Scientists have long wondered why DNA, the genetic code in all living things, always twists in one direction. Their answer may have fallen out of the sky.

Many molecules come in two forms that are mirror images of each other, twisting both clockwise and counter-clockwise. The master molecules that make up life, however, tend to exist in one form or another. DNA spirals clockwise. The amino acids that build proteins, however, spiral counter-clockwise. One way to make these single-direction molecules is by using an opposite “template” molecule.

Sandra Pizzarello is searching for a template that could have been around when life began on Earth. The ASU chemistry professor studied isovaline, an amino acid found in the Murchison meteorite, a piece of rock that fell from space in 1969. The meteorite is more than 4.5 billion years old, just like Earth itself.

Pizzarello says that the isovaline and other amino acids found in the meteorite spin predominantly counter-clockwise. To learn more, Pizzarello mixed isovaline in the proportions found in the meteorite with glycoaldehyde and formaldehyde, two carbon-based chemicals believed to have been common on the early Earth.

The reaction produced threose, a simple sugar found in living things. Pizzarello found there was about 5 percent more clockwise threose than counter-clockwise.

This might have been the beginning of directionality in the molecules of life. While threose doesn’t exist in DNA, it is part of a similar molecule called TNA. Life may have begun with TNA as a base, later evolving to use DNA. Pizzarello and coauthor Arthur L. Weber of the SETI Institute published their findings in the February 20, 2004 issue of Science.—Diane Boudreau

Go to: Physical Science: Chemistry | Life Science: Evolution

Home Page | Arts | Business | Education | Engineering | Health | Life Sciences | Physical Sciences | Social Sciences

Who we are | Contact Info