Thursday, July 13, 2006

The versatile molecule

An extraordinary story from this week's Science. Researchers have identified a gene in DNA extracted from a 43,000 year-old frozen woolly mammoth carcass that likely determined the color of the mammoth's fur. Woolly mammoths have been extinct for nearly 5000 years.

Genes. The ineffable, ineluctable agency of genes.

It has now been just over half a century since James Watson and Francis Crick announced their discovery of the structure of DNA.

What they proposed had a compelling simplicity: A molecule in the form of a spiral staircase. The side rails of the staircase are linked sugar and phosphate molecules. The treads are molecules called organic bases, arranged as pairs. The bases, four in kind, are adenine, thymine, guanine and cytosine, usually labeled simply A, T, G and C. Base A always pairs with T; G always pairs with C. The sequence of pairs along the staircase is the genetic code. The counterclockwise twining of a morning glory, the color of a woolly mammoth's coat, written in a four-letter code.

Watson and Crick added an almost parenthetical remark to their announcement: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

This is what they had in mind: When a living cell replicates, the DNA staircase unzips down the middle of the "treads." Each unpaired base attracts its complement; each half of the staircase completes itself. One set of genes becomes two, two become four, four become eight. Life copies itself in an unceasing dervish dance of recombination.

We do not know how the first DNA molecules came about, but they have been a sturdy and versatile basis for life on Earth. And now engineers are plugging into DNA's remarkable combination of simplicity and fecundity. Almost every issue of Science and Nature seems to have a report of some new engineering use of DNA, from computation to nanotechnology. However it happened that the first DNA molecules arose on the early Earth -- if indeed they had their origin here -- our little blue planet hit the chemical jackpot.