A comparison of the B, A, and Z forms of DNA. (a) In each case, the sugar–phosphate backbones wind around the exterior of the helix (red and blue), with the bases pointing inward. The same 25 base pair DNA sequence is shown in all three forms. (As described later in the chapter, double-stranded RNA typically assumes the A-form geometry shown here.) Differences in helical diameter can be seen in the end-on views (top); differences in helical rise and groove shape are apparent in the side views (bottom). B-DNA, the most common form in cells, has a wide major groove and a narrow minor groove. A-form helices, common for RNA and certain DNA structures, are more compact than B-DNA. The major groove is deeper and the minor groove is shallower than in B-DNA. Z-DNA, which forms only under high salt conditions or with C≡G-rich DNA sequences, is left-handed, and its backbone has a zigzag pattern. It is less compact than B-DNA, with a very shallow major groove and a narrow and deep minor groove. (b) The table summarizes some properties of the three forms of DNA.