What is the structure of a DNA molecule?

• A. A double helix formed by two antiparallel polynucleotide strands
• B. A single strand of RNA
• C. A circular molecule with a single backbone
• D. A triple helix involving three strands

Answer: (A)

The main idea being tested is how DNA is built. DNA is a double helix formed by two polynucleotide strands that run in opposite directions (antiparallel). Each strand has a sugar-phosphate backbone, with the sugar being deoxyribose. The bases—adenine pairing with thymine and cytosine with guanine—project inward and form hydrogen bonds between the two strands, holding the helix together. Because the strands are complementary, the sequence on one strand determines the sequence on the other, which is crucial for accurate replication. This structure is stable due to hydrogen bonding and base stacking, and it gives DNA its characteristic uniform shape. It isn’t a single RNA strand, nor a triple-helix arrangement, and while some DNA can be circular in bacteria or organelles, the standard description is a double helix of two antiparallel strands.