Help! Physicist needs Chemist's help!

Hiranya Roychowdhury hroychow at NMSU.EDU
Wed Aug 13 11:18:43 EST 1997

At 12:00 AM 8/13/97 GMT, Stew2 wrote:
>I need a little info on DNA finger-printing.  I need to know why S35 is
>used, and if possible, can it be substituted with a stronger radioactive

I am no chemist in the strictest sense, but I know why 35S is prefered over
stronger (32P) nucleides. This is a purely physical reason: because of the
hard-hitting betas of 32P, their is scattering and secondary emmissions
resulting in much broader bands. Hence, very closely spaced bands on the
X-ray film are not resolved satisfactorily. Because of the saofter emmission
from 35S, this the bands developed on the film are much more sharp. Lately,
33P is being used by some researchers in conventional hybridizations and
this is said to offer better resolution than 32P, but I have never used it
and I do not know much about it.

 I would also like to know if anyone knows the smallest (on the
>average) size of a "line" on the x-ray plate is.

I am not sure what you mean by '"line" on the x-ray plate', but if you are
alluding to something similar to the lines on an oscilloscope screen, I
believe the X-ray films do not have any such "lines". The emulsion quality
(ie. colloid size and the composition) is what makes films differ in their
sensitivities and resolutions. But then I may be wrong, since I am not a

  I would appreciate it if
>you could e-mail me any answers, or info on where I can find answers. 
>email----Stew2 at

Dr. Hiranya Sankar Roychowdhury
Plant Genetic Engineering Lab.
New Mexico State University
Las Cruces, NM 88003
Ph. (505) 646-5785
hroychow at

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