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|Chapter Five: Cell Reproduction|
Before we can fully discuss cell reproduction, we must look briefly at the organization of DNA in cells. Recall from Chapter One that DNA is an indirect way of controlling what proteins a cell will produce; based upon the DNA's genetic code (which will be discussed in Chapter Six), different proteins will be produced.
When a cell reproduces, it basically just splits down the middle. While the process is of course a bit more complex than that, what is important to realize is that two cells form from the original parent. The parent cell cannot simply cut it's DNA in half and pass half to either of the new cells; doing so would result in cells which do not have the "code" for all of the proteins. Instead, cells must actually make a complete copy of their DNA so that each new cell has a full copy of the entire "code."
In prokaryotes, the organization of DNA is fairly simple. It is just one long circular molecule. As you might expect, eukaryotes have developed a more complex way of organizing their DNA. When the cell is not dividing, the DNA of a eukaryote appears is a large mass called chromatin. However, immediately before the cell reproduces, the chromatin (which has already been replicated) condenses into distinct structures. Each of these structures contains two identical copies of portions of the cell's DNA. The individual copies are known as sister chromatids, and they are attached at a region called the centromere. During division, the chromatids split, and each half is referred to as a chromosome. In this manner, each new cell gets an exact copy of the original cell's DNA.