BiologyDifficulty 2-4

DNA

Deoxyribonucleic acid: the molecule that carries the genetic instructions used in the growth, development and reproduction of all known living organisms. DNA is shaped as a double helix of two strands, each made of a sugar-phosphate backbone with attached bases — adenine (A), thymine (T), guanine (G) and cytosine (C) — that pair specifically (A with T, G with C) to hold the two strands together. Its structure was determined by James Watson and Francis Crick in 1953, building on X-ray diffraction data produced by Rosalind Franklin and Maurice Wilkins.

Gene Expression: From DNA to Protein

A gene's information is expressed in two stages. During transcription, RNA polymerase reads one DNA strand and builds a complementary messenger RNA (mRNA) copy. During translation, a ribosome reads the mRNA three bases (a codon) at a time and assembles the corresponding chain of amino acids into a protein.

Protein-coding genes make up only a small proportion of the human genome — commonly cited estimates put it at roughly 1-2% of the total 3.2 billion base pairs, with the human genome containing on the order of 20,000 protein-coding genes.

Info: This two-step flow — DNA transcribed to mRNA, mRNA translated to protein — is the central dogma of molecular biology; it explains how a purely chemical sequence of bases ultimately specifies a cell's proteins.

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Cell

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Evolution

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Cell — Part of

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Sources and methodology

DNA's structure, determined by James Watson and Francis Crick in 1953, is a right-handed double helix formed by two antiparallel polynucleotide strands, each built on a sugar-phosphate backbone.
- Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid
Base pairing in DNA is specific: adenine (A) pairs with thymine (T) via two hydrogen bonds, and guanine (G) pairs with cytosine (C) via three hydrogen bonds.
- Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid
DNA replicates semiconservatively: each newly formed DNA molecule retains one original parental strand and synthesises one newly made strand, as demonstrated by Meselson and Stahl's 1958 density-gradient experiment.
- The replication of DNA in Escherichia coli
Watson and Crick's 1953 double-helix model built directly on X-ray diffraction data produced by Rosalind Franklin and Maurice Wilkins.
- Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid

Content status: published, last reviewed 30 September 2026.