| environ. |
mutation A change in the chemical constitution of the DNA in the chromosomes of an organism: the changes are normally restricted to individual genes, but occasionally involve serious alteration to whole chromosomes. When a mutation occurs in gametes or gametocytes an inherited change may be produced in the characteristics of the organisms that develop from them. Mutation is one of the ways in which genetic variation is produced in organisms. A somatic mutation is one that occurs to a body cell, and is consequently past on to all the cells derived from it by mitosis. Natural mutations, at this stage of biological evolution, when they occur in the cells of higher animals, almost always produce deleterious characteristics. Both natural and artificial mutations can be brought about by ionizing radiation (hence the genetic and carcinogenic dangers of nuclear weapons) and by certain chemical substances called mutagens (生物体中染色体DNA的化学成分的变化：变化通常限于单个基因，但偶尔涉及严重的改变整个染色体。当突变发生在配子或配子体时，遗传的变化可能产生。突变是生物体产生的遗传变异的途径之一。一个细胞体发生突变是一个体细胞，并因此传递到所有从它派生的有丝分裂细胞。自然突变，在这个生物进化阶段，当他们出现在高等动物的细胞中时，几乎总是产生有害的特性。自然和人为的突变可通过电离辐射(来自核武器的基因和致癌危险)以及某些称为诱变剂的化学物质而产生。) |
