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BIOC 2580 Study Guide - Erwin Chargaff, Photo 51, The Double Helix

Course Code
BIOC 2580
E P Wijekoon

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BIOC 2580 Final Exam Notes
4. DNA Double Helix
Identifying DNA as the Genetic Material
Friederich Miescher (1868)
First isolated DNA
Called it Nuclein
Oswald Avery (1944)
Identified DNA as genetic material
DNA extracted from virulent strain of bacteria injected into non virulent strain of same bacteria
resulted in bacterium being transformed into virulent strain
Confirmed that the information for virulence was carried in DNA
Discovery of Structure of DNA
Erwin Chargaff (1940s)
Found that in all cellular DNAs the number of A residues were equal to the number of T residues
Same for C and G
Known as Chargaff’s Rules:
Sum of purine residues = sum of pyrimidine residues
% of GC and AT varied among different organisms
Rosalind Franklin
Applied methods of X ray crystallography to study DNA
Obtained first useful X ray diffraction images
Photo 51
Contains a lot of information about average structure of components of the DNA
Showed that DNA molecules are helical
Had 2 periodicities along the long axis: primary - 3.4 A secondary – 34 A
First need to isolate DNA for X Ray crystallography
Difficult DNA is very long in chromosomes and easily sheared
Watson and Crick
1953 were able to come up with DNA secondary structure from previous data
Built models
From photo 51: DNA had to be a helix of some sort
Figured out width of helix and spacing of bases
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