natural purification (자정 작용
| natural language processing | Computer processing of a language with rules that reflect and describe current usage rather than prescribed usage. (12 Dec 1998) |
|---|---|
| natural mutation | A mutation which occurs by itself without first being affected by a mutagen, for example during the process of DNA replication. Spontaneous mutations arise at a remarkably constant rate. The rate that spontaneous mutations arise has been used as an evolutionary clock to estimate how closely related two (or more) separate species are to each other. (09 Oct 1997) |
| natural passive immunity | Immunity conferred by the mother on the foetus or newborn. (14 Nov 1997) |
| natural pigment | A naturally occurring coloured compound; absorbs light in the visible range of the electromagnetic spectrum. Compare: structural colour. Synonym: biochrome. (05 Mar 2000) |
| natural products | Naturally occurring compounds that are end products of secondary metabolism; often, they are unique compounds for particular organisms or classes of organisms. (05 Mar 2000) |
| natural selection | The hypothesis that genotype environment interactions occurring at the phenotypic level lead to differential reproductive success of individuals and hence to modification of the gene pool of a population. (18 Nov 1997) |
| immunity, natural | The capacity of a normal organism to remain unaffected by microorganisms and their toxins. It results from non-specific mechanisms such as genetic endowment, naturally occurring antiviral and bactericidal substances, phagocytosis, and constitutional factors such as body temperature, oxygen tension, and age. (12 Dec 1998) |
| killer cells, natural | Cells responsible for spontaneous cytotoxicity of a variety of tumour cells without prior immunization. These natural killer cells are found in non-immune humans and experimental animals and are thought by some to be the same as killer cells (killing by antibody-dependent cell cytotoxicity), but they can also kill in the absence of antibody. (12 Dec 1998) |
| abnormalities, radiation-induced | Congenital changes in the morphology of organs produced by exposure to ionizing or non-ionizing radiation. (12 Dec 1998) |
| accidents, radiation | Accidental dispersal of radioactive materials from a radiation source. Accidents at nuclear reactors can involve large groups of the population from dispersion of radioactivity into the environment and through fallout or a few individuals with high injurious doses. (12 Dec 1998) |
| acoustic radiation | The fibres that pass from the medial geniculate body to the transverse temporal gyri of the cerebral cortex by way of the sublentiform part of the internal capsule. Synonym: radiatio acustica. (05 Mar 2000) |
| acute radiation syndrome | <syndrome> A syndrome caused by exposure of the body to large amounts of radiation, (e.g., from certain forms of therapy, accidents, and nuclear explosions; it is divided into three major forms which are, in ascending order of severity, the haematogic, gastrointestinal, and central nervous system-cardiovascular forms; its clinical manifestations are divided into prodromal, latent, overt, and recovery stages. (05 Mar 2000) |
| adaptive radiation | <chemistry> The evolution of new speciesor sub-species to fill unoccupied ecological niches. (06 May 1997) |
| alpha radiation | <physics, radiobiology> The most easily absorbable type of radiation, it consists of a stream of alpha particles, doubly ionised helium nuclei which are electrically charged and produce intense ionisation in matter. Alpha radiation can be deflected in electromagnetic fields. (09 Oct 1997) |
| annihilation radiation | The radiation resulting when a positron from beta positive decay comes to rest. It encounters an electron, and they annihilate each other and convert their rest mass into two 0.51-MeV gamma rays emitted in exactly opposite directions. (05 Mar 2000) |