What structure did Watson and Crick help define for DNA?

• A. Single-stranded
• B. Triple Helix
• C. Helix-less strand
• D. Double Helix

Answer: (D)

The idea being tested is how DNA is shaped. Watson and Crick proposed that DNA is a double helix—two long polynucleotide strands wrapped around a common axis. The sugar-phosphate backbone sits on the outside, while the bases pair on the inside. The strands run antiparallel and twist in a right-handed manner, with a regular diameter and about ten base pairs per turn. Specific base pairing (adenine with thymine, guanine with cytosine) explains Chargaff’s rules and makes the two strands complementary, so each strand can serve as a template to recreate the other.

This model aligns with X-ray diffraction data that show a regular, helical pattern and with the observed consistency in width, which single-stranded, triple-helix, or nonhelical structures cannot explain. Thus, the double helix is the structure Watson and Crick helped define.