If we're talking about cell division, at some point, the cell should actually divide. But so far, all mitosis has done is produce a cell with two nuclei. Cytokinesis is the process in which the cytoplasmic content of the cell is divided into two new daughter cells. While it's easier to consider the events of cytokinesis separately from mitosis, in actuality, there is some overlap between the two. Luckily for us, the mechanisms that govern cytokinesis are far less complicated than those of mitosis.
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Mitosis was all about ending up with a precise number of chromosomes. On the other hand, division of the rest of the contents of the cell is far less precise. Having an unequal number of chromosomes in the resulting daughter cells would be a bad thing, but having an unequal number of cytoplasmic content, like ribosomes or mitochondria, isn't. The total number of organelles, proteins and other molecules floating in the cytoplasm is so big that even an arbitrary division of the cytoplasmic content in the cell gives each daughter cell enough functionality to survive.
For instance, if a dividing human cell had 1000 mitochondria, a division of 450 in one cell and 550 in another cell still means that both cells can produce enough energy, even though one daughter cell still has ten percent less energy producing capacity than its mother had. So, how do we turn one cell into two cells? It turns out that the strategy used for animals cells is different from what plant cells do, so let's see what type of strategy each cell employs.
We talked about how animal cells are like water balloons. The cytokinesis strategy employed by animal cells is reminiscent of this fact. As mitosis is ending, cytokinesis begins to divide the cell and its contents into two new daughter cells. This basically looks like you looped a string around the outside of a water balloon and started to constrict it like a drawstring. Obviously, the real-life example doesn't end up so well for us, since we end up with a face full of water. Luckily, what happens for the cell isn't quite as messy.
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During cytokinesis, the role of the string is played by the contractile ring. Now, the contractile ring is composed of actin filaments that are assembled just below the cell membrane. A protein called myosin works with the assembled filaments to constrict the ring until the cell membrane touches in the middle. Once those membranes pinch together, they fuse, and the result is two new cells. Each cell has one nucleus, compliments of mitosis, and each cell has about half of the cytoplasmic content, compliments of cytokinesis.
So, why do plants utilize a different cytokinetic strategy? It's not just that they're trying to be different. Remember that plant cells also have a cell wall. Since that cell wall won't bend, a contractile ring isn't going to work. But nature's nothing if not creative. Remember that the Golgi apparatus is an organelle that plays a major role in the secretion of molecules from the cell. It accomplishes this by secreting tiny, membrane-like vehicles called vesicles. A vesicle containing a molecule destined for secretion buds off from the Golgi apparatus and heads for the cell membrane. Since it's made of phospholipids just like a cell membrane, the vesicle can fuse with the cell membrane so the molecules can be released.
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This reveals a way that the plant cell can form a cell membrane without a contractile ring. In a plant cell, the new membrane is formed by vesicles, which fuse together in the middle to form what's called a cell plate. As the cell plate expands to the outer reaches of the cell, it can fuse with the cell membrane and form two new cells in the process.
We've solved one problem, but we've just created another: we have two cells, but they're stuck in the same cell wall enclosure. Remember how we said that the Golgi apparatus is useful in transporting molecules outside of the cell? Well, what if we packed some cell wall ingredients in the vesicles used to make the cell plate? If we do that, once the cell membrane is formed, these materials in the vesicles can secrete into a new cell wall, and, in this way, the two plant daughter cells can separate.
Cytokinesis is the process in which the cytoplasmic content of the cell is divided into two new daughter cells. The mechanism by which cytokinesis is achieved differs between animal and plant cells.
A contractile ring is a ring of actin filaments below the cell membrane that constricts the middle of a dividing animal cell to complete cell division.
The cell plate is the area where the new cell membrane and cell wall form between dividing plant cells during cytokinesis.
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