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DNA Double Helix

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BIOC 2580
E P Wijekoon

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:  A=T C=G  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 • Recognized that purine and pyrimidine bases of DNA can form complementary pairs linked by H bonds  A on one strand T on the other forming a double helix • Each DNA strand encodes the same biological information in complementary fashion  Each strand serves as template of partner strand Watson and Crick Base Pairs • CG has 3 H bonds • AT has 2 H bonds
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