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.

Cajal body function in genome organization and transcriptome diversity

“…These data showed that HLBs not only concentrate essential limited factors for histone pre-mRNA processing but that the absence of HLBs results in specific transcriptional disruption and a small but significant level of spurious polyadenylation of unprocessed histone pre-mRNA. PML NBs and nucleoli have also been suggested to be important for transcriptional activation and repression of physically-associated gene loci, but the exact role of the PML NB in this process is still unclear [39,91]. These studies indicate that the formation of specialized nuclear microenvironments allows cells to produce large quantities of specific RNAs through enrichment of limited but essential protein factors.…”

Ribosomal RNA gene functioning in avian oogenesis

“…Keeping in mind the multifunctional nature of the nucleolus (Boisvert et al 2007;Hernandez-Verdun et al 2010;Shaw and Brown 2012;Grummt 2010Grummt , 2013Olson and Dundr 2015), we are tempted to assume that the nucleolus breakdown and disappearance at the mid-lampbrush stage oocytes is a sign of the so-called Bnucleolar stress^ (Boulon et al 2010;Grummt 2013;James et al 2014;Olson and Dundr 2015). This stress results in p53 transcription factor stabilization followed by cell growth arrest and ultimately apoptosis (Horký et al 2002;Manfredi 2010;Pederson 2011;Grummt 2013).…”

Coming to terms with chromatin structure

“…One of these sub-compartments is the nuclear lamina; the lamina-associated domains (LADs) are large genomic regions that are generally transcriptionally repressed and are located along the nuclear envelope (Guelen et al 2008). Other nuclear structures that associate with specific loci include the nucleolus (Nemeth and Langst 2011;Olson and Dundr 2001), transcription factories (Osborne et al 2004) and a variety of nuclear bodies, such as promyelocytic leukaemia (PML) nuclear bodies (Ching et al 2013), Cajal bodies (Smith and Lawrence 2000) and histone locus bodies (Liu et al 2006;Nizami et al 2010). This spatial proximity between specific loci and nuclear factors or sub-compartments may be a consequence of either active movement directed by nuclear skeletal elements (Bridger 2011;Bridger et al 2014;Chuang et al 2006;Dundr et al 2007) or may reflect a passive mass action and selfassembly of functional domains that stabilize once formed (reviewed in Chuang and Belmont 2007).…”