麻豆传媒

LIFE may have begun as a weird molecular hybrid鈥攈alf-protein, half-RNA,
suggest chemists in California and Texas.

Many biologists believe that RNA was the first self-replicating molecule
because, like DNA, it is composed of chemical bases that spell out the genetic
code, but like protein it鈥檚 also capable of a remarkable array of chemical
reactions.

But 鈥渢he death knell for prebiotic RNA is its sugar,鈥� says Matthew Levy of
the University of Texas, Austin. Under the conditions you might find on early
Earth, the phosphorylated sugar groups which make up RNA鈥檚 backbone are unstable
and don鈥檛 easily form chains.

But this may not be true for RNA鈥檚 chemical relative, the chimera-like
peptide nucleic acid. The bases of PNA, which was synthesised by chemists nearly
a decade ago, are joined together with sugar-free links like those in proteins.
Now Levy and his colleagues at the University of California, San Diego, say that
PNA may have been a natural compound on the primeval planet.

They could persuade up to 78 per cent of plausible prebiotic chemicals to
transform into PNA backbone subunits. They also point out that these link up
readily at 100 掳C, which was probably a common temperature four billion
years ago when our planet was rich in volcanic activity. 鈥淲e鈥檝e shown its pieces
would have been available,鈥� says Levy. 鈥淭hat puts it on a par with RNA and
probably a step past it.鈥�