Thursday, June 19, 2003

Scientists gene map Y chromosome - discover more about evolution.

In today's issue of the journal Nature, a 40-person research team describes in detail the "male-specific region" of the Y, which makes up 95 percent of the stubby chromosome. The portrait they paint is of a Yoda-like entity that has physically shriveled over the eons but nevertheless has found a way to keep its unique powers.

Specifically, the Y chromosome has evolved a way to correct harmful mutations -- or preserve and promote useful ones -- without the benefit of having a nearly identical partner with which to trade DNA, as all other chromosomes do.

First, the Y is a "rotten" autosome. Its molecular landscape is littered with the remnants of genes that have not been functional for millions of years. Like hundreds of ancestral Y genes that have disappeared, these fragments may eventually be lost, too.

Sixteen of the genes are in what the researchers call the "X-degenerate region." This is what is left of the ancient pair of autosomes from which the X and Y evolved, and all 16 genes have counterparts that can still be found on the X chromosome. Most have to do with "housekeeping" functions in cells in many organs, not just those involved in reproduction.

Two of the 78 genes are in what the researchers call the "X-transposed region." This is a long stretch of DNA that closely resembles a chunk of the contemporary X chromosome. It was grafted in from the X recently, about 3 million years ago, sometime after chimpanzees and humans diverged. How it got there and why it has proved useful is unknown.

The remaining genes -- about 60 in all -- are located in a number of regions called "ampliconic" because their contents have been amplified and duplicated over time.

The palindromic stretches of DNA can fold back on themselves, lining up the gene pairs in a kind of mirror image. In that position, one of the genes "overwrites" the other, making the two identical if they happen to be different. This can either get rid of a mutation or reproduce it. Natural selection will determine which is better.

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