Shaking the foundations of the central dogma (again)

The first shock that came to the creators of the central dogma (Watson & Crick) was the discovery of retroviruses which had a little trick of reverse transcribing their RNA genome into DNA. This was a shock because it was believed that the pathway of DNA to Protein occured in one direction and could not be reversed.

Now a new discovery has shaken the foundations yet again. It has always been believed that three nucleotide bases in DNA, a triplet codon, code for one type of Amino Acid... until now! A marine bacteria Euplotes crassus was discovered where one triplet (UGA) can code for either cysteine or selenocysteine. How the bacterial genome regulates which AA is coded is not clear but it is something amazing.

Read more here: http://scienceblogs.com/notrocketscience/2009/01/one_codon_two_amino_acids_the_genetic_code_has_a_shift_key.php

Edit (30-01-2009): Apprently the two-amino-acids-for-one-triplet-codon thing is not a new phenomena. I just found this in Concepts of Genetics by Klug et al., Ch. 14.6, p361 -

"Only one codon, AUG, codes for methionine, and it is sometimes called the initiator codon. However, when AUG appears internally in mRNA, rather than at an intiating position, unformulated methionine is inserted into the polypeptide chain (instead of the initiator type of methionine, N-formylmethionine (fmet)). Rarely, another codon (in Bacteria), GUG, specifies methionine during initiation, though it is not clear why this happens."

Furthermore, in transcribed mitochondrial DNA (mtDNA), mtRNA, there is another special behavior:

"In human mitochondria, AUA, which normally specifies isoleucine, directs the internal insertion of methionine. In yeast mitochondria, threonine is inserted instead of leucine when CUA is encountered in mRNA..."

In fact, there turned out to be many such examples over time. See table 14.5 on page 362.

It just goes to prove that whatever rule exists there is almost always something that breaks it. It also brings home the complexity and heterogeneity found in biological systems.