What is the structure of DNA?

• A. A single chain of nucleotides
• B. A double helix formed by two strands of nucleotides connected by base pairs
• C. A triple-stranded configuration
• D. A circular formation found in prokaryotes

Answer: (B)

The structure of DNA is accurately described as a double helix formed by two strands of nucleotides connected by base pairs. This model was elucidated by James Watson and Francis Crick and is fundamental to our understanding of genetics.

In this double-stranded structure, each strand is made up of a backbone composed of sugar and phosphate molecules, with nitrogenous bases (adenine, thymine, cytosine, and guanine) attached to the sugar. The two strands run in opposite directions (antiparallel) and are held together by hydrogen bonds between complementary bases: adenine pairs with thymine, and cytosine pairs with guanine. This pairing mechanism is crucial for DNA replication and transcription processes.

A single chain of nucleotides does not provide the stability or the necessary information transfer mechanism that a double helix does. While circular formations are characteristic of prokaryotic DNA such as plasmids, this description does not capture the classic double-helix structure that is prevalent in eukaryotic organisms. Similarly, a triple-stranded configuration is not a naturally occurring form of DNA; the standard structure recognized in all biological systems is indeed the double helix. Therefore, the choice describing DNA as a double helix