Types of ribosomes, Biology

TYPES OF RIBOSOMES

Ribosomes are of two types:-

1. Procaryotic Ribosomes -

70 s = Occur in cytoplasm of prokaryotes, and also in mitochondria, and Chloroplast of eukaryotes

2. Eukaryotic Ribosomes -

80 s = Occur in cytoplasm of eukaryotic cells.

  • Each ribosome composed of two subunits i.e. larger and smaller subunits.
  • Larger subunit is dome-shaped and smaller is ovoid. Smaller subunit has a platform, a cleft, a head and a base.
  • The larger subunit has a protuberance a ridge and a stalk. Larger subunit also contains a tunnel which opens in cavity of E.R.
  • 40s + 60s = 80s
  • 30s + 50s = 70s
  • Two subunits are held together by Mg+2. Mg+2 form ionic bond with phosphate groups of rRNA of two subunits.
  • Minimum 0.001M Mg+2 concentrations required for structural formation of ribosomes. If Mg+2 concentration increased 10 times then ribosome DIMER are formed.

583_ribosomes.png

 

  • In prokaryotes 100s type of dimers are formed, whereas in eukaryotes 120s dimers are formed.

1036_ribosomes1.png

52_ribosomes2.png

  • At the time of proteins synthesis, several ribosomes become attached to m-RNA with the help of smaller subunits.
  • This structure is called polyribosome or polysome or Ergosome.
  • The ribosomes move along the m- RNA like beads on a string. Larger subunit contains peptidyl transferase enzyme which catalyses synthesis of peptide bond bet amino acids. Larger subunit has two sites.

(i) A-site - Acceptor site for t-RNA

(ii) P-site - site for growing polypeptide chain

  • After synthesis on ribosomes protein are transported in cytoplasm and organelles.
  • The proper folding of proteins following synthesis is assisted by specific proteins called chaperons which also assist in transport of proteins.
Posted Date: 10/5/2012 7:48:32 AM | Location : United States







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