Re: Please educate me

Joel Duff (virkotto@intrnet.net)
Mon, 15 Dec 1997 21:18:46 -0600

At 06:31 PM 12/15/97 -0600, you wrote:
>I ran into an interesting web page that claimed that the genetic code was
>not universal among all living creatures. That peaked my interest. What I
>found was a claim that tRNA (transfer RNA) was different among different
>creatures. I wrote the guy and said that it was my understanding that tRNA
>was not part of the genetic code. He wrote me back and said I, like many
>who accept evolution, have a very poor understanding of how the genetic
>system worked. He said I should do more study and get back with him when I
>could respond intelligently to his paper. While I thought this was a rather
>arrogant dismissal of my question, in the interest of education, I am here
>to learn. What is the definition of the genetic code, (is tRNA part of it)?
>I didn't think tRNA carried hereditary information and so was not part of
>the genetic code but was part of the cellular machinery. Is his reaction due
>to a re-definition of what is called the genetic code by common parlance?
>
Glenn,

Don't worry, you aren't completely ignorant of what the genetic code is:-)
I don't have to even look at the article to guess exactly what this person
is referring to. There are situations in which a particular codon (which
would come from the DNA code) is not translated into the usual amino acid
the tRNA has an anticodon that corresponds to another amino acid. So
instead of the typical chart you see in any basic biology book of what codon
corresponds to which amino acid there are actually exceptions. I wouldn't
be surprised if many more cases of altered tRNAs are found as more and more
genomes are sequenced. I did look up the article you mentioned and he does
make mention of a possible mechanism behind this transition but this isn't
the only one or even the prominent hypothesis. His explanation of mechanism
is far too simplistic. The article states:

"Here is the essence of what evolutionists are proposing. They propose that
every instance of a specific codon in the DNA is mutated and replaced by
another codon. This requires replacement of 1-5% of the entire genetic
sequence of an organism. Although this doesn't seem like a large amount of
the genome, it is the specificity of
replacement that makes this mechanism statistically impossible."

This is certainly an overestimate and doesn't at all appreciate the
tremendous changes in codon usage in many genomes such that many codons are
essentially unused in some organelles. As a result some codons are seldom
used and if changed (from one amino acid to another) would not necessitate
whole scale changes in genome composition.

I won't try to explain the proposed mechanisms for tRNA changes off the top
of my head but will try find the papers tomorrow. I am sure that there are
others though that are probably more familiar with the odd cases and can
illuminate you on this topic.

Joel

One paper I had at home that is really an interesting review of genome size,
codon flexibility, tRNAs and sybiotic bacterial genomes:

Andersson S.G.E and C.G. Kurland. 1995. Genomic evolution drives the
evolution of the translation system. Biochem Cell Biol. 73: 775-787.

>The article in question is:
>
>http://www.geocities.com/CapeCanaveral/Lab/6562/trnamut.html
>

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