Ribosome

           RIBOSOMES

👉 Ribosomes are the granular structures first observed under the electron microscope as dense particles by George Palade (1953). They are composed of ribonucleic acid (RNA) and proteins and are not surrounded by any membrane.

👉 Except mammalian RBC all living cells have ribosomes. (Both prokaryotes and eukaryote)

👉 Ribosomes are smallest cell organelles 

👉 Ribosomes are also called as "Organelle within organelle" and "protein factory of cell"

Types of Ribosomes:- 

(1) Eukaryotic ribosomes:- 80s - Occur in cytoplasm of eukaryotic cells.

(2) Prokaryotic ribosomes:- 70s - Occur in cytoplasm and associated with plasma membrane of prokaryotic cell. Their size is 15 to 20 nanometre.

👉 70s ribosome also present in mitochondria and chloroplast of eukaryotes.

👉 S=Svedberg unit or Sedimentation rate. It indirectly is a measure of density and size.

👉 Each ribosome composed of two subunits i.e. larger and smaller subunits.

                          80s = 60s + 40s

                          70s = 50s + 30s

   

👉 Magnesium ion is essential for the binding the ribosome sub units. Mg+2 form ionic bond with phosphate groups of r-RNA of two subunits. Minimum 0.001M Mg+2 concentration is required for structural formation of ribosomes.

Chemical Composition of Ribosomes 

70s - 60% r-RNA + 40% proteins 

80s - 40% r-RNA + 60% proteins 

60s - r-RNA 28s,5.8s,5s

40s - r-RNA 18s

50s - r-RNA 23s,5s

30s - r-RNA 16s

👉 At the time of protein synthesis, several 70s ribosomes become attached to m-RNA with the help of smaller subunits. This structure is called polyribosome or polysome or Ergosome. Larger subunit (50s) contents peptidyl transferase enzyme (23s r-RNA) which helps in the formation of peptide bond during Protein synthesis. This is an example of ribosome. (Noller 1992)

Cytoskeleton