Fine structure of the nucleolar organizer of salivary gland chromosome of chironomids

Electron microscopic studies of the salivary gland chromosomes of C. plumosus and C. cingulatus have shown that the chromosome region associated with the nucleolus proper is characterized by coils of 100 Å chromosome fibers, pars amorpha, and nucleolonema. These three components form a disklike structure extending through the chromosome. Coils of 100 Å chromosome fibers also extend into the nucleolus proper. The intrachromosomal pars amorpha and nucleolonema resemble those of the nucleolus proper. It is suggested that only the coils of 100 Å chromosome fibers represent the nucleolar organizer.