The Nucleolus

The nucleolus was first recognized by Fontana in 1874. It is usually a spheroidal body situated within the nucleus, either in a central or peripheral position. It is found in close association with the nucleolar organizer region of two or more chromosomes. Actively synthesizing cells have large nucleoli.
Nucleoli were divided into two main groups, plasmasomes and karyosomes. Plasmasomes or ‘true nucleoli’ stain with acidic dyes and disappear during mitosis. Karyosomes or ‘false nucleoli stain with basic dyes and are flakes of chromatin.    

Structure

Four chief components have been recognized: an amorphous matrix, nucleolar associated chromatin, fibrils and granules.

1. The matrix or pars amorpha of the nucleolus is homogeneous. It contains scattered granules and fibrils.

2. Chromatin. Associated with the nucleolus is chromatin which contains abundant DNA. The chromatin is Feulgen positive. The DNA serves as a template for the synthesis of RNA. Surrounding the nucleolus like a shell is perinucloelar chromatin.



3. The fibrils are 80-100A in diameter. They contain RNA and are probably the precursors of the granules.

4. Granules. Rib nucleoprotein granules 150-200A in diameter make up the pars granulose. The primary nucleon undergoes folding to form the secondary nucleolonema. The nucleolonema may also have fibrils.

Function

1. RNA Production. The nucleolus is one of the most active sites of RNA synthesis. It produces of 70-90 percent of cellular RNA in many cells. RNA in many cells. It is the source of ribosomal RNA (rRNA). The nucleolus thus makes ribosomal precursors, rather than whole ribosomes.

CHROMATIN → FIBRILS → GRANULES → RIBOSOMES

The nucleolus may also produce some types of messenger RNA (mRNA), and at least one type of low molecular weight RNA.

2. Protein synthesis. Maggio (1960) and others have suggested that protein synthesis takes place in the nucleolus. It this is true then the ribosomal proteins are produced in the nucleolus.

3. Ribosome Formation. In eukaryotes the gene coding for RNA contains a chain of at least 100 to 1,000 repeating copies of DNA. This DNA is given of in the form of loops from the chromosomal fiber.