Thursday, 12 January 2012

A Possible Precursor To The RNA World

The most fundamental similarity between all organisms on Earth is that genetic information is encoded with the molecule DNA. Scientists around the world have been been trying to come up with the most likely way that the molecule came into existence. Its sheer complexity has given rise to the idea that its more simple cousin RNA evolved first, before giving way to DNA. This is known as the RNA world hypothesis. Yet RNA is also a complex molecule.

This has made some Scientists look for precursors to RNA itself, ancient molecules which are simple enough to synthesise, before being replaced in nature by more complex genetic material. Now a team of scientists believe that they may have found the most likely candidate: TNA or threose nucleic acid. The team, led by John Chaput, a researcher at the Centre for Evolutionary Medicine at Arizona State University's Biodesign institute, looked at how TNA could have been created and how it evolved under Darwinian laws in a pool of random sequences.

An aspect that drew Chaput to TNA is its simplicity in relation to other nucleic acids. DNA is composed of two intertwined nucleotide strands; RNA is composed of just one and the nucleotide chain in TNA is both single and even more simplistic. Part of its repeated structure is a type of sugar called tetrose, which is composed of a main ring of four carbon atoms. DNA and RNA use a 5 carbon ring sugar called pentose. Already TNA is simpler in composition.

An artist's impression of the primordial soup,
where the first nucleic acids were created
Tetrose could be more simply assembled in a prebiotic world, not only because it requires less carbon, but also because it could be generated from two smaller 2 carbon chains. Originally, it was thought that its simplicity meant that it would be unable to bond with more complex nucleic acids. Yet the research showed otherwise: it could have acted as an early form of genetic information storage, before bonding with RNA or DNA and transferring the biotic data.

Chaput conducted physical tests into whether TNA could have acted as a genetic precursor. He used a method of analysis called molecular evolution, which draws upon the startling principle that inert chemicals can also undergo the fundamental Darwinian processes of self-replication and mutation. The results of the test showed that the molecules were not only able to bind with and transfer genetic information to DNA strands, but were also of a sufficient length to be folded into specific sites within cells.

Of course, obtaining physical evidence that this process occured in nature 3.9 billion years ago is almost impossible. However Chaput does point out that TNA has many of the characteristics needed to act as an RNA precursor: the ability to store genetic information, undergo selection processes and fold into structures which can perform complex functions; as well as bind with more complex nucleic acids and transfer information. Chaput remains optimistic that answers to this question will become clearer with time.