Fwd: Re: Genetic Code

Yersinia yersinia at CYBERNEX.NET
Sat Nov 27 20:34:56 EST 1999

Bill writes:

<If you take a look at the chemical structure of DNA you'll see that it's 
based on a ribose sugar containing 5 Carbon atoms which are numbered 1' 
through 5' according to organic chemistry convention.  (I'm not an 
organic chemist so I won't go out on a limb to explain that further).  
Attached to the 3'  Carbon atom is a hydroxyl group.  This hydroxyl is 
the reaction site where the next base in succession is attached.  This 
reaction attaches the phosphate group of the next base to the hydroxyl 
group of the previous base to form the
phosphodiester linkage.  As you may have guessed, the phosphate group on 
each base is
attached to the 5' Carbon of the ribose sugar.  I recommend looking at a  
textbook such as
"Molecular Biology of the Gene" for diagrams that will make what I'm 
saying much
more clear.  The point is that the 3' end is the end of the DNA base 
chain because attachment of the next base occurs at the 3' Hydroxyl (The 
5' phosphate group is already attached to the 3' hydroxyl of the previous 
base - it may help to visualize the 5' phosphate of the very first base 
as unattached to any other bases).  This should also make it apparent why 
the extension (i.e. replication DNA ---> DNA, and transcription DNA ---> 
RNA) moves in the 5' to 3' direction.>

Wow!  You didn't mean this explanation for me, but please accept my most 
enthusiastic thanks for your combination clarification and memory 
refresher. I took Principles of Genetics 4 years ago or so; really loved 
it, and *thought* I understood it at the time (I certainly recall 
replication and transcription going in the 5' to 3' direction, about the 
hydroxyl reaction sites, ribose sugars, phosphodiester bonds, etc.), but 
not like THIS.

<Hope I've been some help. Hope I wasn't doing a homework assignment but 
I'm happy to help.>

Well, you helped ME, if that counts for something - even though my 
present homework is physics (don't worry. I won't bother anyone here with 
it). As to doing other people's homework, I've discovered from 3 1/2 
years of subscribing to this list that if the subject line is "Help!" (or 
some variant thereof), and/or contains a list of questions which appear 
to be directly phrased from a textbook, the querent is indeed someone 
looking for others to do his homework. Those are the people you can 
ignore, or tell them to try it themselves first. 

However, I don't think *this* querent was looking for an easy fix. It 
looked like he was trying to make sense out of lecture material and/or 
his reading assignment, but was having genuine difficulty. From my 
standpoint, this is legitimate and I see nothing wrong with offering 
assistance. I have occasionally had questions which neither lecture notes 
(which I take copiously!) nor textbook reading - nor even taking those 
questions to my professor could answer. In such instances (fortunately 
not *too* often, but...), I find this place a resource I couldn't live 
without. I'd hate to think that my student side wasn't welcome to ask 
those questions here (although if the proposed student list is formed, 
I'll join that too).


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