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Medical Definition of Cell nucleolus
1. Within most types of eukaryotic cell nucleus, a distinct region, not delimited by a membrane, in which some species of rrna (RNA, ribosomal) are synthesised and assembled into ribonucleoprotein subunits of ribosomes. In the nucleolus rrna is transcribed from a nucleolar organiser, i.e., a group of tandemly repeated chromosomal genes which encode rrna and which are transcribed by RNA polymerase I. (12 Dec 1998)