A structure within the nucleus that stains densely with basic dyes and consists of proteins associated with RNA. The number and distribution of nucleoli varies but is usually characteristic for any one cell type. Electron micrographs show a central area of short fibres surrounded by a matrix of protein material with granules embedded in the peripheral region. Nucleoli are closely associated with the regions of chromosomal DNA that code for ribosomal RNA (see nucleolar organizer
). The transcription of the code is dependent on a specific RNA polymerase found only in the nucleolus. A long precursor molecule is formed initially and is processed to produce two shorter molecules. The longer of these associates with proteins in the nucleolus to form the larger ribosomal subunits. The smaller molecules similarly associate with proteins to form the smaller subunits, possibly in the nucleolus or surrounding nucleoplasm. The formation of complete *ribosomes
from the subunits only occurs when the latter reach the cytoplasm.