In his article we will discuss about the role of nucleolus in production of ribosomes (explained with diagram).
The nucleolus is the site for rRNA transcription and processing as well as for assembly of ribosomal subunits. The nucleolus is organised at the nucleolus organising regions (NORs) of some chromosomes. These chromosomal regions contain the genes for 5.8S, 18S and 28S rRNAs.
There are four types of RNAs present in eukaryotic ribosomes, namely, 5S, 5.8S, 18S and 28S rRNAs. Among these, three, that is, the 5.8S, 18S and 28S are transcribed as a single unit within the nucleolus by RNA polymerase I producing a 45S ribosomal precursor RNA.
The 45S pre-rRNA undergoes processing to yield 18S rRNA for the small ribosomal subunit as well as 5.8S and 28S rRNAs for the large ribosomal subunit. The 5S rRNA which is also present in the large subunit of ribosomes is transcribed outside the nucleolus by RNA polymerase III.
Ribosomes are central to protein synthesis and present in very large numbers, about 5 to 10 million in actively growing mammalian cells. The synthesis of ribosomes requires transcription of very large numbers of rRNA molecules. Cells meet these requirements by the presence of multiple copies of rRNA genes.
The human genome contains about 200 copies of the gene that encodes the 5.8S, 18S and 28S rRNAs and about 2000 copies of the gene that encodes 5S rRNA. The genes for 5.8S, 18S and 28S rRNA are clustered in tandem arrays on 5 different human chromosomes, namely chromosomes 13, 14, 15, 21 and 22, while the 5S rRNA genes occur in a single tandem array on chromosome 1.
Electron microscopically three regions can be distinguished in a nucleolus, the fibrillar centre, dense fibrillar component and the granular component. The rRNA genes are located in the fibrillar centre. Transcription occurs at the boundary of the fibrillar centre and the dense fibrillar component.
Processing of the pre-rRNA initiates in the dense fibrillar component and proceeds into the granular component. In the granular component, rRNA is associated with ribosomal proteins to form pre-ribosomal particles that are exported into the cytoplasm.
Assembly of Ribosomes:
The formation of ribosomes is completed by assembly of ribosomal precursor RNA with both ribosomal proteins as well as 5S rRNA. The genes that encode ribosomal proteins are transcribed outside the nucleolus by RNA polymerase II to produce mRNAs that are translated on cytoplasmic ribosomes. Ribosomal proteins thus formed, are transported from the cytoplasm to the nucleolus where they associate with rRNAs to form pre-ribosomal particles.
Although the genes for 5S rRNA are transcribed outside the nucleolus by RNA polymerase III, the assembly of 5S rRNA into pre-ribosomal particles takes place in the nucleolus. Ribosomal proteins associate with rRNAs while pre-rRNA is still being synthesised.
The smaller ribosomal subunit which contains only 18S rRNA matures earlier than the large subunit which contains three types of rRNAs, 5S, 5.8S, and 28S rRNAs. The pre-ribosomal particles are then transported out into the cytoplasm where active 40S and 60S subunits are formed (Figure below).
