The minor groove in B-DNA is about 1.2 nanometers wide.
DNA’s two sugar-phosphate backbones do not sit directly opposite one another, so the double helix creates two unequal channels: the wider major groove and the narrower minor groove. In the standard B-DNA form, the minor groove measures approximately 12 Å, equivalent to 1.2 nanometers, while the major groove is about 22 Å, or 2.2 nanometers, wide.
These grooves are biologically important because they expose chemical features of the base pairs without requiring the DNA strands to separate. The major groove usually provides more sequence information and is the principal binding site for many DNA-recognizing proteins. The minor groove can also bind proteins, small molecules, and dyes such as Hoechst 33258.
The tempting wrong answer of 2.2 nanometers is actually close to the major-groove width, not the minor groove. Groove dimensions are approximate rather than perfectly fixed: DNA sequence, bending, hydration, bound proteins, and local structural changes can alter them. The values refer to ordinary B-DNA geometry.