What is the basic structure of DNA?

• A. A double helix of two antiparallel polynucleotide strands with A-T and C-G base pairing.
• B. A single strand with random pairing.
• C. A triple helix arrangement.
• D. Circular DNA only in bacteria.

Answer: (A)

DNA's basic structure is a double helix formed by two polynucleotide strands running in opposite directions. The sugar-phosphate backbones are on the outside, while the bases pair inside: adenine with thymine (two hydrogen bonds) and cytosine with guanine (three hydrogen bonds). This base pairing creates two complementary strands, so the sequence on one strand determines the sequence on the other. The strands are antiparallel, with one 5' to 3' and the other 3' to 5'. This arrangement enables accurate replication because each strand can serve as a template for a new partner strand. While many bacterial DNAs are circular, that describes a form rather than the defining structure—the key idea is the double-stranded, antiparallel helix with specific base pairing.