why, Why, WHY???

syvanen at ucdavis.edu syvanen at ucdavis.edu
Mon Apr 12 07:28:20 EST 1999

In article <7emc1r$a41 at net.bio.net>,
  Tim Tillman <tillman at ithink.net> wrote:
> Tim Tillman <tillman at ithink.net> wrote in message

> > DNA utilizes deoxyribose (5C) in its sugar-phosphate backbone.  The
> question
> > is why is this so?  Why wasn't arabinose (4C), any hexose or levo-hexose
> > (6C) used?

> OK, if the fact that ribose was chosen for incorporation is a 'frozen'
> accident, what was the selective pressure that caused ribose to prevail and
> kept it on the job lo these many years?

Your question seems somewhat arbitrary in that you could ask it
for any number of hundreds of metabolites.  Why D-sugars and not L-
sugars? why glucose as life's major carbon source? why lysine and not
ornithine? and on and on.  Formally there are two possible explanations:
1) frozen accident or, some would say "historical contingency" and
2) functional constraint.  A little bit of each explanation
is probably true for each case.  For example consider your question
on deoxyribose in DNA.

We know that polymerized deoxyribose is more stable against spontaneous
hydrolysis than is polymerized ribose; hence selection for stable
genetic material probably resulted in deoxyribose replacing ribose.
This would be functional constraint.  There are some sugars that
would not allow the two strands of DNA to form a semi-stable DNA
duplex; these would lose out to deoxyribose because of functional
constraint.  However, there may be a sugar that would work just as
well but was simply not present for that purpose when DNA evolved.
Hence, frozen accident.

For each metabolite that this question is posed, we can only speculate
about the answers and intelligent speculation requires considerable
knowledge of the underlying chemistry or, even more difficult,
of the chemisty of choices not made.

Mike Syvanen

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