Double-helical structure of DNA was given by whom?

Watson and Crick proposed the double helix model in 1953 in the journal Nature. They built it using Rosalind Franklin's X-ray diffraction data (Photo 51) and Chargaff's base-ratio rules. Two antiparallel strands are held together by hydrogen bonds, with adenine pairing with thymine and guanine pairing with cytosine.

• A) Jacob and Monod worked on gene regulation and proposed the operon model (lac operon, 1961). This has nothing to do with DNA's physical structure.
• C) H.G. Khurana is known for his work on gene synthesis and the genetic code. He shared the 1968 Nobel Prize in Physiology or Medicine with Holley and Nirenberg. He did not propose the double helix.
• D) Darwin gave the theory of natural selection in On the Origin of Species (1859), decades before DNA's structure was known.

Core concept: the double helix explains how DNA stores information and replicates semi-conservatively. For Mains, this links to GS-III biotechnology, genetic engineering, and the ethics of gene editing.

Trap: candidates pick by name association, such as 'Khurana and genetics' or 'Jacob-Monod and genes'. Anchor the double helix to 1953 and Watson-Crick.