Tracking Down the Mystery of Life's 'Handedness'

Within the general question of the chemical origin of life lies the question of handedness, or chirality (a term derived from the Greek word for hand). Critically important molecules that make up life — including DNA, RNA, polysaccharides and proteins — are chiral. This means there are mirror image forms that are not identical, much as one's left and right hands are different and are mirror reflections.

"The molecules of life have mirror images, which can be synthesized by organic chemists, but the mirror images don't exist in life any more than a person's mirror image does," explains Mark M. Green, professor of chemistry at Polytechnic Institute of New York University (NYU-Poly).

But why should this be the case? Scientists have tried to answer this for many years.

One theory is that at the very beginning, there existed both mirror forms of the molecules that would lead to life, but with a tiny excess of perhaps one extra left-handed molecule in a hundred or even a thousand or more right- and left-handed molecules. If that small excess gets amplified or concentrated, the mirror form that's in excess could reasonably become the basis of the chirality in the life that evolved. "Ronald Breslow at Columbia University has recently supported this idea by discovering chemistry mechanisms in which tiny excesses of the chirality seen in meteors raining down on earth, prevailed to control all of the chirality in life we see today," says Green.

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