UK scientists suggest that primordial soup was hot
A UK team of researchers has exposed a new theory that explains why the language of our genes is more complex than it needs to be. Moreover, their paper suggests that the primordial soup where life began on Earth was hot and not cold, as many scientists believe. In a paper published this month in the Journal of Molecular Evolution, researchers from the University of Bath describe a new theory, which they believe could solve a puzzle that has baffled scientists since the language of DNA was deciphered. In 1968, M. Nirenberg, HG Khorana and R Holley received a Nobel Prize for establishing how proteins are produced from genetic code. They discovered that three letter 'words' - known as codons - are read from the DNA code and then translated into one of 20 amino acids. These amino acids are then strung together in the order dictated by the DNA code and folded into complex shapes to form a specific protein. As the DNA 'alphabet' contains four letters - called bases - there are as many as 64 three-letter words available in the DNA dictionary: all the possible mathematical combinations of four letters taken in threes. Why there is this redundancy of 64 possible words in the DNA dictionary has mystified scientists ever since, and none of the dozens of theories proposed to solve the puzzle have proved true. Dr Jean van den Elsen from the Department of Biology and Biochemistry says: 'Why there are so many more codons than amino acids has puzzled scientists ever since it was discovered how the genetic code works. It meant the genetic code did not have the mathematical brilliance you would expect from something so fundamental to life on Earth.' One of traits of the genetic code is the groups of codons which all translate to the same amino acid. Leucine, for example, can be translated from six different codons. However, other amino acids with equally important functions, translated into the same amount, have just one codifying codon. The new theory builds on an original idea suggested by F. Crick - one of the fathers of the DNA structure - that the three-letter code evolved from a simpler two-letter code, although Professor 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, the scientists can replicate the table of amino acids - explaining why some amino acids can be translated from groups of two, four or six 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. 'This simple theory explains many unresolved features of the current genetic code.' The new theory also highlights two amino acids that can be excluded from the doublet system and are likely to be relatively recent 'acquisitions' by the genetic code. As these amino acids - glutamine and asparagine - are unable to hold their shape in high temperatures, this suggests that heat prevented them from being acquired by the code at some point in the past. 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. 'There are still relics of a very old simple code hidden away in our DNA and in the structures of our cells, ' explains Dr van den Elsen, pointing to several molecules involved in protein synthesis that only look at pairs of bases in triplet codons. 'As the code evolved it has been possible for it to adapt and take on new amino acids. Whether we could eventually reach a full complement of 64 amino acids, I don't know. A compromise between amino acid vocabulary and its error minimising efficiency may have fixed the genetic code in its current format,' he concludes.
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