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Video transcript

in a previous video we talked about interphase which is the bulk of a cell's lifecycle as it grows and its DNA replicates and it grows some more and now we're going to talk about the actual cell division we're going to talk about mitosis and if we wanted to be precise mitosis is the process by which this one nucleus will turn into two nuclei that each have the original genetic information now as we exit mitosis we get into cytokinesis which will then split these two nuclei into or they'll put each of the nuclei into a separate cell when we split the cytoplasm right over here when we split the or or the the cell essentially turns into two cells but let's see how all of this let's see how all of this happens so the first phase and I'll leave the end of interphase right over here we have this big cell our DNA has been our DNA has been replicated we have two centrosomes right over here the first phase of of mitosis involves the cell and I might draw it a little bit smaller just so I have enough space here so involved so this is the cell right over here so we're going to go to this phase right over here and a few things start happening one the DNA the chromosomes go from being in there chromatin form where they're all spread out to kind of a more condensed form that you can actually see from a light microscope so for example that magenta chromosome which is now made up of two sister chromatids after replication we talked about that in the interphase video it might look something like this in a it might look something like this if you were to look in a in a microscope it wouldn't it's unlikely to be magenta but it's going to have kind of that classic chromosome shape that you're used to seeing in textbooks and it has the centromere that connects these two sister chromatids right now both of this both of these two sister chromatids combined are considered to be one chromosome even though before replication it was still considered the magenta stuff was still considered to be one chromosome and we could draw the blue chromosome once again it's now in the condensed form that's one sister chromatid right over there that's another sister chromatid they are connected at the centromere at the centromere so they're condensing now as we enter into mitosis and the nuclear membrane the nuclear membrane starts to starts to go away so the nuclear membrane is starting to go away and these two centrosomes start to migrate to opposite opposite sides of the cell so one of them is going over here and one of them's maybe going to go over here so they're migrating migrating to opposite sides of the cell and it's pretty incredible you know when we it's easy to say oh this happens and then that happens but remember this this cell doesn't have a brain this is all happening through chemical and thermodynamic reactions and the ways we you know just based on certain triggers of where the cell is in its lifecycle it's amazing that this is happening it's amazing that the structures and what sometimes we consider to be a simple thing but this is actually incredibly complex thing is actually it kind of quote-unquote knows what to do even though it has no intelligence here and a lot of what I'm talking about the general overview of the process is well understood but scientists are still understanding exactly how do the different things happen when they should happen and by what mechanism and what's actually happening sometimes in a molecular or atomic basis but anyway this first phase of mitosis the nuclear envelope the nuclear membrane starts to disappear the centrosomes migrate to the opposite ends of the cell and our DNA condenses into kind of the condensed form of the chromosomes we call this prophase prophase prophase of mitosis now in the next phase in the next phase let me draw my cell again during that same green color in the next phase your your cell your nuclear membrane is now is now gone and the chromosomes start lining up in the middle of the cells so you have the blue one right over here the blue one that's one sister chromatid here's another sister chromatid and they are connected at the centromere not to be confused with a centrosome and then here's the magenta one of the magenta sister chromatids and another one and once again it's not magenta in real life I'm just making it in magenta because it's it looks nice that's the centromere right over there our centrosomes our centrosomes are at opposite ends of the cell at this point at opposite ends of the cell and you might have heard the word let me let me label this again I labeled it in a previous video that's centrosomes centrosomes where the two sister chromatids are connected that's a centromere centromere now you might have heard the word centriole centrioles are actually they help they are exist inside the centrosomes there are these two kind of cylindrical looking structures each of the centrosomes have two centrioles but for the sake of this video you could say well the centrioles are just part of the centrosomes but i'm just listing you all the words that involve century something centrioles right over there and you have two of them per centrosome so hopefully that that helps clarify some confusion but these things line up and a lot of what you're about to see this orchestration these things moving around and the cell things lining up and soon things pulling apart these are all coordinated with actually a fairly sophisticated mechanism of kind of a scaffold of these kind of ropes these these microtubules and the centrosomes role until now I've just been kind of drawing them you're like well what do they do well the centrosomes role is these microtubules extend from them to each other and to the centromeres and to the centromeres of of these chromosomes and to a large degree although they're not the only actors here they help pull and push things in the right way so these help keep make sure that the two centrosomes push away from each other and then well as we'll see in the next phase that they're able to pull one of each of the sister chromatids four from each of the pairs towards towards towards each of them so this right over here where you see where where the nuclear envelope is now gone the chromosomes have been lined up just like this and your centrosomes are on opposite sides of your opposite sides of your cell we call this the metaphase we call this the metaphase of mitosis and then you can imagine what's going to happen next what's going to happen next what's going to happen next is let me I don't want to draw it too big because I want to be able to fit it all in one page what's going to happen next is that those microtubules are going to start pulling on each of the sister chromatids let me draw that so so let me drew so you have this centrosome right over here you have all these microtubules in your cell you have this centrosome right over here all those microtubules and this one is going to be is going to be pulling is going to be getting one of the blue chromatids to pull towards it or to move towards it so let me draw that so this is one blue chromatid right over here and this one is going to be pulling the other blue chromatid towards it and same thing for the magenta and same thing for the magenta so that one's being pulled that way and this one is being pulled that way and just in case you're concerned about the the some of some of more of the words the vocabulary involved the point at which these microtubules connect to what used to be sister chromatids but now that they're part we now call them chromosomes this is when it were merged this was one chromosome and they have two sister chromatids but now they're apart we would actually now consider these each and independent chromosome so now you actually have four chromosomes over here this point right over here we call a kinetic or kinetic or even exactly how that interface works and exactly how things move is not fully understood some of this stuff is understood but some of this is still an area of research so something even as basic as cell division not not so basic after all so this right over here where you can start to see DNA kind of migrating to their respective sides of the cell we call this the anaphase we call this the anaphase and then that takes us to the last formal phase of mitosis and that is called telophase talo telophase and in telophase in telophase let me do my best job to draw draw it and you could already see I've started to draw that the cells that the the cellular membrane starting to pinch in kind of in preparation for cytokinesis in preparation for the cell splitting into two cells so I'll do it a little bit a little bit more cytokinesis is usually described as kind of being a separate process than mitosis although it is obviously they are they kind of together help the cell fully turn into two cells so now in telophase so you have this what used to be a sister chromatid now we could call it a chromosome on its own and we have this chromosome and we have and we have this chromosome right over there let me actually do it on both sides so you have there and you have you have this right over here and actually let me draw it a little bit different because at this phase you're really starting to unwind what happened at prophase so prophase you have the disappearance of the nuclear membrane and you have the condensation of the chromosomes into this form here telophase that's that's unwinding a little bit so actually let me draw the two centrosomes so you have one centrosome that same color that i had before so you have one centrosome right here you have another centrosome right over there and now the DNA this blue DNA this chromosome is now here but starting to get an unwind a little bit same thing on this side right over here magenta chromosome is on here but once again starting to get a little bit unwound same thing over here and you start to have nuclear membranes forming forming around the DNA so once again it's kind of its kind of redoing what was undone in prophase undone in prophase and so when you're done you essentially you're going to these nuclear membranes the DNA is going to go back to its chromatin form and then you have cytokinesis and cytokinesis is the process by which this gets fully pinched together and you have two separate cells and some folks will say oh it kind of begins in anaphase and you know it finishes after telophase but it's kind of happening near the end of mitosis in parallel with it so this is cytokinesis cytokinesis cytokinesis right over here this essentially is how the the this larger cell that had two nuclei how this divides fully divides into two into two cells and at that point you're back to you're back to this phase of the cell cycle now each of these now two cells are going to go through interphase g1 s phase replicate their DNA g2 phase grow some more and then go through mitosis again and then these two will turn into four cells anyway hopefully you enjoyed that
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