Structure of nucleoli in first-order spermatocytes of selected free-living animal species.

Nucleoli are the product of the activity of nucleolar organizer regions (NOR) in certain chromosomes. Their main functions are the formation of ribosomal subunits from ribosomal protein molecules and the transcription of genes encoding rRNA. Nucleoli are present in the nuclei of nearly all eukaryotic cells because they contain housekeeping genes.