Bozeman science: Mitosis and meiosis

hi it's mr. Andersen and welcome to biology essentials video 28 this is on the cell cycle mitosis and meiosis in other words how we go from one cell to all the cell's the trillions of cells inside our body meiosis is important because that's how we make sex cells now let me digress a little bit this is a whiptail lizard from the desert Southwest and what's interesting about this is that it's a female lizard and when it went wants to make more lizards it will simply use mitosis to make an exact copy of a cell inside its body it's called parthenogenesis or virgin birth and it will make a brand-new number of baby lizards and they're all females so they don't have males it's rare to not have males it's rare to not have meiosis and the reason why is that all the whiptail lizards are genetically the same but it works and if you live in a fairly stable environment it tends to work now we're not going to go into the specifics of mitosis and meiosis you can look I've got videos on each of those that talks about the different phases what I want to talk about this is in general what a mitosis and meiosis do and how does the cell cycle work and how is it control and so a diplo diploid cell is going to be a typical human cell or a typical cell inside an organism it's going to be 2n and what that means is it has two complete sets of chromosomes so for example in humans 2n equals 46 that means we have 46 pair of chromosomes and so the goal of the cell cycle in mitosis is to make a copy of that cell in other words to make a diploid cell that diploid cell can enter into the cell cycle again make more cells and more cells and more cells and so the way that we make set new cells in our body or replace cells that are damaged is mitosis and meiosis we're going to take a normal diploid cell and we're actually going to make a haploid cell we're gonna make sperm and egg and so in humans and now equals 23 now if we were to just stop there we wouldn't have diploid cells anymore but fertilization where egg meets sperm is going to combine those two cells to make a diploid cell and now that diploid cell can enter into the cell cycle again and so we got kind of two loops we've got the mitosis loop in the meiosis loop mitosis is used to make all the cells in our body meiosis just makes gametes or sex cells I also in this video I want to talk about how we control the creation of of diploid cells that may be better to come from here and how we use cycling's an example of that would be the mitosis promoting factor to control the cell cycle and where it is and where it's headed next now if you're talking about cell cycle the best place to watch is start is with videos of actually cells dividing and so this is a cell undergoing division so we start with one cell and it makes an exact copy of itself now when you're watching it let me go back a second on these first two videos what you don't see is everything that actually happens before the cell makes a copy of itself in other words before the cell is actually able to divide it has to duplicate all of the DNA and all the machinery of the cell this last one we're looking actually inside the cell itself and so the cell division actually has two parts to it part one is going to be the division of the nucleus and we call that in general mitosis and then after we divided the nucleus you'll see the chromosomes actually separate here then we actually have the division of the rest of it and that's called cytokinesis cytokinesis is the the break apart of all the other parts of the cell so the machinery of the cell the mitochondria cytoplasm things like that and so let me see if I can start that so first of all there we go first we have the division of the nucleus you can see the chromosomes pulling apart and then you have cytokinesis or the division of the cell itself and so all mitosis is is one cell forming two cells and those two cells are identical to that first cell and that's how we go from that first fertilized egg inside us to the trillions of cells that we have inside an adult body so when you're replacing cells in your body you're doing it through mitosis so let's look at the cell cycle and and so what happens is you'll have a sow enter into this cell is gonna look like this it's going to enter it in to the cell cycle as one cell we make a little better arrow and then it's gonna eventually exit out as two cells each of those cells remember could go back into the cell cycle and so this is how we make all the cells in our body now I've heard that a lot of the dust in a room actually are dead skin cells and this is what skin looks like so skins our skin is going to be created new cells and then they're going to migrate up to the surface and then we're eventually gonna lose skin cells at the top but we keep replacing those cells and to do that we use the cell cycle so let's look at the parts of the cell cycle if we start first with that cell entering in the first thing that will do is will actually enter into the g1 phase g1 phase the cell is actually going to grow it's going to get bigger and bigger and bigger and bigger we then enter into the S phase during the S phase we're going to actually using DNA replication we're going to copy all of the DNA inside a cell it then goes into the g2 or the growth 2 phase where it continues to grow and gets ready for division of the actual cell and so g1 s and g2 are all part of what's called interphase and if you look at a cell it's generally in interphase it's growing it's copying its DNA it's growing or maybe it's just working it's doing what a normal cell does and so most of the life of a cell is in interphase it's in these 3 g1 s and g2 and the actual copying of the nucleus and copying of the cell this mitotic phase is actually gonna be really really small if it never divides again it'll actually stall out in something called the g0 phase and so we've got cells inside our body you've heard of this maybe cells of the central nervous system cell muscle cells for example that never copy themselves during your whole lifetime they're in what's called the g0 phase or they're just waiting and they're not going to make divisions and so how does a cell know when it's time to divide and when it should go on and when it shouldn't the best analogy I can think of as it works kind of like an hourglass clock and so there are little proteins inside a cell and as those proteins accumulate throughout the life of the cell eventually you get a critical number of these proteins at the bottom and once we have enough of those then it actually tells the cell to advance to the next stage and those proteins are called cycling's and so let's look at what cyclones look like a cyclin I'll represent it here it simply as a protein but if we look here at the g1 phase the S phase the g2 phase and mitosis there's a set of cycling's or a set of these different types of proteins and what they're going to do is they're going to build up and so cyclin a and B are ones that I'm really familiar with cyclin a will actually build up during the S phase and then it'll drop off right as the cell divides as it going into this mitotic phase and so cycling's will actually accumulate and so those are like the sands through an hourglass they're gonna get more and more and more and more cycling's as a cell goes through the cell cycle now the other chemical that I want to talk about is something called c d k c d k is simply a cyclin dependent kinase and the kinase is simply going to be a chemical that can speed up actions within a cell and so CDKs are found in all living organisms and you can actually take C decays from a yeast and put them in our cell and they work just as fine so they show homology through through evolution and so cyclin dependent kinase if you look at their name are simply dependent upon cyclin so I made a little of animation of how that works and so a typical cell in your body is going to have a bunch of CDKs in it or cyclin dependent kinase 'iz and so we could say this is like right here but throughout the life of the cell the cell is going to start building up and accumulating larger amounts of cyclin so the amount of cyclin is going to get larger and larger and larger so eventually what happens is the cyclin is going to fit into the cyclin dependent kinase ha's now we have an activated cdk-cyclin complex what does that mean we have something a protein that's able to do things so now you can think of we've like mustered this army and now the army is ready to do something and so what does it do well we're right here so we're just about to enter to the M phase that we're just about to do mitosis and so what these cyclin dependent kinase is do is they act on the cell itself so they are an a specific type of cdk is called the mitosis promoting factor or MPF and what that does is when it builds up enough of these cyclin dependent kinases they're actually going to work on the cell so one thing they'll do is they'll actually break apart the nucleus so we were able to start dividing that cell another thing that CDKs will do is they'll actually work on the microtubules that build this spindle and so all of these together will work on pushing that cell through into the mitotic phasor to this next start step of the cell cycle the neat thing about each of these is that after they've actually done that they'll actually gobble themselves up they'll disappear and then the whole cycle begins over again okay so if we kind of talk a big picture about what happens in the cell cycle a typical cell right here is going to be let's say this is a typical cell typical cell right here is actually going to be 2n its diploid you have one chromosome from mom one chromosome from dad so in this case it's going to be 2n equals two it's then going to duplicate itself so during the S phase it's going to make copies of itself and so at this point right here we'd actually have a four and so it's made copies of that and at this point we can either take the path of mitosis or the path of meiosis and so in the past path of mitosis that'll simply split in half and now we'll have to 2n cells and if you look at these two two cells to 2n cells they're exactly the same as that first cell so this is what's happening to the chromosomes if we look at that for set for n cell and as it goes into mitosis it'll actually line up it'll split in half and then it'll split in half again and so what you have is actually n cells those are called haploid cells and we have four of those in meiosis and so the cell cycle will take a diploid cell make two diploid cells in mitosis or make four haploid cells in meiosis got a little messy so let's look at it in a little bit more detail so mitosis is how we replace cells in the body so cells that are broken cells that have broken down cells that we need to replace we do that through mitosis or when we want to grow how do we go from a very small organism to a very large organism it's just making more cells and so this would be a real to make it simple we're gonna make start with a simple cell this is going to be a 2n it's diploid but 2n equals 2 so this would be that first parent cell it will then duplicate all of the DNA so now we have this characteristic shape it has two sides to it and this side and that side are copies that's what happened during the S phase they'll then meet in the middle they'll divide in half so this would be our mitosis phase and now we have two diploid cells and if you look at those cells they're exactly the same as that original cell and each of those are 2n equals 2 so that's mitosis what happens to these 2n cells will they can enter into the cell cycle again and it goes over and over and over again in meiosis what happens is a little more detailed remember meiosis only deals with making sex cells from in reproduction and so in this case to make it to show you my O società increase the number of cells in this case or chromosomes 2n equals 4 in other words we have two chromosomes from mom two chromosomes from dad so they'll copy themselves so now we actually have a 4n cell at this point but since we're doing meiosis this is we're actually crossing over occurs and so parts of this chromosome will swap places with this chromosome and vice versa what that gives us is variability in all of the sperm and the egg they'll divide in half now we have 2n cells and then eventually we'll have n cells equals two and so we started with 2n equals four now we have N equals two each of these four things in a male become a sperm and in a female one of them they'll become an egg and the other ones they'll actually form what are called polar bodies and so what do we get after meiosis well we get sperm and we get egg and we can then go from these original 2n equals to 2 eventually or n equals 2 2 a 2 N equals 4 zygote and so now we're back to a diploid cell and that diploid cell could enter into the cell cycle this is a zygote to make more cells and eventually make sex cells to make the next generation so that cell cycle mitosis meiosis and I hope that's helpful