Lecture11_NABPty2012 (1)

The x pattern is indicative of a helix with a pitch

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Unformatted text preview: teps of 3.4 Å. http://osulibrary.oregonstate.edu/specialcollections/coll/pauling/dna/pictures/franklin-typeBphoto.html H-bonds between Watson-Crick base pairs provide specificity in the double helix but most of the stability comes from base stacking interactions H O N C C N NH N NH N A C O N N CH3 CC C C C N G and C form three H-bonds O H C C C-H C NH N H N C H H H C C C G HN C C H O A forms two Hbonds with T or U C-H HN N T Base-pairing explains Chargaff’s rules for the base composition of DNA: A = T; G = C “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” -- J.D. Watson & F.H.C. Crick, Nature 171: 737 (1953) The B-form DNA helix has a diameter of about 20 Å ~20 Å Base Base pairs fill the center of the helix; the phosphates are on the outside. A base pair is more exposed to the solvent on one side (the “major groove”, at the top in these views) than the other (the “minor groove”, bottom). B-form DNA consists of a right-handed double helix with antiparallel strands 3.4 Å per base-pair 5’ 3’ minor major groove 34 Å (10 bp) per turn major groove minor groove 3’ 5’ These dimensions are for DNA fiber...
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