RIBOSOMES SMG

A brief account of Ribosomes - 70 S and 80S ; functions

1. RIBOSOMES Dr. Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry

2. RIBOSOMES First observed by George Palade (1955). Small, dense and granular particles , not bound by a membrane. Occur in most prokaryotic and eukaryotic cells.

3. In prokaryotic cells , the ribosomes occur freely in the cytoplasm. In eukaryotic cells, ribosomes occur either freely in the cytoplasm or remain attached to the outer surface of the rough endoplasmic reticulum (RER ) and nuclear envelope.

4. Number : Vary from cell to cell – abundant in cells where active protein synthesis takes place - meristematic cells , liver cells, pancreatic cells etc. Chemical composition : Ribosomes are made up of equal amounts of RNA and Protein[ Ribonucleoprotein (RNP) ]. Ribosomal RNA ( rRNA ) represents more than 80% of the RNA present in cells.

5. RIBOSOMES - STRUCTURE A spheroidal particle of 23 nm. Minute, irregular organelles without a membrane around them. Each Ribosome has two sub units - large sub unit, dome – shaped and a small sub unit – occur above the larger sub unit forming a cap - like structure .

6. Sub units occur separately in the cytoplasm and join to form Ribosomes only at the time of protein synthesis. During protein synthesis, many Ribosomes are aggregated on a common messenger RNA and form Polyribosome ( Polysomes or Ergosomes ).

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8. Ribosomes are usually isolated from the cell by differential centrifugation. The sedimentation coefficient of the Ribosomes is expressed in the Svedberg unit , ‘S’ unit. ‘S’ is related with the size and molecular weight of the ribosomal particles.

9. Types of Ribosomes : 70 S Ribosomes : Smaller in size – 20 to 30 nm in diameter. Have the sedimentation coefficient 70S. They have the sub units , 30 S (small sub unit ) and 50S (large sub unit ) – The 30 S sub unit is smaller in size and occurs above the large 50 S sub unit like a cap. Found in prokaryotes – Bacteria, Cyanobacteria (Blue green algae ) and plastids of plants( plastidoribosomes ).

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11. ii) 80 S Ribosomes : Larger in size – 25 to 35 nm in diameter. Have the sedimentation coefficient 80S. They have the sub units , 40 S (small sub unit ) and 60S (large sub unit ) – The 40 S sub unit is smaller in size and occurs above the large 60 S sub unit like a cap. In the absence of a suitable concentration of Mg++, the ribosomes dissociate reversibly into sub units. 80 S ribosomes occur in eukaryotic cells.

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13. RIBOSOMES - ULTRASTRUCTURE A eukaryotic Ribosome shows a groove at the junction of the small and large sub units. From this groove , a tunnel extends through the large sub unit and opens into a canal of the ER. The polypeptides are synthesized in the groove between the two ribosomal sub units and pass through the tunnel of the large sub unit into the ER.

14. RIBOSOME - ULTRASTRUCTURE Image:https :// www.biologydiscussion.com

15. RIBOSOMES - FUNCTIONS Ribosomes are the sites of protein synthesis. – ‘Protein factories’ of the cell. Two or more Ribosomes simultaneously engaged in protein synthesis on the same mRNA strand form polyribosomes . The Ribosomes functions as a template , bringing together different components involved in the synthesis of proteins.

16. Interaction of the t-RNA – aminoacid complex with m-RNA , which brings about translation of the genetic code is coordinated by the Ribosomes . Ribosomes also have a protective function. The m-RNA strand which passes between the two sub units of Ribosome is protected from the action of nucleases. The nascent polypeptide chains passing through the tunnel or channel between the sub units are protected against the action of proteases.

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