fall 09 lecture 06

fall 09 lecture 06 - Back to tertiary structure of DNA Both...

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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 :
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Supercoils And: DNA functions in replication and transcription depend on its topological state. DNA helix becomes topographically linearized (locally uncoiled): Base paring is interrupted DNA molecule exibits supercoiling . Important: Supercoils - reduce the induced stress by twisting the double helix Topological isomers - DNA differing only in their states of supercoiling A circular DNA without any superhelical turn is known as relaxed molecule. Supercoils – important for “packing” of DNA – one way of making DNA more “condense”
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Supercoiling cont. 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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Positive and Negative Supercoils Protein “pushes” in front of the protein two strands become wrapped around each other more than once every 10bp (turns are “shorter”- fewer bases/”new” turn – overwinding of DNA ) Consequence : formation of positive s uperc oils in front of the protein; it occurs when right-handed B-DNA is twisting really tightly about its axis, until the double helix begins to distort and knot into positive left-handed supercoils .
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Positive and Negative Supercoils Protein “pushes” behind the protein two strands become
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fall 09 lecture 06 - Back to tertiary structure of DNA Both...

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