C Z

FIGURE 6-11 Models Of the B, A, and Z forms ot DNA. The sugar-phosphate back bone of eacfi chain is on the outside in ail structures {one purple and one green) with the bases (silver) oriented inward. Side views are shown at the top, and views along the helical axis at the bottom, (a) The B form of DNA the usual fomn found in cells, is characterized by a helicat turn every )0 base pairs (3 4 nm); adjacent stacked base pairs are 0.34 nm apart. The mttpr and minor grooves are also visible (b) The nore compact A form of DNA has ! I base pairs per turn and exhibits a large tilt of the base pairs with respect to the helix axis. !n addition, the A form has a central hole (bottom) This helical form is adopted by RNA-DNA and RNA-RNA helices, (c) Z DNA is a left-handed helix and has a zigzag (hence 7") appearance. (Source: Courtesy of C Kielkopf and P. 8. Dervan.)

a 6 DNA

b A DNA

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