ribosomes

1. By
Subhananthini jeyamurugan
Roll no 18py17
I M.Sc., Microbiology
RIBOSOMES

2. • Cell have a tiny (small)
granular structures known as
Ribosomes
• Ribosomes are Ribonucleo-
Protein Particles.
• Ribosomes are assembly
shops for Protein Synthesis.
What are Ribosomes?

3. • The ribosomes are first noted
in plant cells by Ro Robinson
and Brown in 1953.
• Shortly afterward Palade in
1955 observed them in
animal cells.
• He isolated the ribosomes
and detected the RNA in
them in 1956.
Discovery of Ribosomes
PALADE

4. Ribosomes are occur in the
cells, both prokaryotic and
eukaryotic cells.
In prokaryotic cells the
ribosomes often occur
freely in the cytoplasm. But
in eukaryotic cells attached
to the outer surface of the
membrane of Endoplasmic
Reticulum.
Occurrence

5. • Bacterial Ribosomes are 70S type
• Eukaryotic ribosomes are 80S type
• Fungal mitochondrial ribosomes are 77S type
• Animal cell mitochondrial ribosomes are 60S type.
They differ in size but show the similar structure.
Size and Structure

6. The numbers of Ribosomes differs greatly.
A rapidly growing E. coli cell may have as
many as 15,000 to 20,000 ribosomes per
cell.
Number

7. • The 70S ribosomes is found in prokaryotic cells.
• Its very smaller in size and it has a sedimentation
co-efficient of 70S.
• It has a molecular weight of 3×106 Daltons. It is
composed of rRNA proteins.
• The 70S ribosomes made up of two subunits,
namely a large 50S and a small 30S.
• Each subunits made up of rRNA and ribosomal
proteins. The 50S subunit is composed of 23S
rRNA, 5S rRNA and about 30 different proteins.
• The 30S subunit is composed of 16S rRNA and
about 20 different proteins.
70S Ribosomes

8. • The 80S ribosomes are eukaryotes.
• It is larger in size and has a sedimentation co-
efficient of 80S.
• It has a molecular weight of 5×106 Daltons.
• It is composed of rRNA and Ribosomal proteins.
• The 80S ribosomes made up of two subunits,
namely a large 60S and a small 40S.
• Each subunits composed of ribosomal proteins and
rRNA. The 60S sub unit has 28S rRNA, 8S rRNA,
5S rRNA and about 50 different proteins.
• The 40S subunit has 18S rRNA and 30 different
ribosomal proteins.
80S Ribosomes

9. • In the beginning of protein synthesis, the two
subunits unite together and at the end of protein
synthesis they dissociate.
• Similarly the two ribosomes unite together form a
dimer.
• Many ribosomes together to form a polyribosome.
• The association of subunits as well as ribosomes
occur at a high concentration of Mg ++
• The dissociation is brought about by a low
concentration of Mg ++
Association and dissociation
of ribosomal subunits

10. Smaller
subunit
Larger
subunit
subunits dimer
80S 70S
40S 60S
30S 50S
Fig. association of ribosomes
Fig. dissociation of ribosomes

11. Matrix Ribosomes:
These synthesize proteins destined to
remain within the cell.
Plasma membrane Ribosomes:
These make proteins for transport to the
outside.
Types of ribosomes

12. There are two domains of Ribosomes
• Translational domain: the region responsible for
translation is called the Translational domain
• Both subunits contribute to this domain, located in
the upper half of the small subunit and in the
associated areas of the large subunit
• Exit Domain: the growing peptide chain emerges
from the large subunit at the exit domain
• This is located on the side of the subunit.
Domains of Ribosomes

13. • Ribosomes are spherical in shape.
• The ribosomes of prokaryotes are smaller in size
and those of eukaryotes are larger in size.
1. In prokaryotes, they are 150 Aº and
2. In eukaryotes, they are 250Aº in diameter.
• Ribosomes are spherical bodies.
• Each ribosomes consists of two subunits, namely
1. large subunits 2. small subunits
• The subunits occur separately in the cytoplasm.
• They join together to form ribosomes only at the
time of Protein synthesis.
Structure of Ribosomes

14. Continue…
• Each subunits is constructed from one to two rRNA
molecules and many polypeptides
• 30S smaller Subunit
• 50S larger Subunit.
• 30S subunit is smaller and has a molecular weight of
0.9×106 Daltons
• it is made up of 16 S rRNA and 21 polypeptide chains.

15. • The subunit is larger one and has a molecular weight of
about 1.8×106 Daltons.
• It consists of 5S rRNA, 23S rRNA and 34 polypeptide
chains.

16. • The ribosomes has
three sites for binding
t-RNA
• The peptidyl or donor
site ( P site)
• The Aminoacyl or
Acceptor Site ( the A
site)
• The exit site ( the E
site)
Sites of Ribosomes

17. • The ribosomes contain RNAs, proteins, enzymes and metal
ions.
1. Ribosomal RNA.
2. Ribosomal proteins.
3. Enzymes
a. Initiation factor
b. Elongation factor
c. Termination factor.
4. Metal ions.
Chemical composition

18. 1. Ribosomal RNA
• The RNA present in the ribosomes are called
rRNA.
• In eukaryotic cells, rRNAs are found in four forms,
namely 28S rRNA, 18S rRNA, 5S rRNA and 5.8S
rRNA. 18S rRNA present in small subunit and others
are found in the larger subunit.
• In prokaryotic cells, they are form of 23S rRNA, 16S
rRNA and 5S rRNA. 5S rRNA present in small subunit
and others are found in the larger subunits.
2. Ribosomal proteins:
The 70S ribosomes contain 50 to 60 proteins.
The 80S ribosomes has 70 to 80 proteins. These
proteins are two types, namely
• core proteins (CP)
• Spilt proteins (SP)

19. Continue…
3. Enzymes
Play important role in Protein synthesis.
They are 3 groups of factors.
1. initiation 2. elongation 3. termination factors.
4. Metal ions
Ribosomes contain a number of metal
ions such Mg, Ca, Mn, Fe, etc.

30. 1. Protein synthesis
2. Decoding of mRNA
3. Acceptance of correct Amino Acyl + RNA
4. Peptide bond formation
5. Translocation of peptidyl tRNA
6. Translocation of deacylated tRNA
7. Exit of deacylated tRNA
8. Protection.
Functions of Ribosomes

31. 1. Protein synthesis
Ribosome plays important role in protein synthesis.
• It is assembly shop or engine where amino acids
are linked to produce proteins.
• In the beginning of protein synthesis, the two
subunits unite together and at the end of protein
synthesis they dissociate
• .Similarly the two ribosomes unite together form a
dimer.
• Many ribosomes together to form a polyribosome.
2. Decoding of mRNA
• 30s subunit involved in decoding.

32. 3. Acceptance of correct Amino Acyl + RNA.
The ribosome allows the correct amino acyl t-
RNA to be attached to the A site.
4. Peptide bond formation
During protein synthesis, ribosome links the
amino acids by peptide bonds. The 50S subunit helps in
peptide bond formation.
5. Translocation of peptidyl tRNA
Peptidyl tRNA is shifted from A site to P site. The
30S and 50S subunits are involved in translocation.

33. 6. Translocation of deacylated tRNA
The deaylated tRNA is shifted from P site to
E site.
7. Exit of deacylated tRNA
The deacylated tRNA is removed from E
site.
8. Protection
The mRNA passing through the ribosomes is
protected from nucleases. Similarly, newly
synthesised polypeptide chains are protected from
proteases.