What contribution was made by x-ray defraction towards the
determination of the structure of dna, and what other uses does it
have in forensic science

Hi

 x-ray diffraction (not d-e-fraction) was (and is) primary tool for
study of the structure of solid matter.  It was first developed by
Laue and Bragg to determine structure of crystals.
 
 Basic idea is explained here (Java required).
 http://www.matter.org.uk/diffraction/x-ray/rotating_crystal_method.htm
 
 More detailed and technical explanation given here includes very nice
interactive simulator. (you need to enter some number, like 10 or 100,
into the entry box on the right to see simulation in action).
 http://www.uni-wuerzburg.de/mineralogie/crystal/teaching/ibasic_b.html

 After the war, Dr. Franklin extended method to study graphite
structures (what we today call nanotubes and fullerens and buckyballs)
and later to  organic polymers, which included DNA molecule:

"..At 26, Franklin had her PhD and the war was just over. She began
working in x-ray diffraction -- using x-rays to create images of
crystallized solids. She pioneered the use of this method in analyzing
complex, unorganized matter such as large biological molecules, and
not just single crystals..."
http://www.pbs.org/wgbh/aso/databank/entries/bofran.html

 She was a master of technique and in a series of brilliant
experiments determined the structure of  DNA as double helix.
 Here are some details of that:
 http://www.aip.org/pt/vol-56/iss-3/p42.html
 and a bit about her life. She died early- and Nobel prize cannot be
awarded posthumously. Other people got it and gave her no credit.
http://www.accessexcellence.org/RC/AB/BC/Rosalind_Franklin.html

Polymers (very long molecules) such as DNA have three scales of
structure (primary is chemical formula,secondary is sequence of
repeating units (mers) and tertiary is gross geometry of molecule:
coil, helix, or folded chain...

Secondary is used for forensic ID and that has no direct connection
with tertiary structure discovered by Franklin.
Primary and tertiary are universal, same for all life, but secondary
is different for all humans except for identical twins, clones and
very rare chance.

More about that here:
http://www.accessexcellence.org/RC/VL/GG/
 tertiary on the right
 http://www.accessexcellence.org/RC/VL/GG/dna2.html
 
 Secondary is (almost) unique (see long article given by this very
long URL( all between [] to be pasted into address field of the
browser)
[
 http://images.google.com/imgres?imgurl=http://www.codefun.com/Images/Genetic/seeing/image015.jpg&imgrefurl=http://www.codefun.com/Genetic_see.htm&h=247&w=473&sz=18&tbnid=Nf9Ro3tCi6cJ:&tbnh=65&tbnw=124&start=13&prev=/images%3Fq%3Dgenetic%2Balphabet%26hl%3Den%26lr%3Dlang_en%26ie%3DUTF-8
 ] or more simple one
 http://www.ornl.gov/sci/techresources/Human_Genome/publicat/tko/05b_img.html
 
 primary on the right
 http://www.accessexcellence.org/RC/VL/GG/basePair2.html
 
SEARCH TERMS: Franklin, Laue, X diffraction, DNA structure, copolymers


Hedgie

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Clarification of Answer by hedgie-ga on 03 Apr 2005 09:14 PDT

Appendix

DNA testing for ID purposes (no X diffraction here)
http://www.dnalc.org/resources/aboutdnafingerprinting.html

Use of X diffraction in forensics lab (no DNA here):
 
 It is a standard tool in analytical lab 
http://www.brad.ac.uk/acad/chemistry/research.html

used e.g. to identify drugs
http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=13043246&dopt=Abstract

general review

X-RAY DIFFRACTION IN FORENSIC SCIENCE

http://www.rigakumsc.com/journal/Vol20.1.2003/forensic.pdf

 Hedgie

(rating is appreciated)

Brilliant answer,much more detail than i had hoped for and much
appreciated. very quick