= Solution
Use the standard Watson–Crick edges, with the glycosidic attachments at purine N9 and pyrimidine N1. The A–T pair has \b[two <hydrogen bonds>]: <adenine> N6–H donates to <thymine> O4, and <thymine> N3–H donates to <adenine> N1. The C–G pair has \b[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>.
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