Hi. In this video, we're going to be talking about cytokinesis. So cytokinesis is kind of the last step or maybe right after the enphase, where it actually will cleave the cytoplasm into 2 separate cells. So there are a few things that we need to know about this. The first thing is that the mitotic spindle is disassembling, so it doesn't really play that big of a role. But before it starts disassembling, what it does is it actually positions this complex called the cleavage furrow. The cleavage furrow's position in the cell, which is where the cell is going to divide, depends on positioning by the mitotic spindle. The cleavage furrow is kind of just this puckering of the plasma membrane where it's going to split in 2. So if I were to draw it real fast, the cleavage furrow is here, where the cells have puckered in. And that positioning where that is divided depends on the mitotic spindle. This is super important because we don't want it to pucker here and there because you may be missing some organelles or some chromosomes. The way you want it dividing is exactly where it's supposed to be, so each daughter cell gets half of everything. This positioning is super important. Most of the time, this is going to be symmetrical division to ensure that both daughter cells get exactly half of everything that's been replicated. They get half of the replicated chromosomes, half of the replicated organelles. But sometimes, this can actually be asymmetrical, especially when the cell needs to develop into a different type of cell type. Maybe the cell type needs many more mitochondria than the other. So, most of the time, it'll be symmetrical cleavage, but sometimes it won't. But either way, the positioning is super important.
Another factor of cytokinesis is the contractile ring. This begins forming during anaphase. It's made of actin and myosin, and it works to put this force on the plasma membrane, assisting in the actual cutting of this. So, eventually, that pressure has to actually separate the two cells, so the contractile ring contracts and actually provides that pressure so that the cells will split into 2. There's an important protein here called Rho A. It's a GTPase. Remember, it's going to hydrolyze GTP, and it's super important in triggering the contractile ring formation. This is all super important in dividing cells.
But, when we talk about plant cells, which we haven't really mentioned a lot, so we're just going to mention briefly. Plant cells have an extra step because they need to create a new cell wall after division. So, how they do that is there's this structure called the phragmoplast, and it is formed by microtubules as well. Microtubules are super important in cell division. And it helps assemble the new cell wall. So what it does is it forms a structure called the cell plate, and this is a cell wall precursor. This will eventually become the cell wall, but it's just so simple, and it's a precursor, so it hasn't formed yet. And it forms inside the cell, and, eventually, as the cell matures and completely divides, it will form a real cell wall. So here, we get 2 cells that are about to divide, but what you get here is this contractile ring made up of actin and myosin that comes and contracts this area here, provides that pressure, and will split the cell into 2. And notice here that this is actually symmetrical division. Everything in this cell is present also in this one. So super important. So with that, let's now move on.
