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).

7. Image:https://sites.google.com

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 :
i) 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).

10. Image:https://www.kullabs.com

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.

12. Image:https://www.kullabs.com

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.

17. THANK YOU