Nucleolus, Biology


  • 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.


Posted Date: 10/5/2012 8:06:22 AM | Location : United States

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