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COURSE BIOMOLECULES Lesson DNA and its various types Dr. Praveen Kumar Agrawal
end Double helical structure and specification 3 end HO The structure given by Watson and Crick is of B DNA B-DNA is an alpha helix, meaning that it has a right handed . or clockwise, spiral. . There are 10 base pairs (bp) per 360-degree rotation of 2 nm3.4 nm the helix. . So each base pair is twisted 36 degrees relative to the adjacent bases. The base pairs are 0.34 nanometer (nm) apart 0.34 .So each complete rotation of the molecule T A encompasses 3.4 nm. This length is called pitch of the DNA The diameter of the helix is 2 nm, and the bases are perpendicular to the long axis of the DNA molecule. 3 end 5' end
A and Z DNA A-DNA Another secondary structure that DNA can assume is . the A-DNA structure. .It exists if less water is present. Like B-DNA, A-DNA is an alpha (right-handed) helix. . It is shorter and wider than B-DNA. . Its bases are tilted away from the main axis of the molecule. Z-DNA This form of DNA occurs if DNA is placed in high salt concetration. . It forms a left-handed zigzag helix. It is left handedly coiled.
28A (a) A form (b) B form (c) Z form
dfgd dfg Very Rare Right handedly coiling Parameters B DNA Rare (under high salinity) Left handedly coiling Occurrence Very common Coiling Right handedly coiling (Alpha helix) 10 (Alpha helix) Number of base pairs per turn Diameter 12 2 nm 2.6 nm 1.8 nm Pitch length 3.4 nm 2.86 nm 4.4 nm
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