Why 5'->3' and not 3'->5' ?

Scott McMahan mcmahan at oncology.wisc.edu
Thu Mar 26 20:13:39 EST 1998

In article <6fes6c$jqt at ringer.cs.utsa.edu>, jkumar at lonestar.jpl.utsa.edu
(Jay  . Kumar) wrote:

Quick answer:

Because the triphosphate group is off the 5' and not the 3'

Long answer:

The lose of the pyrophosphate group supplies the energy for the
phosphodiester bond formation (over simplification, but P-Chem isn't my
strong suit).  If the NA builds 3' -> 5', the growing chain supplies the
energy group, not the incoming nucleotide.  This means if an incorrect base
is inserted, a proof-reading enzyme would excise the last base and its
triphosphate group.  Before it could proceed with the correct base, it
would have to put a pyrophosphate group back on, which would require a loss
of energy from somewhere else.  5' -> 3' removes the mismatch and begins
elongating with the next (d)NTP.

                                         Scott McMahan
                                         mcmahan at oncology.wisc.edu

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