Tuesday, August 16, 2005

Why is the DNA Code Based on Codons

As everybody knows the genetic code is based on deoxyribonucleic acid (DNA). The DNA is composed of four different molecules arranged in codons with three per codon. Consequently, there are 64 possible arrangements of three molecules each. Each codon codes for one of 20 different amino acids, which in turn can make an almost infinite number of proteins. Since there are 64 arrangments but only 20 amino acids are used there is a certain amount of redundancy in the genetic code. One of the more interesting questions in genetics is how such a system developed. Recent research is beginning to answer that question.

One of quirks of the genetic code is that there are groups of codons which all translate to the same amino acid. For example, the amino acid leucine can be translated from six different codons whilst some amino acids, which have equally important functions and are translated in the same amount, have just one.

The new theory builds on an original idea suggested by Francis Crick - one of the discoverers of the structure of DNA - that the three-letter code evolved from a simpler two-letter code, although Crick thought the difference in number was simply an accident “frozen in time”.

The University of Bath researchers suggest that the primordial ‘doublet’ code was read in threes - but with only either the first two ‘prefix’ or last two ‘suffix’ pairs of bases being actively read.
By combining arrangements of these doublet codes together, the scientists can replicate the table of amino acids - explaining why some amino acids can be translated from groups of 2, 4 or 6 codons. They can also show how the groups of water loving (hydrophilic) and water-hating (hydrophobic) amino acids emerge naturally in the table, evolving from overlapping ‘prefix’ and ‘suffix’ codons.

“When you evolve our theory for a doublet system into a triplet system, you get an exact match up with the number and range of amino acids we see today,” said Dr van den Elsen, who has worked with Dr Stefan Babgy and Huan-Lin Wu on the theory.

One of the more interesting aspects of this is that it explains how the genetic code can maintain it's integrity. Because, for example, translation errors can result in another amino acid with the same charactersitics. Even more interesting is it's prediction about two amino acids (glutamine and asparagine). Both denature (or melt) at lower melting points than the other amino acids and researchers suggest that both were added somewhat later in evolution:

One possible reason for this is that the Last Universal Common Ancestor (LUCA), which evolved into all life on earth, lived in a hot sulphurous pool or thermal vent. As it moved into cooler conditions, it was able to take up these two additional amino acids and evolve into more complex organisms. This provides further evidence for the debate on whether life emerged from a hot or cold primordial soup.

Support for the theory comes from the fact that several aminoacyl-tRNA synthetases link with pairs - rather than triplettes - of bases during translation:

“There are still relics of a very old simple code hidden away in our DNA and in the structures of our cells,” said Dr van den Elsen

Which, of course, provides an avenue for future research.

I wonder what Intelligent Design advocates will make of this since it is a good example of how information content can be increased via natural selection. Something that Intelligent Design advocates say is impossible.