Molecular Basis of Inheritance Practice Test

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Which description best characterizes the structure and strand orientation of DNA?

DNA consists of a single strand with the sugar-phosphate backbone on the inside and bases on the outside.

DNA is a double helix with two strands oriented in the same direction.

DNA forms a triple helix with bases pairing in a criss-cross pattern.

DNA is a double helix of two antiparallel strands with the sugar-phosphate backbone on the outside and base pairs A-T and G-C forming hydrogen bonds on the inside.

The structure being tested is that DNA forms a double helix made of two antiparallel strands, with the sugar–phosphate backbone on the outside and the bases stacked inside the helix. The bases pair specifically across the two strands: adenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds). This arrangement keeps the helix uniform in width and supports accurate replication because each base on one strand has a precise partner on the other strand. The idea that the molecule is a single strand with the backbone on the inside, or that both strands run in the same direction, or that DNA forms a triple helix, does not fit the observed and well-supported structure of DNA in cells.