Nucleolus: Structure and Function

Abstract The nucleolus is the nuclear subdomain that assembles ribosomal subunits in eukaryotic cells. The nucleolar organiser regions of chromosomes, which contain the genes for pre‐ribosomal ribonucleic acid ( rRNA ), serve as the foundation for nucleolar structure. The nucleolus disassembles at the beginning of mitosis, its components disperse in various parts of the cell and reassembly occurs during telophase and early G1 phase. Ribosome assembly begins with transcription of pre‐ rRNA . During transcription, ribosomal and non‐ribosomal proteins attach to the rRNA . Subsequently, there is modification and cleavage of pre‐ rRNA and incorporation of more ribosomal proteins and 5S rRNA into maturing pre‐ribosomal complexes. The nucleolus also contains proteins and RNAs that are not related to ribosome assembly and a number of new functions for the nucleolus have been identified. These include assembly of signal recognition particles, sensing cellular stress and transport of human immunodeficiency virus 1 ( HIV ‐1) messenger RNA . Key Concepts The nucleolus, whose primary function is to assemble ribosomes, is the largest structure in the cell nucleus. The nucleolus organiser regions of chromosomes, which harbour the genes for pre‐rRNA, are the foundation for the nucleolus. All active nucleoli contain at least two ultrastructural components, the nucleolar dense fibrillar component representing early pre‐ribosomal complexes and the granular component containing more mature pre‐ribosomal particles. Most nucleoli in higher eukaryotes also contain fibrillar centres, which are the interphase equivalents of the nucleolus organiser regions. The nucleolus disassembles at the beginning of mitosis and begins to reassemble in telophase. Ribosome assembly begins with the transcription of pre‐rRNA by RNA polymerase I. Ribosomal and non‐ribosomal proteins and 5S RNA associate with the pre‐rRNA during and after transcription. The pre‐rRNA is modified and processed into rRNA with the aid of non‐ribosomal proteins and small nucleolar RNAs. The nucleolus has numerous other functions including assembly of signal recognition particles, modification of transfer RNAs and sensing cellular stress.