Biology

The basic structural and functional unit of all known living organisms. Every living thing is made of one or more cells, each bounded by a membrane and containing the machinery needed to grow, respond to its environment, and reproduce. Cells fall into two broad types: prokaryotic cells (such as bacteria), which lack a nucleus, and eukaryotic cells (such as those in plants, animals and fungi), which organise their DNA inside a membrane-bound nucleus.

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.

The process by which populations of living organisms change in their heritable characteristics over successive generations. Charles Darwin and Alfred Russel Wallace proposed natural selection as its principal mechanism: individuals with heritable traits better suited to their environment tend to survive and reproduce more successfully, so those traits become more common in the population over time.