Genetic code evolved almost inevitably, study finds

Researchers studying the beginnings of human life on Earth say that the development of the genetic code was an inevitable consequence of naturally-occurring chemical relationships between nucleotides and amino acids.

By piecing together how 'Ida' and 'Luca' - what scientists call the initial Darwinian ancestor and the last universal common ancestor - assembled themselves from the chemicals thought to be present on the primordial Earth, international research teams are closer to understanding how organisms developed the ability to, not only self-replicate, but store genetic information.

Professor Michael Yarus, from the University of Colorado at Boulder, USA, is one such researcher who works with artificial RNA, a close cousin of DNA, which Luca is thought to have been based on. Strands of RNA can act like enzymes, which means organisms could have had a metabolism before proteins allowed more complex life forms to develop.

The nucleotides that make up an RNA strand are grouped into triplets, each of which codes for a different amino acid. Professor Yarus' research shows that the natural chemical bonding means amino acids have a much higher affinity for certain RNA triplets (known as anti-codons) than others, explaining why individual triplets always code for the same amino acid. He has shown this by mixing RNA with amino acids and watching certain matches nestle next to their corresponding triplets.

'Yarus found that anticodons [a type of triplet found in some RNAs] were particularly good in this regard and bind the 'correct' amino acid with up to a million fold greater affinity than other amino acids', said Dr Nick Lane of University College London.

Two other researchers at the Salk Institute for Biological Sciences in La Jolla, California, USA, have taken Professor Yarus' idea further by looking for chemical affinities in real organisms. When the pair looked at where amino acids sit in the ribosome - key components of cellular machinery which assemble proteins from amino acids - they found that 11 of 20 standard amino acids were far more likely than not to be positioned next to the 'correct' RNA triplet, as reported in New Scientist.

These discoveries have led research teams to speculate that Ida, the precursor organism to Luca, may have been made of something very similar to RNA.  The catalyst which could have necessitated nucleotides to assemble genetic chains on their own has long evaded scientists, explains Professor Yarus, however, Dr Lane believes there may be a simpler way to explain how the first chains of RNA appeared.

Professor Ernesto Di Mauro from Sapienza University of Rome in Italy, has found that cyclic nucleotides - a chemical variation of the nucleotides which make up RNA - will spontaneously link together to form RNA chains.
Dr Lane has looked at natural environments such as the Earth's crust for where these cyclic nucleotides may have originated. 'In environments like hydrothermal vents it is likely, but as yet experimentally unproven, that a range of amino acids and nucleotides would be formed by the laws of chemistry', he said.