Biology Reference
In-Depth Information
214
For a physicist, he or she would place a mirror behind the experimental set-
up and look at the mirror-image. There, the right-handed helix turn into left-
handed helix, while the X-ray diffraction picture appears the same. That
picture is symmetric right-to-left. Therefore, the physicist would conclude
that if right-handed helix give that X-ray diffraction picture, left-handed
helix would give the same picture. The solution is thus not unique.
For a mathematician, he or she starts with the equations of the right-handed
helix or of the left-handed helix. There is only one negative sign difference.
On carrying out the calculation for the total amplitude,
A(h)
, the
mathematician discovers that becomes However, the intensity,
I(h)
, remains the same as since the phase drops out. Again, both
the right-handed helix and left-handed helix give the same X-ray diffraction
pattern.
For a biologist, he or she needs to learn some mathematics and physics as
discussed here, before the above question can be answered. As we have
pointed out, without a thorough knowledge of X-ray diffraction and the
properties of Bessel function, it is rather difficulty to understand the entire
problem.
Duality
All the consequences based on the duality of DNA double helix are retained
for the four-stranded model.
Unwinding and re-winding
Since the sugar-phosphate backbones are straight, unwinding and re-
winding are no longer required (Wu, 1969).
DNA Supercoils
The simplest DNA supercoils are the intact double-stranded DNA circles of
plasmids. Under electron microscopes, they appear as rods or branched
rods, each of which consists of two side-by-side DNA double helices
