1. The central controlling body within a living cell, usually a spherical unit enclosed in a membrane and containing genetic codes for maintaining the life systems of the organism and for issuing commands for growth and reproduction. 2. The nucleus of a cell is an organelle (i.e., a cellular organ) that is essential to such cell functions as reproduction and protein
synthesis. It is composed of nuclear sap and a nucleoprotein-rich network from which chromosomes and
nucleoli arise and is enclosed in a definite membrane. See also
Nucleoli.