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In this lecture series we will begin to explore Cell division, why we need it, and how it is controlled
First of All why do we care about cell division?
We care because if cell division and specialization is not controlled, we would just be a blithering mass of cells, that had no coordinated function
It is because of cell division, and its specific controls that we have so many different cells that function in so many specific ways.
Eyeball cells are distinctly different in behavior than the cells lining your small intestine
Although they all have the same genetic material inside -that is pretty amazing if you think about it!
In fact we call it CANCER when these controls of cell division go wrong, and cells begin to divide uncontrollably, and never grow up or become differentiated.
So, next...Why do cells divide?
There are several reasons - They divide for growth (say from an embryo to an adult form) - they divide to repair tissues (like a broken bone) and they devide to replenish themselves (like in the case of skin)
Finally cells also divide in order for us to reproduce - In our case we undergo *** reproduction, so we must have a mechanism to create *** and Eggs.
Mitosis is the process used for the first three cases, while a very specialized process called Meiosis is necessary to produce *** and eggs.
Lets take a look at the genetic material itself
Prior to cell division, the DNA becomes wound up into chromosomes.
Humans Have 23 pairs of chromosomes, 22 of which are autosomes, and one pair of Sex chromosomes which are involved in sex determination.
In the figure here, you can see a Human Karyotype, a karyotype is essentially a picture of all the chromosomes in a cell before it divides. Of course they do not naturally line up like this. They have been stained, and organized in to pairs.
Each Chromosome pair is composed of 2 Homolgous Chromosomes. Homologous means the same...so there are two chromosomes number six's, that both have the same sort of information on them. For example eye color. Each chromosome may have different varieties of the trait, but they both still contain genes for eye color.
Now, in order for daughter cells to have an exact copy of the parent cells DNA, a copy of the DNA must be made. SO right before cell division, each chromosome is coppied, and that copy is stuck to its original by a structure called a centromere. the two copies are clalled sister chromatids....BUT, only when they are stuck together....I'll repeat that....they are only considered sister chromatids when they are stuck together...
Lets take a closer look at the sex chromosomes....they are NOT homologous in males, becasue males are XY, and the Y chromosome is so tiny...poor little thing...In females they are homologoius, because they are bot X's, and thus contain the same sort of information.
Before we get into Cell Division, lets consider the cell when it is not dividing. The cell spends most of its time in Interphase being a normal cell, doing the normal cell things it is supposed to do. Its DNA is all unraveled, not wrapped up in chromosomes, so all the necessary instructions can be read from the DNA.
As you can see, Interphase is broken into several subphases. G1 is the stage right after the cell has divided, and it needs to grow in size to reach maturity. Any cell that is not going to divide again soon (or ever) will go off in to teh G0 loop over here. In G0 the cell is just being a cell, doing its thing.
S phase is where the Chromosomes get coppied before cell division to created those sister chromatids. This process is calles DNA synthesis, hence the S in S phase. Once a cell enters S-phase, it is on the road of no return to cell division.
After DNA is replicated (or synthasized) - as in the chromosomes have been copied, the cell must make preparations for Division. All these prepareations occurr in G2.
Finally the Division - M-phase. You will notice M-phas, or mitosis is divided into several sub phases. The first five are considered to be Mitosis or nuclear division, and the final phase it cytoinesis of cellular division. this is where the actual cell is divided to create two identical cells each with its own nucleus.
Mitosis itself is broken into 5 phases - Prophase, Metaphase, Anaphase and Telophase. We will take a look at the details of each of these phases shortly.