Which statement best describes the structure of a DNA molecule?

• A. A single-stranded helix composed of nucleotides, with bases exposed to the environment and no specific base-pairing rules
• B. Two antiparallel nucleotide strands twisted into a double helix, with sugar-phosphate backbones on the outside and complementary base pairs (A-T and G-C) on the inside
• C. A triple helix of nucleic acid strands in which each base pairs with two complementary bases
• D. Two parallel strands held together by covalent bonds between complementary bases, forming a straight ladder-like structure

Answer: (B)

Verified step by step guidance

1. Recall the fundamental structure of DNA as discovered by Watson and Crick, which is essential to understanding the molecule's properties.
2. Identify that DNA consists of two strands made up of nucleotides, where each nucleotide contains a sugar, a phosphate group, and a nitrogenous base.
3. Understand that these two strands run in opposite directions, meaning they are antiparallel, which is crucial for the molecule's stability and function.
4. Recognize that the strands twist around each other to form a double helix, with the sugar-phosphate backbones on the outside providing structural support.
5. Note that the nitrogenous bases pair specifically (adenine with thymine, and guanine with cytosine) through hydrogen bonds on the inside of the helix, ensuring accurate base pairing and genetic information storage.