Which statement best describes the overall DNA structure described?

• A. DNA is a single strand with random bases
• B. The backbone is sugar-only, without phosphate
• C. DNA forms a double helix with two strands held together by specific base pairing (A-T and G-C)
• D. Bases pair with thymine in all cases

Answer: (C)

DNA is organized as two long polynucleotide strands that twist into a double helix, running in opposite directions. Each strand has a sugar-phosphate backbone, so the backbone includes phosphate groups and is not sugar-only. The two strands are held together by hydrogen bonds between complementary bases: adenine pairs with thymine, and guanine pairs with cytosine. This specific base pairing gives the molecule a uniform width and allows accurate replication, since each strand can serve as a template for the other. Statements describing a single strand, a backbone without phosphate, or non-specific thymine pairing don’t match how DNA is actually structured.