Is There Life Through The Looking-Glass? The Riddle Of Life’s Single-Handedness

The molecules that make up life may have arrived from space, and many are chiral. NASA / Jenny Mottar

It is thought that a small excess of L-amino acids was “rained” onto the ancient Earth by meteorite bombardment, and scientists have found that a small excess of L-amino acids can catalyse formation of small excesses of D-nucleotide precursors. This, we proposed, led to a marginal excess of D-polynucleotides over L-polynucleotides, and a bias to D-chains of longer mean length than L-chains in the RNA world.

In the primordial soup, local excesses of one or other hydrogen peroxide enantiomer would have occurred. Specific interactions with polynucleotides destabilise the shorter L-chains more than the longer, more robust, D-chains.

With a greater fraction of L-chains over D-chains destabilised, hydrogen peroxide can then “go in for the kill”, with one enantiomer (let us say M) preferentially oxidising L-chains.

But could there be mirror-image life made of L-nucleic acids elsewhere in the universe? Shutterstock

Overall, this process works in favour of increasing the fraction and average length of D-chains at the expense of L-species.

But the hydrogen peroxide itself remains a racemic mixture, on average, meaning that over time and space it has a balance of M and P enantiomers. So we have a subtle reinforcement effect: the fraction D/P increases while the fraction L/M decreases over time.

Thus, the emergence of homochirality in itself confers a significant advantage on replicating RNA species.

But could there be mirror-image life made of L-nucleic acids elsewhere in the universe?

Well, all I can say at this stage is that when one reflects on it (or attempts to, so to speak), in a sense we are all vampires, made of molecules that have no natural mirror images on this world, and forever searching the universe for our lost reflections.

Rowena Ball, Associate professor, Australian National University

This article was originally published on The Conversation. Read the original article.

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