Wednesday, April 20, 2011

Structural Variation in DNA among Different People

It is often remarked that no two people except identical twins have the same DNA. As far as scientists now know, the statement is true, but like many generalizations, it hides a lot of the detail. What the human genome project has shown is that about 20 percent of human DNA actually specifies protein structure. Much of the DNA that specifies protein structure is pretty similar among different people. The differences that do exist—polymorphism in DNA—cause protein and enzyme polymorphism.

Polymorphism in DNA is simply a base change here or there from one person to another. A single base change in coding DNA might cause a different amino acid to be inserted into the protein; but some single base changes would not even do that, because several triplet sequences of the genetic code can specify the same amino acid. As long as the base changes do not cause too great a change in the protein structure, the protein is still functional. These single-base changes in DNA over time become mutations, and they occur in all cells. Mutations that cause major disruption in the structure (and thus function) of vital proteins generally do not allow the organism that has them to survive, meaning that the mutation does not survive either. But many mutations do survive and create the polymorphism in DNA and proteins that is so common.

In spite of many single-base differences throughout the genome, there is considerable similarity in the DNA of most people. It is not realistic right now to sequence large segments of DNA just to find the differences between people because the process would be too burdensome. The Human Genome Project has provided considerable information about where the differences are, however, and technology is rapidly being developed that will allow searches for hundreds or thousands of small differences all at once.

There is another kind of variation in DNA, however, that has been exploited for forensic DNA typing. As already noted, about 20 percent of human DNA specifies protein structure. What about the remaining 80 percent? What does it do? No one is sure, but there is something very interesting about much of this remaining “nonfunctional” DNA: It has a lot of repeated sequences.