Alternative helical structures aform bform zform

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Unformatted text preview: el shows how closely the bases are packed within the helix. Note the locaDon of the major and minor grooves. Comparison of A & B forms of DNA Most DNA is in the B form Double ­stranded RNA and DNA ­RNA hybrids are in the A form. The 2 hydroxyl of RNA is too close to the phosphate and carbon 8 on the adjacent base for RNA to adopt a B form. Alternative Helical Structures! A ­form B ­form Z ­form Plasticity of DNA Structure Nucleic acid bases can exist in syn or an0 conformaDons. Z-DNA: A Radically Different Helical Structure! • First crystal structure of defined sequence (Rich, 1979) • Like A ­DNA and B ­DNA, strands are anD ­parallel and base pairs are standard • Helix is narrower and longer 44.6 Å 34 Å • Requires repeaDng sequence of pyrimidine ­purine Z-DNA: A Radically Different Helical Structure! • First crystal structure of defined sequence (Rich, 1979) • Like A ­DNA a...
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