Bioc 2580 final exam notes: dna double helix. 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. 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. Applied methods of x ray crystallography to study dna: obtained first useful x ray diffraction images. Contains a lot of information about average structure of components of the dna. 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.