What Is Nucleolus? Function and Structure of Nucleolus

The nucleolus (/njuːˈkliːələs, -kliˈoʊləs/, plural: nucleoli /-laɪ/) is the biggest structure in the nucleus of eukaryotic cells. It is also known as the site of ribosome biogenesis. Nucleoli also join in the formation of recognition particles and play a vital role in the cell's response to stress. They are made of proteins, DNA and RNA and form particular chromosomal regions called nucleolar organizing regions. Nucleolopathies can be caused to several humans due to the malfunction of nucleoli and the nucleolus is being investigated as a target for cancer chemotherapy.


During the 1830s the nucleolus was identified by bright-field microscopy. Till 1964, very few were aware of the function of the nucleolus, when a study of nucleoli by John Gurdon and Donald Brown in the African clawed frog Xenopuslaevis create an interest in the function and in-depth detailed of the structure of the nucleolus. In the study, they found that 25% of the frog eggs had no nucleolus and those eggs were incapable for life. Partially some of the eggs had one nucleolus and 25% had two. In nutshell, the nucleolus had a function necessary for life. According to Birnstiel in 1966, and other collaborators showed via nucleic acid hybridization experiments that DNA within nucleoli code for ribosomal RNA.

RNA Structure

There are three major components of the nucleolus are recognized: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC). Transcription of the rDNA falls in the FC. The DFC contains the protein fibrillarin, which is necessary for rRNA processing. The GC contains the protein nucleophosmin, (B23 in the external image) which is also mixed up with ribosome biogenesis. Nevertheless, it has been planned that this particular organization is only observed in higher eukaryotes and that it evolves from a bipartite organization with the transition from anamniotes to amniotes. Reflecting the extensive increase in the DNA intergenic region, a new fibrillar component would have divided into the FC and the DFC.
The different structure has identified within many nucleoli (particularly in plants) is a clear area in the middle of the structure pointing to as a nucleolar vacuole. It has various plant species which have very high concentrations of iron in comparison to human and animal cell nucleoli.

With the help of an electron microscope, ultra structure of nucleolus can be easily seen, while the organization and dynamics can be studied through fluorescent protein tagging and fluorescent recovery after photobleaching (FRAP). Antibodies against the PAF49 protein can also be used as a marker for the nucleolus in immune fluorescence experiments. Although usually only one or two nucleoli can be seen, a diploid human cell has ten nucleolus organizer regions (NORs) and could have more nucleoli. Most often multiple NORs participate in each nucleolus.

Function and Ribosome Assembly

In ribosome biogenesis, two of the three eukaryotic RNA polymerases are required, and these functions are done in a step by step manner. In the first stage, the rRNA genes are transcribing as one unit within the nucleolus by RNA polymerase I. In order for this transcription to occur, several test pol I-associated factors and DNA-important trans-acting factor are needed. In humans, a similar PIC has assembled with SL1, the promoter selectivity factor (composed of TBP and TBP-associated factors, or TAFs), transcription beginning factors, and UBF (upstream binding factor). RNA polymerase I transcribe most rRNA transcripts 28S, 18S, and 5.8S) but the 5S rRNA subunit (the component of the 60S ribosomal subunit) is transcribed by RNA polymerase III.
Transcription of rRNA yields a long originator molecule (45S pre-rRNA) which still holds the ITS and ETS. In a later stage, more processing is needed to produce the 18S RNA, 5.8S and 28S RNA molecules. In eukaryotes, the RNA-update enzymes are brought to their individual recognition sites by communication with guide RNAs, which join these exact sequences. RNAs guide belongs to the group of small nucleolar RNAs (snoRNAs) which are complicated with proteins and exist as less-nucleolar-ribonucleoproteins (snoRNPs). Once the rRNA subunits are passed, they are ready to be organized into larger ribosomal subunits. On the other hand, an extra pair of rRNA molecule, the 5S rRNA, is also necessary. 

In yeast, the 5S rDNA series is localized in the intergenic spacer and is transcribed in the nucleolus by RNA Pol.
As you go high in eukaryotes and plants, the condition is more complicated, for the 5S DNA sequence lies outside the Nucleolus Organiser Region (NOR) and is transcribed by RNA pol III in the nucleoplasm, after which it finds its way to enter into the nucleolus to contribute in the ribosome assembly. This assembly not only includes the rRNA but ribosomal proteins as well. The genes encoding these r-proteins are transcribed by pol II in the nucleoplasm by a "conventional" pathway of protein synthesis (transcription, giving out of pre-mRNA, nuclear export of mature mRNA and translation on cytoplasmic ribosomes). The mature r-proteins are carried out, “imported" back into the nucleus and finally into the nucleolus. 

Association and maturation of rRNA and r-proteins result in the formation of the 40S (small) and 60S (large) subunits of the complete ribosome. 

These are exported through the nuclear pore complexes to the cytoplasm, where they remain free or become connected with the endoplasmic reticulum, forming a rough endoplasmic reticulum

Key Concepts

• The nucleolus, whose key function is to hold ribosomes together, is the main structure in the cell nucleus.
• The nucleolus controller regions of chromosomes, which harbor the genetic factor for pre‐rRNA, are the basis for the nucleolus.
• All active nucleoli comprise two ultrastructure constituents, the nucleolus thick febrile constituent representing early pre‐ribosomal complexes and the granular constituent having some additional mature pre‐ribosomal particles.
• Mostly the nucleoli in higher eukaryotes also have fibrillar centers, which are the interphase equals of the nucleolus organizer areas.
• The nucleolus disassembles at the starting of mitosis and arises to reassemble in telophase.
• Ribosome assembly starts with the transcription of pre‐rRNA by RNA polymerase I.
• Ribosomal and non‐ribosomal proteins and 5S RNA links with the pre‐rRNA during and after transcription.
• The pre‐rRNA is altered and processed into rRNA with the aid of non‐ribosomal proteins and minor nucleolar RNAs.
• The nucleolus has many other functions containing an assembly of signal recognition particles, alteration of transfer RNAs and sensing cellular stress.