actually helps to protect itself. So cell division allows a single-celled fertilized egg to grow into an organism with trillions upon trillions of cells. And again, this starts with one cell.

And then we get something as complex as our cells, a redwood tree, a whale, your pet dog. It's all the same thing. Somatic cells. Somatic cells are often called whole body cells. And When we do the first part of this, when we do mitosis, we're talking about these somatic cells.

This is every cell that you can see on the outside of your body. The skin, the cells that make up your eyes, your lungs, even a lot of the internal stuff are all somatic cells. Now, a lot of these cells actually continue to divide constantly as we live, and then some do not.

For example, there are portions of your nervous system that once those cells mature, they will not divide again. That's why damage to the nervous system is so severe in some cases. But blood cells, blood cells are constantly replicating themselves.

Skin cells and cells that line the digestive tract and your respiratory tracts constantly have to replace themselves because they're constantly being abraded. Blood cells, red blood cells might last 120 days, but you've got to replace those. And so the cell cycle is responsible for that. Apoptosis. Now apoptosis is programmed cell death.

Now this might be a timed event where an internal timer is going to go off and then once that cell has reached that time it will simply disintegrate and essentially die. But also it's also a way for the body to check itself. If you have a mutation in a cell, a cell that might become cancerous, apoptosis is one way the body can actually deal with those.

Some examples of programmed cell death, if you've ever had the fortune to have a pond or been by a lake or stream and seen tadpoles. Tadpoles are the young of frogs and toads and a tadpole very young in age has a tail so it can swim through the water. Frogs and toads don't have that and so this is one way that helps the transformation of this organism.

And believe it or not, when you're young, when you're in the uterus, you do have a little bit of skin between all of your fingers and toes. And normally that goes away again by apoptosis. Now the cell cycle is how we're going to organize what a cell goes through on its journey to division. And just like any cycle, we have to do one thing first, then we're going to do the next and then the next.

And then eventually we're going to get two copies. And so the result are daughter cells. Now, gender is not implied here.

They're not all female cells. But what these are are the offspring of the cells that came before them, literally two new cells. So the cell cycle, we're going to break it down into two primary stages.

We have interphase and we have mitosis or the mitotic phase. Interphase is in between the actual division, and it can be divided into three stages. We have G1. G1 is the first stage after the cell has divided from a previous mitotic event. And so the cell is going to grow, it's going to mature itself, it's going to make sure it has all of its organelles.

Now once a cell is on its way to division, it's going to enter the S phase. First thing that cell is going to do is replicate its DNA. It's going to replicate its coding information.

It needs to make two copies. Because once this cell divides, you have to put a copy in each one of those cells. Right before division, that cell is going to enter a complete G2. This is the stage that's going to prepare the cell for division.

We're going to get some more proteins. We're going to make some more organelles if necessary. Now after the G2 stage, we have the mitotic stage.

This is broken down into two sub-stages called mitosis. This is the active division of the nucleus. And that's the whole key point is we want to divide the nucleus.

We want to have copies of the DNA so that when we do get two cells, they're essentially a carbon copy of each other. They're going to be identical. Now cytokinesis is going to happen right at the very end before we have two cells, after the nucleus has divided, and this is going to be the final division of the cytoplasm and the organelles that are contained in that cytoplasm. So here's a graphic explaining all of those details. So first stage is the G1 phase.

This is growth. This is what the cell is going to do right after division. And I mentioned some cells that don't go any further. I mentioned the nerve cells, parts of the nerve cells. They will stop right here.

They're not going to go any further. This is how you're going to spend the rest of their lives, literally your life. They're going to stay suspended in this G1 phase.

If the cell is going to divide, it's next going to go into this S phase. This is where it's going to continue to grow, and we're going to replicate the DNA. We're going to make copies of that. G2 is getting ready for division or mitosis. This is where we're going to mix the proteins.

We're going to get things all in order so the cell can divide. Now mitosis we're going to get to, and this is where we're going to spend a bunch of time on the first part of this chapter, but the first phase is going to be prophase. We're going to move into metaphase, anaphase, telophase, and then finally before we separate the cells, we're going to go into cytokinesis, and we're going to get two identical cells. So again, G1, this is where it's going to double its organelles. It's going to grow.

It's going to become a mature cell. And then, as I mentioned before, there's an additional step in here called the G0, such as some nerve cells, some muscle cells. They're not going to divide again.

They're going to just stay the way they are. In fact, some of the nerve cells in your body, if you live to be 80 years old, you're going to have 80-year-old nerve cells. If the cell is going to continue on, it's going to enter into the S phase.

It's essentially going to make a copy of itself and those copies are going to be called sister chromatids. And again gender is not implied. This just indicates that these are exact copies of the DNA that gets copied in the S phase. And these are now called chromatids.

Chromatids are two copies joined together. G2 the cell is getting ready for Mitosis is going to finalize all the proteins in accessory organelles to help it divide. Back to apoptosis, again this is timed cell death, but also it's a way for the body to help kind of clean itself up. It does need chemical signals and most cells are held in check by what are called inhibitors that don't allow the cell to degenerate or kill itself.

unless there's a signal for it to do so. Severe mutation, like it was mentioned before, during growth, the body needs to reorganize itself. So it's going to absorb some of that tissue, and it does do that by apoptosis.

Now, after we've been in interphase, we're going to enter the mitotic phases. Essentially, by definition, mitosis is the division of the nucleus. That's the whole punchline. We're trying to take those two copies of those sister chromatids, and we want to distribute them into two daughter cells, two identical cells. Cytokinesis, this again is the division of the cytoplasm.

Any organelles get divided up here too. And then at the end of this whole cell cycle, you have two daughter cells that are identical to the another cell. and they can go on to divide again if necessary. So very important with mitosis is what we start with is what we get, meaning the number of chromosomes in that cell is going to be the exact same number that we're going to get after mitosis. If you remember, eukaryotic chromosomes are composed of chromatin.

Chromatin is the long nucleic acid informational fibers that once it starts to coil up, are going to be the chromosomes and this is DNA, essentially the coding region for everything that your body is going to do. Also to help keep it together there's a protein called histones and that's going to give DNA its structure. Now in a cell that's not dividing, part of that cell's job is to make copies. It needs information to make proteins or substances and so it's important that it is dispersed or not condensed into those chromosomes while it's working. And so it can be read.

Essentially you can take the information from it. But when it gets ready for division, it's going to be condensed and then these are going to be chromosomes. So each species, and again, it doesn't matter what it is.

It can be the tree in your backyard, it could be your cat, your dog, the fly on the wall, yourself. Each species has a unique chromosome number and composition and mitosis has to maintain that. Because once we divide a cell, we want to make sure it's exactly the same cell.

So some more definitions. When you hear the word diploid, this means that that cell has a complete set of chromosomes. It is also known as 2N, so two pairs of chromosomes.

In the human body, this would equal 46 total or 23 pairs. And important to note, and we'll get back to it in meiosis, 23 pairs, each of these 23, you get one set for mom, you get one set for dad. When you add 23 up, you get a total of 46. Now, haploid number is only one set of chromosomes.

Again, in humans, this is going to be 23. And we're only going to find these types of cells when the body's making human eggs in the females or sperm in the males for reproduction. Again, the whole punchline of mitosis is to divide the nucleus. So we start with a 2N cell, and then we're going to end with two identical 2N cells.

During DNA replication, we're going to duplicate the chromosomes. Remember, that happens in the S phase. Each duplicated chromosome is composed of two sister chromatids.

So these are twins. These are identical copies. They are held together in the middle by a structure called a centromere.

And that centromere is going to hold them together until about the middle of mitosis. So remember, sister chromatids are genetically identical. They have exactly the same information.

And then after mitosis, each one of those sister chromatids is going to end up in different daughter cells. But those daughter cells and the daughter chromosomes are identical. to the cell that it just came from. So here's one chromosome. This is one chromatid.

So this would be in our G1 phase. G2, or I'm sorry, the S phase would be the replication. And then the result of that replication, we're going to have two sister chromatids held together by a centromere, kind of forming this X structure.

And again, to note, this side this side exactly the same information. So in our example, as we go through the rest of mitosis, we're just going to follow four chromosomes. So this is a very simplistic cell. We also remember that the centriole is present only in animal cells, and so it is important to mitosis in animal cells. So we're going to follow four chromosomes.

This is a somatic cell. It hasn't divided. And so it is a diploid cell.

with a total of four chromosomes. If we assign mom the blue color, or I'm sorry let's assign dad the blue color, I guess it just doesn't matter. So we have the blue chromosomes, we have the red chromosomes. Remember red comes from one parent, blue comes from the other parent.

Again in that S phase during interphase we're going to make identical chromatids. We still have four chromosomes just like we have here it's just now that we have copies attached to each other still have the 2n equals 4 2n equals 4. we enter mitosis and again the whole punchline is mitosis is to divide the nucleus and during that division we get exactly the same four chromosomes in each cell that we started with up here at the top So now let's just kind of go through and highlight the important things in each step of mitosis. So again, mitosis, the whole point is to form two daughter nuclei, exactly the same number of chromosomes and the same kind of chromosomes.

The daughter cell, the result of mitosis, is going to be genetically identical to the parental cell. And a structure called the spindle is going to help us move those chromosomes around. Until we have division.

So we're going to start with prophase. To move around the chromosomes, the nuclear membrane is going to disappear. This is going to allow those chromosomes to move through the cytoplasm and eventually into the two different cells.

Proteins called spindle fibers are going to appear. These are going to help move the chromosomes in the cell. The chromatome that was in the nucleus is going to condense and now we're going to have chromosomes. And with a good microscope, these are actually visible through that microscope.

When DNA is in chromatin, you can't see it. It just looks kind of hazy inside of the nucleus. So each of those chromosomes has two sister chromatids. Remember, those are carbon copies. Sister mirrors are going to attach to those spindle fibers.

And then eventually, we're going to move those chromosomes throughout the cell. Here's a micrograph of an animal cell. This is early prophase. We can start to see the condensation of the chromosomes. And then the asters, these are the spindle fibers.

This is the complexes fibers along with the centrioles that are going to help move those chromosomes around. We get into prophase, the middle of prophase. We see that we have chromosomes. You can really distinguish each one of these apart if you look close. The other noted thing that happens in prophase is this nuclear envelope disappears.

We have to get rid of the nuclear envelope because the chromosomes are much too large to go through those nuclear pores. And so in prophase we'll get rid of that. Now towards the end of prophase, before we go into the next one, those spindle fibers are starting to attach to the central mirrors and they're going to start to push and pull the chromosomes. within the cell.

Metaphase. When you hear metaphase, think middle because that's what's going to happen to the chromosome. They're going to line up at a structure called the metaphase plate which is essentially the middle of the cell.

Most animal cells are roundish and so we can actually assign two different poles. Kind of like the poles on the earth, you have a north pole, you have a south pole. That's where eventually the chromosomes are going to be pulled toward. But right now in metaphase, we're going to put them on the metaphase plate or the equator, if you will, of that cell.

Like we have the equator of the Earth right in the middle. After metaphase, we have anaphase. The centromeres are going to divide and then the sister chromosomes are going to separate and then move to the opposite poles or the opposite sides of the cell. Canita core spindles.

polar spindle fibers. These are going to shorten. They're going to pull and push until those chromosomes are at the opposite ends.

So metaphase, everything lines up in the middle. Anaphase, we're going to pull those sister chromatids apart and start to move them to the opposite poles. Now telophase, separation of the sister chromatids is complete.

And we're going to start making the individual nuclear envelopes around those chromosomes. Spindle fibers are going to disappear. And then after that, we're going to enter cytokinesis, where the cell eventually will divide. In telophase, the chromosomes are going to go back down into chromatin.

They're going to unwind. And then the nucleolus, which also disappeared in prophase, is going to reappear. in each one of the daughter cells. So let's look at that again.

We have animal cells in a micrograph and then we have our cartoons and we're still in our example just following these four chromosomes. Metaphase, everything's on the middle and then we have a pull at each end. Now these spindle fibers are gonna start to pull and push until those sister Chromatids are now being pulled apart and now they're going to be dragged towards each end of the pole.

So this is anaphase, separation of those sister chromatids. Telophase, all of the chromosomes are now at the poles. We're going to have reformation of the nuclear envelope and also the nucleolus is going to reform as well. And then in telophase, we're going to start cytokinesis.

Now mitosis in plant cells is very very similar except for cytokinesis. What's the difference again between plant cells and animal cells? Somebody evidently just said it.

It is a cell wall. Cell wall we have to get around that especially during cytokinesis and also remember plant cells do not have any centrioles. They don't have those structures. That was one of the definitions that we saw in a previous chapter. So here's a series of micrographs.

Here's a plant cell in prophase. I have chromosomes. They're still in the nuclear region, but we don't see a nuclear envelope. Metaphase, they've more or less ended up on the middle of that cell.

Antiphase, the sister chromatids are now being pulled apart, going to the opposite poles. And then in telophase, the nucleus is beginning to reform. And then something called a cell plate is now forming between those two new nuclear regions. This is eventually going to become the new cell wall that we find in plant cells. So this is the start of cytokinesis in plant cells.

Now in animal cells there's no cell wall and so after the nuclei are reformed something called a cleavage furrow is going to form. And essentially what this is, is there's a little fiber that wraps around the cell that becomes tighter and tighter and tighter. It's kind of like closing a bag that has a drawstring on it until it's fully closed.

So here's actually a scanning electron micrograph showing the outside of two animal cells. Cleavatero is right here. It's a nice division.

This one's actually almost done. There's that little protein. You can almost see it as the little line in between them. And it's going to keep contracting until literally this cell pinches itself apart. So cytokinesis in plant cells, again, we have to first have a new cell wall.

It starts by originating from a flattened small disc that appears between the daughter cells. This is known as that cell plate that I showed you on a previous slide. Vesicles coming from the Golgi are going to bring new cell wall material in and add it until that cell wall reaches each side.

And then other vesicles are going to bring across plasma membrane material and put it on each side of the cell wall. Here's a transmission micrograph. new cell membrane this is the cell plate that's forming between the new cells now meiosis so meiosis is for reproduction and again this happens in humans it happens in flowers crickets flies anything that sexually reproduces is going to go through this process now the difference between mitosis amiosis is at the end we're going to have a reduction in the chromosome number in half and again this is important because when a male and a female if anything come together each one is going to contribute an egg the other is going to contribute the sperm and when those join in fertilization you want to come back to the exact same number of chromosomes that you started with because remember we're trying to make more flowers we're trying to make more humans we're trying to make more crickets if you will and so we can't change the number and so for meiosis this is the important outcome for that so it all begins with a diploid parental cell so we have 46 chromosomes in the human so in your reproductive system you have diploid cells that are going to do this and eventually become reproductive cells To do that, we're actually going to go through two different cell divisions. And then the punchline is we're going to end with four haploid cells.

Remember, haploid is half the number of chromosomes. And again, daughter cells, this doesn't imply gender, but these are just offspring of that original parental cell. It involves pairs of chromosomes called homologs.

Now, homologue... is a chromosome that you got from dad, one from mom, and they're going to pair up during various stages of meiosis. So again, homologs, this is a pair of chromosomes, one from mom, one from dad.

During meiosis one, they're going to line up at the middle. This is called synapsis. When homologs separate, each daughter cell receives one member of the pair.

So either mom's chromosome or dad's chromosomes. And then at the end of meiosis one, we have two haploid cells. Meiosis two...

No replication of DNAK is going to occur between meiosis I and meiosis II. So we're not making copies anymore. We're just going to separate the sister chromatids that were formed in the S phase. So the centromeres are going to divide, and sister chromatids are going to migrate to the opposite poles to become individual chromosomes.

And so at the end of meiosis II, we're going to have four daughter cells. each with the haploid number of chromosomes again. So let's look at an overview and then we'll go through the steps. So we start with a diploid cell.

Remember 2N is diploid and we're only going to follow again four chromosomes. Believe me, you don't want to follow 46. One pair from mom, one pair from dad. meiosis one they're going to form homologs meaning moms and dads chromosomes that are identical are going to line up they're going to form that homolog again mom and dad's joining together exactly the same chromatone lining up at the end of meiosis one we're going to separate the homologs and so you're going to get one chromosome you from dad and then one chromosome from mom in each one of these cells.

They're not going to travel together, but notice we still have the sister chromatids. We still have the carbon copies of these chromosomes. Meiosis 2, we're going to separate the sister chromatids and then we still end up with two chromosomes in each of the four cells.

So we start with a diploid number. And as we enter meiosis one, we're going to go and exit with a haploid number. Four chromosomes here, four here, four here.

Now we're at two, so this is haploid, this one's haploid. We divide the sisters, and we still have two chromosomes in each one of these. And so, fertilization. These are daughter cells as a result of meiosis are going to mature into gametes. Again, sperm for the males and eggs for the females.

And the act of fertilization is fertilization or the fusing of their two nuclei. This is going to restore the diploid number. And again, this is important because if we breed a couple dogs, we want to get dogs back.

And so we don't want to increase the number of chromosomes. We don't want to reduce it. We have to get exactly the number we started with to get dogs out of that development.

And so again, meiosis requires two nuclear divisions. It's going to result in four daughter nuclei, and then each of those nuclei are going to have half the number of chromosomes as the parental cell. Meiosis I is divided into four phases. prophase 1, metaphase, anaphase, telophase.

And just like we saw in mitosis, the chromosomes are going to be doing very similar things, except with a couple exceptions that I'm going to point out as we go through this. One thing that meiosis does is it helps ensure genetic variation. And so you're a result of your parents, but you don't look exactly like either one of your parents.

Probably you can see traits. from both your parents in yourself and just when you have kids your kids are going to be a result of a union with your with your spouse and so you're going to see genetic variation in your children too and through meosis this is how this is accomplished first way something called crossing over that i'll illustrate with a graphic but this is the sharing of genetic information between two chromosomes and then they divide so you get something unique and then independent assortment is something that happens during cell division because there's no specific way that the homologs have to line up and so we can get a lot of variation out of that as well So let's look at that and we'll come back to those two other points in a minute. So first thing that happens, DNA replication is going to take place.

We're going to have our sister chromatids form. We're entering to prophase 1. Homologs are going to line up. In metaphase 1, anaphase, the homologs are going to be separated.

We're going to join into telophase 1 where we're going to rebuild the nuclear envelope and then cytokinesis is going to happen and two chromosomes here two chromosomes here again we started with four right here we're going to separate those homologs and eventually we're going to have a single set of chromosomes in each one we're going to have a haploid cell and then two haploid soils are the result of the first division so again prophase one synapses occurs these are where the homologs are going to come together membrane is going to break down around the nucleus spindle fibers are going to appear and then we're going to start moving the homologs to the center now it's in pro phase one when these homologs are lining up that we're going to have crossing over and I'm going to show you a graphic in a second how that works. But after crossing over the chromatids held together are no longer identical. We've actually made a brand new combination.

One of them is going to have the original genetic combination. The other one is going to have something called recombine where we've changed the genetic material. So again this happens in Pro Phase 1. Synapse.

are when the chromosomes again come together. Again one from mom, one from dad. They're going to line up and pair up. Now sometimes the individual chromatids will cross over when they come into close contact.

If that happens and if these two pairs stick, there can be an exchange between moms and dads resulting in a brand new combination on each one of these chromosomes. This is what we started with. This is all blue. This is all that reddish orange color.

But after crossing over, this is different. The genetic information is going to be different on each one of these chromosomes. So that's going to lead to genetic variation.

And then that's going to be carried through right to the end of meiosis all the way to the end. Metaphase 1, anaphase 1, homolog pairs are going to line up the metaphase plate, again the equator. And this is where we're going to have independent assortment.

It doesn't matter what side of the cell that mom's chromosome or dad's chromosome is facing. Moms could all face left. Dads could all face right. But it's random. So maybe 50% and 50%, maybe 25 and 25. And so that independent assortment is also going to add toward diversity.

So let's talk about that just for a second. Let's see, these are our homologues. And in this example, what they have is all of moms lining up, going to the left.

All of dads are going to go to the right. And this is our result. But there's nothing to say that all the blues can be here, all the reds can be here. And then there's nothing to say that they all have to be on the same side. And so we could have this combination where we actually get two.

different combinations out of that. That's independent assortment. There's no rule saying how these have to line up.

This is going to be differently genetically because mom's chromosomes, dad's chromosomes, they're different. They look different. And so it's going to have different information and then we're going to mix it up here too. You add in crossing over, we can really have some different combinations. So again, we're going to replicate the DNA.

We've got the sister chromatids. Prophase 1, remember we're going to lose the nuclear envelope. All this material is going to condense into chromosomes.

Metaphase 1, we're going to have everybody lining up in the center. And then in this example, you can see that there's going to be some swapping. We also have some examples of crossing over.

And so that when we get to the end of this, each four of these cells is going to be genetically different from one another. So the first division, metaphase, anaphase one, homologs. align at the metaphase plates.

And again, it doesn't matter how they align. Independent assortment tells us that moms and dads don't have to be on the same side. Independent assortment is going to generate cells with different combinations of material and parental chromosomes. Now, for example, in humans with 23 pairs of chromosomes, the number of possible combinations In that is over 8 million different combinations just by the way mom's chromosome and dad's chromosome lines up on that plate. It doesn't even include what can happen in crossing over.

This number could go up another couple numbers out here. We could go to nine numbers. We go to billions of different combinations when we include crossing over. When we enter into telophase 1, the nuclear envelope is going to reform. Nuclei are going to reappear.

Cytokinesis might occur or it might just go into the second division, getting ready for those four eventual cells. Here's anaphase 1. Homologs have separated. Remember, we still have the two sister chromatids.

Telophase 1. nuclear envelope appears cytokinesis is starting and again at the end of the first division we have two cells with different genetic combinations. Now interkinesis this is the time or the gap between the period of time between meiosis 1 and meiosis 2 and when we go into the second division we're not going to do anything with the DNA again we're not going to do We're not going to duplicate it again. Now the phases are going to be similar, but this time we're separating out the sister chromatids. Prophase 2. Cells have one central chromosome from each homologous pair. A spindle appears and the nuclear envelope disassembles again.

Each duplicated chromatid attaches to the spindle and that nucleolus is going to disappear again. Metaphase 2, sister chromosomes are, sister chromatids, I'm sorry, are going to line up. Anaphase, those sister chromatids are going to separate.

Now they're going to be called daughter chromosomes, and they're going to go to each end of the cell to the poles. Telophase 2, the spindle is going to disappear. Nuclear envelope is going to reform.

And then cytokinesis is going to occur, and then we're going to have cell division at that point. point. And so this is meiosis two.

Remember we had two cells at the end of that. We have different combinations of genetic information in here. Metaphase two, those chromosomes are going to line up. Antiphase two, the sister chromatids separate. telophase two we're going to reform the nucleus and then at the end we get four daughter cells each one haploid and each one with a different genetic combination so again meiosis produces haploid cells from the diploid ones this is a way we have to do it otherwise fertilization would not work we wouldn't get the same thing In this process, genetic variation produces cells no longer identical to either parental cell.

And again, this happens in two ways. We have that crossing over, which happened in prophase I, and then we have the independent assortment during anaphase I of the homologs. Now when fertilization happens, the combination of the chromosomes from genetically different gametes help ensure the offspring are not identical to the parents. And so that's again why we all look a little bit like our parents, or maybe somewhat quite a bit, but we're not identical.

The genetic variability is the main advantage of sexual reproduction, and for that long-term genetic variation actually is beneficial. It helps some Species survive different situations. Now meiosis, mitosis.

DNA replication occurs only once prior to either one. Meiosis requires two divisions, mitosis one. Meiosis produces four daughter cells, mitosis produces two, which are identical.

Those four daughter cells are going to be different. Again, the four daughter cells for meiosis are haploid, and then the two from mitosis are diploid or have a full set of chromosomes. Daughter cells from meiosis are genetically variable, meaning they're going to have different chromosomes, different genes in them, and then the ones from mitosis are identical.

They're going to be carbon copies. Again, we're going to start with prophase, metaphase, anaphase, telophase, essentially the same thing is happening. We're condensing the chromosomes in both. We're getting rid of the nuclear envelope in both. Metaphase, we're aligning the chromosomes.

Anaphase, we're pulling apart the pairs. It's just the difference in meiosis. We're separating homologs. In mitosis, we're separating the sister chromatids.

And then the end result of mitosis again are two identical diploid cells. And then after meiosis two, we get four haploid cells that are genetically different. So meiosis occurs at only certain times of the life cycle in reproduction organisms. This happens in humans right around puberty.

In other organisms, it also happens around when the organism is ready to reproduce. They have to be a certain size. They have to be a certain point of maturity. Mammatosis happens from conception, literally the point of fertilization, all the way through the end of your life. You're constantly, again, replacing your skin cells.

You're replacing the cells. of your digestive tract and this won't stop until you die and then meiosis only happens when that organism is reproductively ready so homologous chromosomes pair and cross over during prophase 1 of meiosis 1 but not during mitosis this is why we get carbon copies all the time in mitosis here at homolog Homologous chromosomes align at the metaphase plate during metaphase 1. Sister chromatids align at the metaphase plate during mitosis. Homologous chromosomes separate and move to the opposite poles during anaphase 1. And it's the sister chromatids that are going to move to the opposite poles in anaphase of mitosis. Now here's a chart just basically doing the same thing.

Meiosis 1 pairing of homologs, mitosis, prophase. We're not going to pair the chromosomes. And I'll let you use this as kind of a way to kind of condense and study for that material that we just covered.

Same thing, moving into meiosis 2 and comparing it to mitosis. And then that is going to complete chapter 5.

Transcript for:Understanding Cell Division Processes

Transcript for:
Understanding Cell Division Processes