Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson on mitosis. Today, we are going to be discussing the processes that happen during mitosis. Mitosis is a part of the cell cycle. And the cell cycle describes events that happen from the time a cell is formed until it divides. So it includes mitosis as a part of the cell cycle.
Mitosis is a type of cell division that happens in somatic cells. Somatic cells are our body cells, so all the cells in your body except for sex cells. The process of mitosis actually produces new cells.
Cells are constantly going through the cell cycle and producing new cells. As certain cells grow old and die, they need to be replaced by new ones. The process of mitosis helps to do that.
Mitosis actually includes four phases, which we are going to be discussing today. But before we discuss that, we are going to discuss the first part of the cell cycle, which is called interphase. Interphase is the first part of the cell cycle, but it's not considered to be a part of mitosis. Because interphase is a part of the cell cycle where the cell is getting ready to divide but it's not dividing yet.
Interphase is the first part of the cell cycle. And it's actually the longest phase of the cell cycle. So this is where the cell spends most of its life and, actually, all of its life when it's not dividing. So any time the cell is not dividing, it's an interphase.
There are actually three subphases to interphase. And those are G1, S, and G2. The G1 part of interphase is when the cell will start to increase in size and grow in preparation for cell division. During the S phase, the DNA is copied and chromosomes are duplicated, again in preparation for division. And then in the G2 phase of interphase, the cell is basically making its final preparations in order to get ready to divide.
Normally in interphase, chromosomes are not visible. Genetic information, or DNA, is found in the form of chromatin, which is kind of like this thread-like ball of yarn that's found within the nucleus. But as a cell is preparing to divide, that DNA will then condense into these condensed chromosomes. And then those will be copied-- chromosomes are duplicated-- in preparation for division.
I'm going to give you a quick overview on the structure of chromosomes before we talk about the phases of mitosis. We're going to take a look at this diagram, right here.
OK. So here, we have a structure of a chromosome. These, here, are called sister chromatids. As a whole, this is a chromosome, but each of these individual parts of the chromosome is called a sister chromatid. Sister chromatids are attached at this pinched-in point in the middle, here, called a centromere. Just so you have a little bit of an overview on the structure of chromosomes, as we move through these phases of mitosis.
The first phase of mitosis then is prophase. We're actually starting mitosis with prophase. It's the first step in mitosis. And during this phase, the nuclear envelope that normally surrounds the genetic information will start to break down.
At this point, the chromosomes then are condensed, so they're visible. And centrioles, which are organelles in the cell that play a role in cell division, will start to move towards opposite poles. I'm going to zoom in here, just so you can see what's going on a little bit more. The green organelles, here, are our centrioles. So they'll begin to move towards opposite poles of the cell.
Our next phase, after prophase, comes metaphase. Metaphase is our second phase of cell division. And during metaphase, the chromosomes will line up on the metaphase plate, which is kind of this invisible line in the middle of the cell. So they're lining up in the middle of the cell.
The way that I always remember metaphase is that metaphase and middle both start with the letter, m. So you can remember what's happening in metaphase-- the chromosomes are lining up in the middle of the cell on the metaphase plate. And then spindle fibers are attached to the centromeres of the chromosomes. The spindle fibers will attach to that centromere in order to prepare these chromosomes to be pulled apart to separate ends of the cell.
Following metaphase, we have anaphase. And in anaphase, sister chromatids are separated and moved to opposite ends of the cell. So we have those spindle fibers that were attached to the centromere, and now those sister chromatids, that were attach, are pulled to opposite ends of the cell. So that's what's happening during anaphase. One sister chromatin is going this way, one sister chromatids is going this way.
Our next phase is telophase. And I've actually also heard it pronounced telephase. So I guess it just depends on how you pronounce it. But telophase or telephase is our next phase.
And during this phase, the nuclear envelope will begin to reform around the chromosomes. And the plasma membrane is also starting to pinch off. This area where it's pinching off is called the cleavage furrow.
Then the final step-- so telophase is technically the final step of mitosis, but then what we end up with, as a result of mitosis, are two separate cells, so two completely individual cells. And we call this cytokinesis.
In cytokinesis, we end up with two individual cells. These two separate cells are diploid daughter cells. The cells that were produced from the process, we call them daughter cells. And they're diploid, because they contain 46 chromosomes-- so the same number as the parent cell. They are also identical to the parent cell.
At this point, the nuclear envelope has completely reformed. And the DNA will return to its thread-like form that I had mentioned earlier, called chromatin. But just for the sake of showing you that it's identical to the parent cell, I left them as chromosomes in cytokinesis.
You can see that the cell we started with here in interphase is identical to the cells that we ended up with. Mitosis produces new cells that are identical to the cell you started with. The same number of chromosomes, everything is identical.
This lesson has been an overview on the process of mitosis.
