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).
Infectionately,
Yersinia.
