Re: How do organisms choose how to interpret their genetic codes ?

First off I will say that you have asked a very good question. The answer 
I am going to give you is biologically sound but I can not be 100% sure 
it's all correct! As I am sure you are aware there is no simple website 
explaining this, I used my own biolgical knowledge to form a conclusion. 
However I am confident the answer is correct because I can find no 
counter example to the explanation I am about to give, that is, an 
organism that is able to interpret a codon in more than one way.


So in order to answer your question we must first review the process of 
translation: the way in which the messenger RNA (created from the DNA) is 
translated by the ribosom into a strong of amino acids which go on to 
form a protein. Below is a simple picture of the process:



As you can see the tRNA contains 3 bases pairs which are complementary to 
the codon on the mRNA. They also carry the specific amino acid for that 
mRNA codon. The ribosome then joins the amino acid onto the growing 
polypeptide chain.

To put it simply, the reason why some organisms are able to interpret a 
codon as being a different amino acid is because within their genetic 
code, they encode a different tRNA. 

The tRNAs for each amino acid are different and a whole class of enzymes 
ensure the correct amino acid is partnered with the correct tRNA. It is 
true that mitochondria have a slightly different interpretation of the 
genetic code, and because of this the mitochondrial (and chloroplast) 
genomes encode tRNAs and enzymes so that the correct amino acid is used 
in these special instances.

As far as my knowledge goes, an organism does not choose how to enterpret 
it's genetic code. That is dictated by whatever tRNAs it has evolved. 

Most organisms that have different triplet code tend to be very ancient, 
the mitochindria and chloroplasts are both thought to have evolved from 
bacteria that were engulfed by larger cells. Hence why they have their 
own genomes. The differences in their genetic codes simply did not spread 
to other organisms. 

I hope this answers your question!

Melanie