the heterochromatic region of a chromosome that retains the tightly spiralized structure of a chromonema between two successive cell divisions in the interphase. When stained with nuclear dyes, the chromocenter appears under the microscope to be a solid body.

The size and number of chromocenters differ in the interphase nuclei of different organisms and in different tissues of the same organism. Large chromocenters are generally formed by grouped regions of the pericentromere, nucleolar, and telomere hetero-chromatin, as well as by sex chromosomes. In some organisms the number of large chromocenters coincides with the number of chromosomes; in others it is smaller, owing to the fusion of chromocenters, or larger. If polyploid nuclei come into being during the differentiation of somatic cells, complex chromocenters may appear owing to the uniting of the chromocenters of homologous and nonhomologous chromosomes. In Drosophila and in some other dipteran insects, a single large chromocenter is formed in cells with giant polytene chromosomes through the uniting of the centromere regions of all the chromosomes.

A given group of chromocenters reflects the number of chromosome regions inactive in ribonucleic acid (RNA) synthesis and, accordingly, the functional characteristics of the nuclei of different types of cells. The functions of the chromocenter have not been adequately clarified. Frequently repeating sequences of deoxyribonucleic acid (DNA) are generally localized in chromocenters formed by pericentromere heterochromatin.