06 - Back to tertiary structure of DNA Both circular and...

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Primary structure of DNA: sugar-phosphate “chain” with purine and pyrimidine bases as side chain(s) Secondary structure of DNA double helical structure (hydrogen bonding between A-T and G-C; stacking interactions; phosphate backbone “outside”) Tertiary or higher order structure double stranded DNA (both circular AND linear) makes complexes with proteins – supercoil Supercoiling = coiling of a coil. Back to tertiary structure of DNA… Both circular and linear DNA : Both circular and linear DNA :
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Topological isomers - DNA differing only in their states of supercoiling A circular DNA without any superhelical turn is known as relaxed molecule . Important for packing of DNA one way of making DNA more “condensed” DNA helix becomes topographically linearized (locally uncoiled) during replication and transcription • Base paring is interrupted • DNA molecule exibits supercoiling . Supercoils reduce stress on DNA by twisting the double helix
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One DNA supercoil (superhelical turn) forms in the double helix for every 10 bp opened (for B DNA). Superhelical tension Strain is released mainly by writhing into superhelical turns it is easier (energetically “cheaper”) to bend long DNA then to untwist it. Situation in circular DNA molecules AND locally in long linear DNA molecules
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Right handed double helix Left handed supercoil http://www.youtube.com/watch?v=k4fbPUGKurI&feature=related
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Protein “pushes two strands in front of the protein become wrapped around each other more than once every 10bp (turns are “shorter”- fewer bases/”new” turn – overwinding of DNA) Consequence : formation of positive supercoils in front of the protein; occurs when right-handed B-DNA is twisted really tightly about its axis, the double helix begins to distort and knot into positive left-handed supercoils .
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Protein “pushes two DNA strands behind the protein become wrapped around each other less than once every 10bp underwinding of DNA (turns are ”longer”- more bases/ “new” turn) Consequence : formation of negative supercoils
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06 - Back to tertiary structure of DNA Both circular and...

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