Nucleolus, Biology

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.

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







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