"BIO 201 - Kerry Henrickson 4.2 Cell Cycle - Mitosis Review This"
BIO 201 - Kerry Henrickson 4.2 Cell Cycle - Mitosis Review This lecture is a review of the cell cycle and mitosis and again it’s a review of concepts that you should already be familiar with but I thought if you wanted to just be quickly refreshed on these concepts that I would post this up for you. I don’t go into lot of details in this, because I assume that you already had it in other classes and it’s again just to jog your memory. So let’s start out with talking about why it is we have mitosis in the first place. Why should cells divide? And the answers to that are two fold, one is human being for example can’t grow from being a baby into a full size adult without the having cells in the body of the baby be able to divide and grow and also replace those cells that die and mitosis or a sexual reproduction in cells is the means by which we replace dead cells and also the means by which we increase the number of total cells we have. That is the process we are going to talk about in this particular lecture. There is another type of cellular division that is called meiosis, which we will talk about during second semester in biology-II and hopefully you have heard about it before in past biology classes, but this is the special type of reproduction cells in which we produce gametes and in other words sperm cells and egg cells and its distinctly different from mitosis. In mitosis, I always get two identical daughter cells as the product of mitosis, but in meiosis, I get four daughter cells and they are all genetically distinct from one another and none of them are like the parent cells. Now before we are going to talk about mitosis, I need to talk to you about the events that lead up to mitosis and those are the events that are part of the cell cycle. Now human beings, we have a life cycle. We start out as an egg in a sperm fused together to create something called as zygote that develops into an embryo and then into foetus, into a baby, and a toddler, and a child, and a teenager, and an adult, and then we die. And that’s a very predictable sequence of events. It’s not as if suddenly you have somebody that is a toddler and then they become an old person and then they become an adult. There is a rigid sequence of events that we go through in our life cycle. A cell is the same way. It has its own life cycle called the cell cycle and it has a number of stages that it goes through and it has to go through them in a particular order and it repeats that order over and over again. It starts out with a process called interphase. An interphase is a very long phase of the cell cycle. It has three subparts to it the G1, the S, and G2 phases, which we will talk about in a second. It would then go through the process of mitosis, which we will talk about later in this section and then follow up with something called Cytokinesis. Now what is interesting with cells is that not all cells go through the cell cycle all the way to completion. Now as humans we know, we definitely all die right, but some of us don’t make it all the way to old age, may be an illness or an accident causes us not make it to old age, may be a person dies as an adult, because of an accident or they die as a child, because of some disease or something. Cells are the same way they do not always to through all other phases of the cell cycle. Some cells will stop in interphase and never progress past interphase. For example neurons, your nerve cells in your body, once you are born for the most part they do not divide any more, they don’t go through mitosis, they just stay in interphase for the rest of your life. Muscle cells are similar. On the other hand, there are other cells that very rapidly go through the entire cell cycle over, over, and over again. Skin cells for example, because you are constantly losing and sluffing of skin cells of the outside your body, they have to be replaced very quickly. Your blood cells have to be replaced all the time. Your cells of your liver about once a year are replaced and then again like I said there are other cells that never undergo replacement such as neurons. So let us very quickly look at these three parts of interphase, before we all head into mitosis. The three phases of interphase take up the majority of the life cycle or the cell cycle. Okay, look at this, this is all interphase, all of this. So interphase is a long part of the life span of a cell and it consists of the G1 phase, ‘G’ can stand for growth or gap, your choice, that the G1 phase is one that cell grows and then it replaces organelles etc. Now if you remember, if I am just entering the G1 phase, I just mean that I want your mitosis where one parent cell gave rise to two daughter cells and those two daughter cells will be exactly half the size of the parent cell. Those two daughter cells. Oops, they are supposed to be the same sizes but you get the idea. Those two daughter cells, there first order of business is to grow back up to adult size, so that’s what the G1 phase is all about. Replenishing organelles that didn’t get, making extra copies of things, growing in size, and normal metabolic activity and then it will undergo the S phase. The S phase is one I copy all of the DNA, S stands for synthesis, DNA synthesis and at this point I may go into the G2 phase or the gap 2 phase. And this is where I make specific proteins that are needed for mitosis and something called the centrioles replicate. We’ll talk about the centrioles in a second when we get to mitosis. So if I go through all three phases of the interphase, the next thing is I should be ready to undergo mitosis. Mitosis is also called nuclear division, because it is literally the process by which I divide up the copied DNA into two daughter cells. Remember the DNA was made during the S phase or the synthesis phase okay. I am in a skip for now, talking about duplicated versus unduplicated chromosomes. We are gonna talk about that in classroom, we talk about human genetics, so just hang tie with sister chromatids, staying in the duplicated and unduplicated chromosomes; we will get to that later. After I undergo mitosis, I am going to go through the steps of mitosis in a second here. The second stage after that is cytokinesis and that’s right divide up the contents of the cytoplasm okay. So mitosis is about dividing up the contents of the nucleus between the two daughter cells, cytokinesis is about the division about the rest of the stuff in the cells between the two new daughter cells. So what happens in mitosis? There are four stages to mitosis; you will probably remember this that shorthand for the four phases is PMAT, prophase, metaphase, anaphase, and telophase. In prophase, this is the time when the cells prepares itself to divide up the contents of the nucleus and in order to do that, I have to get rid of the nuclear envelopes so it’s not in the way. The chromatin, which is not visible under the microscope, condenses into visible chromosomes that we can see such as what you see over here. The centrioles will start to separate and go to opposite ends of the cell, so you’ll see those centrioles on either side of the genetic material in the centre here and the centrioles, remember will send out those spindle fibers that are gonna reach out and grab on to individual chromosomes, in preparing getting ready to pull them to opposite sides of the cell. The next phase is metaphase and that’s when the all other chromosomes neatly line up along the middle of the cell as you can see here. The chromosomes are in red and the spindle fibers and the centrioles are in blue okay. So we move all of the chromosomes along the metaphase plate or the midline of the cell in preparation for anaphase, which is where I pull apart the copied chromosomes and divide half of the chromosomes go to one daughter cell and half go to the other daughter cell and then the last phase is Telophase, this is where I start to do everything in reverse that I did in prophase. So in prophase for example we lost the nuclear envelope, in telophase we reform the nuclear envelope. The spindle fibers that appeared now disappear. The centrosomes disappear okay. The chromosomes go back into chromatin, where they can’t be visible. So everything you did in prophase, you just reverse in telophase and once you have done that, that is the end of the mitosis and that is the end of nuclear division, dividing up the contents of the nucleus. Then the next step is cytokinesis, where I divide all the contents of the rest of the cell and it always follows right after telophase and it involves a ring called the contractile ring forming right down the middle of the cell here and that pinches the two daughter cells and separates them each from the other. If you want to go through review of this, I have mitosis movie available for you or animation I should say, available for you under resources next to this lecture, if you want to see the whole process in actual or realtime in an animation rather than just a description from me.