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.

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

• 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.