Biology Reference
In-Depth Information
209
Unfortunately, they discarded it by looking at the “width of layer line
streaks”, instead of the locations of the intensity spots on the various layer
lines of the original picture obtained by Franklin and Gosling (1953) for
sodium salt of DNA fibers at 92% relative humidity.
AN ALTERNATIVE HELICAL STRUCTURE OF DNA
For a single helix of pitch 2P, there will be twice the number of layer lines
on the X-ray film representing the reciprocal space. Layer lines appear at
instead of In order to reduce the number of layer line by a
factor of two, another helix of pitch 2P is placed along the same helical axis,
but displaced by a distance P. With destructive interference, intensity spots
on all odd layer lines will be cancelled, thus giving layer lines again at
n/P. As originally mentioned by Wilkins, Stokes and Wilson (1953), this
combination of two helices will give spots on layer lines with intensities
correspond to etc. Based on our previous
discussion, the intensity spot on the first layer line will thus be located
directly on top of that on the equatorial layer line. This is indeed the case
for the X-ray diffraction picture shown in Fig. 8-1.
At the same time, the model must also result in reduced intensity on the first
layer line and eliminate that on the fourth layer line. Thus, another similar
set of two helices can be placed along the same helical axis and displaced
relative to the first set by 3P/8 as discussed before (Figs. 8-4 and 8-5). The
X-ray diffraction pattern based on this four-stranded structure, in its
molecular details, is in perfect agreement with that shown in Fig. 8-1 (Wu,
1969). Another way of looking at that structure is to first consider one DNA
double helix. Unwind it completely to a straight ladder with base-pairs on
one side of the ladder. Take another ladder, and mutually intercalate the two
ladders together. Then twist them down slightly. The mass per unit length
remains the same.
In order to understand the intricacy of how various combinations of helices
can give rise to different X-ray diffraction patterns, the following helices or
combination of helices are illustrated:
