Ribosome synthesis occurs in special regions of the nucleus called nucleoli (singular = nucleolus). Most eukaryotic genomes contain large numbers of copies of the genes for ribosomal RNAs, and these are clustered in the nucleolus. Here enzymes and other proteins facilitate rRNA transcription, and it is within nucleoli that the ribosomal subunits are formed. Newly transcribed rRNA is processed into functional sequences that bind with dozens of proteins, and these subunits then move from the nucleolus to the cytoplasm via nuclear pores. Cells vary widely in the number and size of nucleoli they contain, and this can depend on the activity of the cell. Large nucleoli are often found in cells busily engaged in protein synthesis, and cells may increase the number of nucleoli at certain times. Oocytes of some amphibians may have thousands of nucleoli, which can produce up to a trillion ribosomes during egg maturation.
Related Molecular Functions
Related Biological Processes