transition

1. Passage from one condition or one part to another. 2. In polynucleic acid, replacement of a purine base by another purine base or a pyrimidine base by a different pyrimidine. [L. transitio, fr. transeo, pp. -itus, to go across]

- cervicothoracic t. the junction between the last cervical vertebra and first thoracic vertebra.

- isomeric t. the t. of a nuclear isomer to a lower quantum state; e.g., ^131mXe → ¹³¹Xe + γ.

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tran·si·tion tran(t)s-'ish-ən, tranz-, chiefly Brit tran(t)s-'izh- n

1) passage from one state or stage to another esp an abrupt change in energy state or level (as of an atomic nucleus or a molecule) usu. accompanied by loss or gain of a single quantum of energy

2) a genetic mutation in RNA or DNA that results from the substitution of one purine base for the other or of one pyrimidine base for the other

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tran·si·tion (tran-zĭґshən) [L. transitio crossing over] 1. a passage or change from one state or condition to another. 2. in molecular genetics, a point mutation in which one purine base replaces another purine or one pyrimidine base replaces another pyrimidine. Cf. transversion.