Th e complementary chains are parallel, but run in opposite directions, and together they twist into a right-handed double helix. Th e helix forms two grooves of unequal size (Figure 2.10). Th e wider and more accessible major groove (Figure 2.11) allows regulatory proteins or other molecules to gain access to nucleotide functional groups on the edges of the groove. Th e narrower, less acces- sible minor groove (Figure 2.12) allows much more limited access to the func- tional groups lying within the groove.