Ribosomes are small organelles found in most prokaryotic and eukaryotic cells that are the sites of protein synthesis. They are composed of RNA and protein and exist in two subunits - a small subunit that binds to the mRNA and a large subunit where protein synthesis occurs. In prokaryotes, 70S ribosomes freely float in the cytoplasm, while in eukaryotes 80S ribosomes are either free in the cytoplasm or bound to the endoplasmic reticulum. Ribosomes bring together mRNA and tRNA to translate the genetic code and synthesize polypeptide chains.

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.

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