On Sun, 15 Jun 2003 22:21:35 +0000 (UTC), Michael Ragland 
 wrote:
> Larry Moran wrote:

[snip]

>> I suspect that you don't understand the meaning of the words "Genetic
>> Code." The Genetic Code refers to the rules governing the
correspondence
>> between nucleotide sequences in mRNA (or a gene) and the amino acid
>> sequence in a protein. The three-letter code for tryptophan, for
>> example, is UGG and the three-letter code for valine can be any one of
>> four codons: UGU, UGC, UGA, UGG. All living species use the same
"code"
>> to translate mRNA into amino acid sequences, with only a few minor
>> exceptions. The Genetic Code is the rule by which information transfer
>> works to decode a message. It is not the message. 
>> You don't want to alter the "Genetic Code" in humans - that's
>> impossible. What you want to do is to alter the content of the message. 
> 
> I'm learning Mr. Moran. When I replied to you there was a "feeling" in
> me my statements about the so-called universal genetic code were
> erroneous. This came about from the mere realization that if the code is
> "universal" then how can it be changed. So, I did a little bit of
> research (and there is a post of mine in this thread reflecting that)
> which states the genetic code IS NOT "univesal". 

This is old news to anyone who has studied biochemistry or molecular 
biology. I'm having trouble understanding why you think this is so
significant.

> It's unfortunate but
> apparently it was Francis Crick, one of the co-discoverers of DNA, who
> advanced the so-called "Frozen Accident theory. 

This refers to ideas about the *original" evolution of the genetic code
3.5 billion years ago. One of the key questions is whether particluar 
codons have some sort of physical significance so that the matching of 
codons to amino acids might be due to selection. If this is true then  
when we replay the tape of life we'll come up with the same genetic code. 
The consensus nowadays is that there is no particular reason why GGG, and 
not AAC, encodes glycine. The modern genetic code is mostly an accident 
that is now "frozen" into place because changing it too much would have 
serious consequences. This idea does not rule out genetic code variants 
as you seem to believe - the emphasis in Crick's idea is on "accident" 
and not on "frozen."

> Although scientific
> evidence shows the code is not universal and can evolve there are 
> still those in the scientific community who adhere to the concept 
> of a universal genetic code.

No there aren't - at least not in the sense that you imply. The existence 
of genetic code variants has been in the textbooks for twenty years.
It's unlikely that there is anyone in the "scientific community"
who hasn't heard of it.

> The genetic code is "standard" and predominates across life but it is
> not universal and can evolve. So I'm not referring to the content of the
> message (which can't be seen as totally independent from the standard
> genetic code) but to the standard code which plays a role IMO in
> determining the content of the message. 

Nobody doubts that the genetic code could evolve and everybody knows
that there are minor variations that exist in a few specialized species
or organelles. These people also understand why you can find a non-
standard genetic code in an organelle that only encodes a few dozen 
proteins. They appreciate the fact that changing standard genetic code
for hundreds of nuclear-encoded proteins might be a tad difficult.

> Of course, currently it is impossible to change our standard genetic
> code. But if the standard genetic code is not universal and can evolve
> then that indicates we will likely have the ability one day to change
> our genetic code through genetic engineering.

We could do it today if we want. I think it's been done in order to
incorporate certain unusual amino acids in bacteria. But why would we 
want to do this in humans? Why do you think this is such a big deal?



Larry Moran
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