Tóm tắt
The discovery of the double helix structure of DNA led immediately to
questions
on the mechanics of unravelling its intertwined strands during replication.
If a
parental DNA is to be duplicated into two progeny molecules by separating
its two
strands and copying each, then the strands must untwine rapidly during
replication (Watson & Crick, 1953).That DNA indeed replicates in such a semiconservative fashion was soon
demonstrated by the Meselson–Stahl experiment (1958). At first, it
appeared that
the unravelling of the intertwined strands should not pose an insurmountable
mechanical problem. The two strands at one end of a linear DNA, for example,
can be pulled apart with concomitant rotation of the double-stranded portion
of
the molecule around its helical axis. If the strands of a DNA double helix
are to
separate at an estimated replication rate of 100000 base pairs (bp) per
minute, then
the speed of this rotation would be 10000 revolutions per minute from the
10 bp
per turn helical geometry of the double helix. This speed, though impressive,
seemed reasonable: owing to the slender rod-like shape of the double helix,
the
estimated viscous drag for this rotational motion is actually rather modest
(Meselson, 1972).
