In this article we will discuss about the structure and functions of nucleolus.
Structure of Nucleolus:
Estable and Sotelo (1951) described the structure of a nucleolus under the light microscope. According to them, nucleolus consists of a continuous coiled filament called the nucleolonema embedded in a homogenous matrix, the pars amorpha. The first description of nucleolar ultra structure was given by Borysko and Bang (1951) and Bernhard (1952).
They described two main nucleolar components, a filamentous one corresponding to the nucleolonema and a homogenous one corresponding to the pars amorpha (matrix).
Later on, Gonzales- Remirez (1961) and Izard & Bernhard (1962) demonstrated that the nucleolonema consists of a spongy net work in place of a continuous filament. The ultra structure of the nucleolus have been reviewed by Day (1968), Bernhard and Granboulan (1968) and Bush and Smetana (1970).
Four chief components have been observed:
(i) An amorphous matrix or pars amorpha.
(ii) Chromatin containing abundant DNA.
(iii) Fibrils containing RNA, 80-100 Å in diameter, precursor of granules.
(iv) Granules- Ribonucleoprotein granules 150-200 Å in diameter.
Functions of Nucleolus:
(i) Ribosome formation or biogenesis of ribosomes.
(ii) Synthesis and storage of RNA:
It produces 70-90% of cellular RNA in many cells. It is source of RNA. The chromatin in nucleolus contains genes or ribosomal DNA (rDNA) for coding ribosomal RNA. Chromatin containing DNA gives rise to fibrils containing RNA. Granules containing RNA already produces ribosomes.
(iii) Protein synthesis:
Maggis (1960) and others have suggested that protein synthesis takes place in nucleolus. Other studies confirm the above views. In eukaryotes the gene coding for RNA contains a chain of at-least 100-1000 repeating copies of DNA. This DNA is given off from the chromosomal fibre in the forms of loops. The DNA loops are associated with proteins to form nucleoli.
The DNA seems as a template for 45S rRNA. Half the 45S rRNA is broken down to form 28S and 18S RNA. The other half is broken down further to nucleotide level. Within the nucleolus the 28S rRNA combines with proteins made in cytoplasm to form the 60S ribosomal sub-unit. The 18S rRNA also associates with proteins to form the 40 S subunit of the ribosome.