Biology Reference
In-Depth Information
213
In Figs. 8-8 to 8-12, the same scale is used for the reciprocal space
coordinate system,
h
, and the spot on the equatorial layer line for R = 1/2r is
shown. Then, it becomes obvious which combinations of helices give
patterns similar to the experimental findings shown in Figs. 8-1 and 8-5. To
study detailed molecular structures represented by these combinations will
require massive amount of computing time. Numerical sums of various
orders of Bessel function with double precision will be essential (Cochran,
Crick and Vand, 1952).
Several other four-stranded helical structures of DNA have been proposed
over the years. As pointed out by McGavin, Wilson and Barr (1966), there
is no steric hindrance of the base-pairs in the middle of the helices in their
model. Wilson is the same author for both this 1966 paper and the 1953
paper mentioned before. Similarly, Gehring, Leroy and Gueron (1993) have
also proposed a sterically allowed four-stranded structure. Others consist of
side-by-side double helices, some with one right-handed and another left-
handed.
If DNA is double helical at 66% relative humidity and in crystals, and is
quadruple helical at 92% relative humidity and in solution, a logical
extrapolation to 100% relative humidity inside cell nucleus suggests that the
sugar-phosphate backbone of DNA can exist in a straight configuration.
This possibility can thus resolve many of the difficulties resulting from the
universally accepted double helical structure, especially for unwinding and
re-winding.
POSSIBLE BIOLOGICAL IMPLICATIONS
Possible biological implications of the above mathematical analysis of the
X-ray diffraction pictures of DNA fibers will now be discussed.
Right-Handed or Left-Handed Helix
Do the X-ray diffraction pictures shown in Figs. 8-1 and 8-5 indicate that
the DNA helix has to be right-handed?
