Proteins conduct the processes of life. Each protein is encoded in one "master document", a gene, in the cell's "library", the genome. The genome consists of DNA. No cell can function without the proteins that it requires, so it cannot function without its genes, its DNA. If some of the "master documents" are missing or defective the consequences are potentially serious. Every cell must begin with a very accurate copy of its parent's or parents' DNA. So it is important for the DNA to be stable, to resist change, otherwise inaccuracies will appear.
This is a challenge. Recall our discussions of the shape and size of a DNA molecule (chapter 2) and the control of gene expression (chapters 7-9). The cell's DNA looks fragile and it is continually being transcribed, replicated, packaged and unpackaged. A very long, thin thread that is incessantly being coiled and uncoiled, stretched and compacted, decorated with adhesive proteins and freed of them again, is vulnerable to damage. DNA is continually harassed and battered by the processes of life. Unless they were constantly subjected to repair and maintenance, the cell's "master documents" would become degenerate.
If a gene is altered, the cell's metabolism or internal structure might be changed, or perhaps its transport mechanisms; in short, its internal state. Or there might be a change in the way it responds to stimuli from the environment or controls the expression of other genes. The long term survival of an organism depends on its ability to prevent or forestall such damage, or to buffer itself against the effects of gene modification.
On the other hand, if genes never changed at all, there would be no variation among organisms, no evolution, no diversity of life. The evening in the woodland would be a far less magical experience. Indeed, there would be no woodland and no one to appreciate it. We owe our existence, and we owe the abundance of life around us, to the capacity of DNA for change. In this chapter we shall examine this seeming conflict between the need for DNA to remain stable and the need for it to undergo permanent alterations.
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