BIO206H5 Lecture Notes - Lecture 3: Nucleic Acid Double Helix, Chemical Polarity, Pyrimidine

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Native state of dna: 2 antiparallel polynucleotide chains, chemical polarity, (cid:1007)" 5" phosphodiester li(cid:374)kage, double helix, sugar phosphate backbone. Why are base-pairs only between purine and pyrimidines: because they fit in the space, purine + purine = too big, pyrimidine + pyrimidine = too small. Why only 4 bases: 4 bases allow for 4n different combinations, which is a lot considering a human cell usually has 3x109 nucleotides. Hydrogen bonds: 2 between a t, 3 between c g. C- g pairs are more stable, harder to denature because they have more hydrogen bonds. Length a-t = c-g: 1. 085 nm or 10. 85 angstroms. Center of helix not center of base pairs: because base pairs are of different length, creates the turn, and the major and minor grooves. Helical structure: to unwind, dna must separate its strands, bases lie (cid:862)flat(cid:863, dna approx. 2 nm thick: 1 rotation = 3. 4 nm, 10 nucleotides per turn.

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