the major groove in the A-form is deeper and narrower than it is in B-form DNA, while the minor groove is wider and shallower (Figure 2.18 and Figure 2.19). Th e narrowing and deepening of the major groove in A-form helices means that it is more diffi cult for proteins to read out the sequence-specifi c information at the edges of the bases in A-form helices (that is, α helices cannot readily enter the narrower major groove of the A-form helix).