Prokaryotic Ribosome: structural subunits and function

 Ribosomes present in all living cells. Ribosomes are made up of several RNAs and proteins. They act as site of protein synthesis where mRNA translation takes place. Ribosomes link each amino acid in the order specified by the codons on mRNA to form a polypeptide chain.

Bacterial ribosome structure

  • Several ribosomes come together to form polysome during the protein synthesis. The rate of protein synthesis depends on the number of ribosomes.
  • Bacterial cell contains about     10000 ribosomes and they contribute 30% of total dry weight of the cell.
  • In Bacterial cell ribosomes are present in the cytoplasm.
  • More number of ribosomes provide granular appearance to the cytoplasm.
  • Ribosomes are of two types 70s and 80s.
  • The 80s ribosomes are present in eukaryotic cells.
  • Bacterial cell contains 70s ribosomes
  • Here the "s" denotes svedberg units.
  • Svedberg units indicate the rate of sedimentation during ultracentrifugation.
  • The sedimentation rate depends on size, shape and weight of the molecule, for example svedberg value is more for the heavier molecules than the lighter molecules.
  • These units are named after Theodor Svedberg from sweden who discovered the principle of ultracentrifugation. He was awarded the nobel prize in 1926.

Structure of 70s Ribosome

  • The 70s ribosome is made of two subunits a small and a large subunit.
  • Smaller subunit is of 30s and the larger one is a 50s.
  • Similarly the 80s ribosomes in eukaryotic cell consist of a 40s and 60s subunits.

30s Subunit of Ribosome

  • The smaller subunit in 70s ribosome(30s) has 16 sRNA and comprise of 1540 nucleotides bound to 21 proteins.

50s Subunit of Ribosome

  • The larger subunit has 5 sRNA and 23 sRNA.
  • The 5sRNA comprise of 120 nucleotides bound to 31 proteins.
  • The 23sRNA comprise of 2900 nucleotides bound to the 31 proteins.

Protein synthesis by Ribosome

  • During the protein synthesis the mRNA binds to the smaller subunit(30s).
  • The larger subunit(50s) links amino acids by providing the peptide bonds to the polypeptide chain.
  • The two subunits are combined together. The strength of the attachment depends on the concentration of Mg++ ions.

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