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12.2 The Structure of DNA
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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
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% 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
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Chargaff (1949) – studies A always same % as T C always same % as G - so, _______________ = Chargaff’s Rules
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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
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Two strands run in opposite directions
= antiparallel Are complementary P | covalent bond S – B ~ B – S | | P P |
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