recombination

Recombination is a general term for processes in which two or more components join to form a single product. Different fields use the term in related but distinct senses: genetic recombination reshuffles DNA; cosmological recombination concerns the formation of neutral atoms in the early universe; and chemical or plasma recombination describes the combination of oppositely charged or reactive fragments to yield neutral species.

Genetic recombination occurs during meiosis and DNA repair. It is initiated by double-strand breaks, followed by processing and strand invasion that allows exchange of genetic material between homologous chromosomes. This creates new allele combinations, increases genetic diversity, and can repair damaged DNA. Recombination is guided by enzymes such as Spo11 in eukaryotes and involves crossover and non-crossover outcomes.

In cosmology, recombination happened about 380,000 years after the Big Bang, when electrons combined with protons and nuclei to form neutral hydrogen and helium. The photons released during recombination decoupled from matter, producing the cosmic microwave background. After recombination, the universe became transparent to radiation. Reionization later reintroduced free electrons as the first stars formed.

In chemistry, recombination reactions involve radicals, ions, or fragments combining to yield neutral molecules. Examples include radiative recombination of a free electron with a positive ion, or three-body recombination in dense environments. Recombination processes are important in atmospheric chemistry, plasma physics, and semiconductor device operation.