DNA Structure: Puzzle #8 (Version 2)

Clive Delmonte clived at ndirect.co.uk
Fri Apr 2 12:14:14 EST 1999


The Crystallographers' Paradox

Several teams of nucleic acid crystallographers have reported on the
interconversion of the B to the Z form under very mild conditions inside
fibres of drawn DNA (22,23), as well as the B to form D conversion under
similar, very mild conditions (24).

The very mild conditions are interpreted by some (23,24) to mean that the B
and Z forms, and the B and D forms, respectively, must have the same helical
handedness.  (Another team (22) seems to offer no interpretation of its
observation of the B to Z transition inside a fibre.) The reversible
conversion inside fibres of the A to, and from, the B form has been known
for many years, and therefore the A, B, D and Z forms would all seem to have
the same helical handedness.

However, there are many studies of diffraction from true oligonucleotide
crystals which purport to show that the A and B forms are right-handed, for
example (25,26), and purport to show that the Z form is left-handed, for
example (27).

How can all the crystallographers be right ?
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22        Polymorphism of DNA Double Helices; A.G.W. Leslie, S. Arnott, R.
Chandrasekaran & R.L. Ratliff; J Mol Biol Vol 143 (1980) 49 - 72

23       B to Z Transition in a DNA Fibre: The Question of Handedness of
the Duplex; V. Sasisekharan & S.K. Brahmachari; Current Science Vol 50
(1981) 10 - 13

24       Time-resolved X-ray Diffraction Studies of the B to D Structural
Transition in the DNA Double Helix; A. Mahendrasingham, V.T. Forsyth, R.
Hussain, R.J. Greenall, W.J. Pigram & W. Fuller; SCIENCE Vol 233 (1986)
195 - 197

25       Conformation of d(GGGATCCC) x 2 in crystals and in solution
studied by X-ray diffraction, Raman spectroscopy and molecular modelling; H.
Fabian, W. Holzer, U. Heinemann, H. Sklenar & H. Welfle; Nucleic Acids
Research Vol 21 (1993) 569 - 576

26       Crystal Structure Analysis of a Complete Turn of B-DNA; R. Wing,
H. Drew, T. Takano, C. Broka, S. Tanaka, K. Itakura & R.E. Dickerson; Nature
Vol 287 (1980) 755 - 758

27       Molecular Structure of a Left-handed Double Helical DNA Fragment
at Atomic Resolution; A.H.-J. Wang, G.J. Quigley, F.J. Kolpak, J.L.
Crawford, J.H. van Boom, G. van der Marel & A. Rich; Nature Vol 282 (1979)
680 - 686
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Clive Delmonte







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