12.2 The Structure of DNA

DNA = deoxyribonucleic acid Made of repeating nucleotides (subunit) Nucleotide = sugar (deoxyribose), phosphate, base 4 bases – adenine, guanine, cytosine, thymine A & G = larger purines C & T = smaller pyrimidines

% of each nucleotide base ANIMALS A T G C Human 30.4 30.1 19.6 19.9 Sheep 29.3 28.3 21.4 21.0 Sea Urchin 32.8 32.1 17.7 17.3 Salmon 29.7 29.1 20.8 20.4 PLANTS Wheat 27.3 27.1 22.7 22.8 BACTERIA E. coli 24.7 23.6 26.0 25.7 VIRUSES Lambda 21.3 22.9 28.6 27.2

Chargaff (1949) – studies  A always same % as T C always same % as G - so, _______________ = Chargaff’s Rules

Franklin – (1952) – X-ray diffraction - showed DNA as a tightly coiled spring = helix Watson & Crick (1953) – used Franklin’s and Chargaff’s info to determine structure Double helix held together by weak Hydrogen bonds

Two strands run in opposite directions = antiparallel Are complementary P |  covalent bond S – B ~ B – S | | P P |