 Solution

Use the standard Watson–Crick edges, with the glycosidic attachments at purine N9 and pyrimidine N1. The A–T pair has two hydrogen bonds: adenine N6–H donates to thymine O4, and thymine N3–H donates to adenine N1. The C–G pair has three: cytosine N4–H donates to guanine O6, guanine N1–H donates to cytosine N3, and guanine N2–H donates to cytosine O2. The middle and right panels above show these atoms and the sugar-attachment directions.

The major-groove side is the edge toward purine N7 and toward thymine O4/its methyl group or cytosine N4. The minor-groove side is toward purine N3 and adenine C2–H or guanine N2, and toward pyrimidine O2 and the sugar attachments. Groove labels refer to the exposed outer edges of the paired bases; the hydrogen bonds joining the bases are not themselves empty grooves. The resulting patterns differ even when the bases remain paired inside duplex DNA.