NUCLEOLUS

• Discovered by Fontana and name given by Bowman.
• It is naked structure that is attached to chromatin at specific point called nucleolar organiser region or NOR.
• NOR is a type of heterochromatin.
• Nucleolus known as ribosomal factory, ribosomal protein is synthesized in the cytoplasm.
• There is normally one nucleolus with each set of chromosomes.
• Somatic cells of man may have five nucleoli (on 13, 14, 15, 21, and 22 chromosomes) while oocyte of Amphibians have approximately 2000 nucleoli. Nucleus of onion cells have 4 nucleoli.
• True nucleolus is also termed as Plasmosomes and they are stainable by acid dyes. Cell actively engaged in protein synthesis have large nucleolus. For example oocytes, neurons etc.
• Nucleolus remains absent in sperm cell, muscles cell and blastomeres during cleavage.
• Nucleolus contains following parts -

(1) Fibrous part - Central part of nucleolus composed of fibers called nucleonema. Nucleonema mainly composed of r-DNA and proteins.

(2) Granular part - Peripheral part of nucleolus composed of granules (r-RNA + proteins) which represents maturing ribosomes.

(3) Amorphous matrix or Pars amorpha - A proteinaceous matrix which contains both fibers and granules. r-RNA 5.8s, 18s and 28s are synthesized in nucleolus. Ribosomal protein is synthesized in cytoplasm. Organisation of r-RNA and protein to form ribosomes occurs in nucleolus. Completely formed ribosomes then move to cytoplasm through nuclear pores. Nucleolus also play important role in initiation of cell division.