Triple-stranded DNA Structure

clive delmonte clived at
Fri Sep 11 10:36:41 EST 1998

The formation and properties of triple-stranded DNA are now described in a
substantial body of literature.  However, I have been searching for early
research reports identifying triple-stranded DNA and I have come upon a
problem.   I  would be grateful to hear what thoughts others have had about

Leslie et al.(J Mol Biol 143 (1980) 49-72) 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 molecule also gave sharp spots in its
X-ray diffraction pattern, and this new pattern did not have any of the
original spots.  Therefore the conversion was complete, or substantially

Very similar results were reported for fibres of poly(dA).poly(dT)
converting to poly(dT).poly(dA).poly(dT) (J Mol Biol 88 (1974) 509-521) and
for the formation of poly(U).poly(A).poly(U) (Nature New Biology 244 (1973)

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.
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 ?

None of the original workers offered any explanation as to how this
conversion might be possible, nor, indeed, remarked that there might be
something to explain.

Naturally I have given this problem some thought, but I have been driven
towards a daunting conclusion.   I shall be glad to share my thoughts with
anyone who lets me know they are interested, or anyone who offers their
thoughts on this problem.

clived at

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