DNA Structure: Puzzle Number 6

clive delmonte clived at ndirect.co.uk
Thu Dec 3 02:37:37 EST 1998

Leslie et al.(Puzzle 5, re. 26) reported that a fibre of poly(dI).poly(dC),
which gave sharp X-ray diffraction spots shortly after being drawn, was
unstable and
transformed irreversibly after a few days into poly(dC).poly(dI).poly(dC+).
This new, three-stranded molecule also gave sharp spots in its own X-ray
diffraction pattern which did not have any of the original spots.

Therefore the conversion was complete. Very similar results had been
reported for fibres of poly(dA).poly(dT) converting to
poly(dT).poly(dA).poly(dT) (9) and for the formation of
poly(U).poly(A).poly(U) from poly(U).poly(A) (10).

The problem is evident:  How and where would the torque arise inside a
solid fibre that would permit one double helix to rotate so as to unwind one
strand and rewind it onto an adjacent triple helix having a different
diameter and rotating at a different angular velocity ?

Inside the fibre, individual molecules would have a random axial translation
so only very rarely would two adjacent double helices have the same axial
starting point down the fibre, and they would be unlikely to be perfectly
straight inside the fibre but would probably be entwined around neighbours.

How would there be a very high conversion of double to triple stranded
molecules when any rotation of adjacent double helices would normally find
the free ends of one of them axially displaced down the fibre compared to
its neighbour ?

9      Structures for the Polynucleotide Complexes Poly(dA).Poly(dT) &
Poly(dT).Poly(dA).Poly(dT); Struther Arnott & E. Selsing; J Mol Biol Vol 88
(1974) 509-521

10      Structures for Poly(U).poly(A).poly(U) Triple Stranded
Polynucleotides; Struther Arnott & P.J. Bond; Nature New Biology Vol 244
(1973) 99-101

Clive Delmonte

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