BIOL 180 Chapter Notes - Chapter 4: Phosphodiester Bond, The Double Helix, Hydrogen Bond

• School: UW
• Department: Biology
• Course: BIOL 180
• Professor: Freeman Scott

Document Summary

Bond angles and measurements suggest with of helix = 2. 0 nm and distance between bases stacked in spiral = 0. 34 nm. Watson and crick constructed series of physical models: allowed them to mess around with different types of helical configurations. They arranged two dna strands side by side going in opposite directions. Strands with this type of orientation are antiparallel. If antiparallel strands are twisted together to form double helix, coiled sugar phosphate backbones end up on outside of spiral and nitrogenous bases on the inside. Bases must form purine-pyrimidine pairs to fit in interior of 2. 0-nm-wide structure. Key point: pairing allows hydrogen bonds to form between certain purines & pyrimidines, adenine forms hydrogen bonds with thymine, guanine forms hydrogen bonds with cytosine. A-t and g-c bases are complementary 2 hydrogen bonds: a(denine)-t(hymine) = 2 hydrogen bonds, g(uanine)-c(ytosine) = 3 hydrogen bonds. A-c and g-t pairs allow none or only one hydrogen bond.