04 June 2009

O noua descoperire despre originea vietii

Discutie intre John Horgan si George Johnson, trec in revista teoriile despre originea vietii:

Articolul lui Nicholas Wade din NY Times.

Scientists had long suspected that the first forms of life carried their biological information not in DNA but in RNA, its close chemical cousin. But despite 20 years' work, they had found no plausible way in which nucleotides could have been assembled.

A nucleotide consists of a chemical base, a sugar molecule called ribose and a phosphate group. Chemists quickly found plausible natural ways for each of these constituents to form from natural chemicals. But there was no natural way for them all to join together. (...)

In an article being published Thursday in Nature, Dr. Sutherland and his colleagues Matthew W. Powner and BĂ©atrice Gerland report that they have taken the same starting chemicals used by others but have made them react in an order and in combinations different from those of previous experiments. They discovered their recipe, which is far from intuitive, after 10 years of working through every possible combination of plausible chemicals.

The starting chemicals, they found, will naturally form a compound that is half-sugar and half-base. When another half-sugar and half-base are added, the RNA nucleotide called ribocytidine phosphate emerges. A second nucleotide is created if ultraviolet light is shone on the mixture.

Dr. Sutherland said he had not yet found natural ways to generate the two other types of nucleotide found in RNA molecules, but synthesis of the first two was thought to be harder to achieve.

If all four nucleotides formed naturally, then they zipped together easily to form an RNA molecule with a backbone of alternating sugar and phosphate groups. The bases attached to the sugar constitute a four-letter alphabet in which biological information can be represented. (...)

Darwin, in a famous 1871 letter to the botanist Joseph Hooker, surmised that life began ''in some warm little pond, with all sorts of ammonia and phosphoric salts.'' But the warm little pond has given way in recent years to the belief that life began in some exotic environment like the fissures of a volcano or the deep sea vents that line the ocean floor.

Dr. Sutherland's report supports Darwin. His proposed chemical reactions take place at moderate temperatures, though one does best at 60 degrees Celsius. ''It's consistent with a warm pond evaporating as the sun comes out,'' he said.

And because his proposal requires ultraviolet light, it would rule out deep sea vents as the place where life originated.