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The
technique first developed by Maurice
Wilkins
and Rosalin
Franklin
and used by James Watson and Franics Crick to
determine the double
helix
structure of DNA is known as X-ray
crystallography. X-rays are synchronized so that the
crests and troughs are in identical locations. These rays
are then bounced off of the DNA molecule to form an image
on photographic film. X-rays are used due to their low
wavelength, between 0.05 angstrom and several hundred angstroms.
The wavelength of normal light is hundreds of
times longer, making the offset caused by the molecule so
small that it is of no use. As the X-rays impact the DNA,
they are reflected at different angles and at different
times in their cycles (because the distance traveled is
different for each wave). This causes a pattern to form.
However, when only one molecule is used, the image is
very faint. To enhance and amplify it the image, the
molecules must be arranged in order, as in a crystal. DNA
is arranged in crystal form by turning it into a gel. A
small object can then be inserted into  the mixture. When twisted, the object will
pull out a strand of several DNA molecules, all in order.
This strand can then undergo X-ray crystallography, and
produce a much stronger image. X-ray crystallography image courtesy of
the British
Biophysical Society.
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