DNA double helix

The DNA double helix is the three-dimensional structure of deoxyribonucleic acid, first deduced by James Watson and Francis Crick at the Cavendish Laboratory, Cambridge, in 1953, in a single-page paper published in Nature on 25 April 1953. Their model rested decisively on the X-ray diffraction photographs—most famously "Photograph 51"—produced by Rosalind Franklin and Raymond Gosling at King's College London, and on Erwin Chargaff's earlier finding (Chargaff's rules) that the molar quantity of adenine equals that of thymine, and guanine equals cytosine. Watson, Crick and Maurice Wilkins shared the Nobel Prize in Physiology or Medicine in 1962; Franklin, having died in 1958, was ineligible. The discovery is regarded as one of the foundational moments of molecular biology.

Structurally, DNA consists of two polynucleotide strands wound around a common axis to form a right-handed helix. Each strand is a backbone of alternating deoxyribose sugar and phosphate groups, with one of four nitrogenous bases—adenine (A), thymine (T), guanine (G) or cytosine (C)—projecting inward. The strands are held together by hydrogen bonds between complementary base pairs: adenine pairs with thymine via two hydrogen bonds, and guanine with cytosine via three. The two strands run antiparallel, one in the 5′→3′ direction and the other 3′→5′. The dominant biological form, B-DNA, completes one full turn roughly every 10.5 base pairs (about 3.4 nanometres) and exhibits major and minor grooves where regulatory proteins bind. This complementary base pairing provides the chemical basis for semiconservative replication, demonstrated experimentally by Matthew Meselson and Franklin Stahl in 1958, whereby each strand serves as a template for a new partner strand.

The double helix underpins the central dogma of molecular biology, articulated by Crick in 1958: information flows from DNA to RNA to protein. It enabled the Human Genome Project (1990–2003), modern DNA fingerprinting (developed by Alec Jeffreys in 1984 and now standard in forensic identification), recombinant DNA technology, and the gene-editing tool CRISPR-Cas9, for which Emmanuelle Charpentier and Jennifer Doudna received the 2020 Nobel Prize in Chemistry. As of 2026 the double-helix model remains universally accepted, with active applications spanning personalised medicine, ancestry testing, and the DNA-based identification regimes adopted by several states.

For competitive examinations the DNA double helix appears chiefly in the General Science and General Studies components—UPSC Prelims and the science portions of CSS, BCS and the Guokao—as well as in current-affairs questions on biotechnology, gene editing and forensic DNA legislation. Typical question angles include: the year 1953 and the names Watson and Crick; the contribution of Rosalind Franklin; the complementary base-pairing rule (A–T, G–C) and the number of hydrogen bonds in each pair; Chargaff's rules; the antiparallel and right-handed nature of B-DNA; and the distinction between DNA and RNA (deoxyribose versus ribose, thymine versus uracil). Candidates should also be able to connect the structure to applications such as the Human Genome Project and CRISPR.