Origin, Chemical composition, Structure, and Function

1. Ribosomes – Origin, Chemical composition,
Structure and Function
Assignment presentation on
Submitted To
Mr. Manish Sharma
Assistant Research Scientist
Pulse research station,
S.D.A.U.,
Sardarkrushinagar-385 506.
Submitted By
Damor Kalpeshkumar Mohanbhai
Reg No: 04-AGRMA-01983-2019
C. P. C. A., S.D.A.U.,
Sardarkrushinagar-385 506.

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. Discovery of Ribosomes
• The ribosomes are first noted in plant cells by
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.
• Albert Claude, Christian de Duve, and George Emil
Palade were jointly awarded the Nobel Prize in
Physiology or Medicine, in 1974, for the discovery
of the ribosome.
• The Nobel Prize in Chemistry 2009 was awarded to
Venkatraman Ramakrishnan, Thomas Steitz and
Yonath for determining the detailed structure and
mechanism of the ribosome.

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. • It is the process of making
Ribosomes in the nucleus
• The protein parts are made in the
cytoplasm (Ribosome)
• Then transferred to the nucleus
(Nuclear Pores)
• rRNAs are transcribed in the
nucleolus
• The ribosomal proteins and
rRNAs bind together
• Small and large subunits are made
• They are transported out of nucleus
(Pores)
Ribosome Biogenesis

6.  Ribosomes are spherical in shape.
 The ribosomes of prokaryotes are smaller in size and those of eukaryotes are larger in
size.
o In prokaryotes, they are 150 Aº and
o In eukaryotes, they are 250Aº in diameter.
 Ribosomes are spherical bodies.
 Each ribosomes consists of two subunits, namely
o 1.large subunits
o 2. small subunits
 The subunits occur separately in the cytoplasm.
 They join together to form ribosomes only at the time of Protein synthesis.
 Each subunits is constructed from one to two rRNA molecules and many polypeptide
Structure of Ribosomes

7. 30S small 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

8. 50S large subunit
• 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

9.  The S in 70S and similar values stand for Svedberg units
 The faster a particle travels when centrifuged, the greater its Svedberg
value or Sedimentation coefficient
 The sedimentation coefficient is a function of a particles molecular
weight, volume and shape
 Heavier and more compact particles normally have larger Svedberg
numbers or sediment faster
Svedberg unit

10. 70S Ribosomes
 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.

11. 80S Ribosomes
 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, 5.8S rRNA, 5S rRNA and about
50 different proteins.
 The 40S subunit has 18S rRNA and 30
different ribosomal proteins.

12.  Matrix Ribosomes (Mitochondrial Ribosomes ):
• Smaller than cytoplasmic ones
• Sedimentation coefficient of 55S with 35S and 25S subunit
• More similar to bacterial ribosome
• Protein synthesis takes place in mitochondria with the help of mitochondrial
ribosome
• These synthesize proteins destined to remain within the cell.
 Plasma membrane Ribosomes or Cytoribosomes:
o These make proteins for transport to the outside.
Types of ribosomes

13. There are two domains of Ribosomes
 Translational domain:
o The region responsible for translation is called the Translational domain
o 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:
o The growing peptide chain emerges from the large subunit at the exit domain
o This is located on the side of the subunit.
Domains of Ribosomes

14. • The ribosomes has three sites for binding t-RNA
• The peptidyl or donor site (P site) – Which
holds the tRNA with the nascent peptide chain
• The Aminoacyl or Acceptor Site ( the A site) –
Which accepts the incoming aminoacylated
tRNA
• The exit site (the E site) – which holds the
deacylated tRNA before it leaves the ribosome.
Sites of Ribosomes

15. The ribosomes contain RNAs, proteins and metal ions.
1. Ribosomal RNA.
2. Ribosomal proteins.
3. Metal ions.
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.
Chemical Composition

16. 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
a.core proteins (CP)
b.Spilt proteins (SP)
3. Metal ions
Ribosomes contain a number of metal ions such Mg, Ca, Mn, Fe, etc.

17. Functions of Ribosomes
• Main function of the Ribosome is involved in the process of Protein
Synthesis
• Protein Synthesis is divided into three stages:
1. Initiation
2. Elongation
3. Termination

18. Initiation
• The necessary Components Assemble:
• The two ribosomal subunits
• A tRNA with the anticodon UAC
• The mRNA molecule to be translated
• Along with several additional protein factors
• In E.coli and most bacteria translation begin with specially modified aminoacyl tRNA,
N-formylmethionyl tRNA
• Because the α-amino is blocked by a formyl group, this aminoacyl tRNA can be used
only for initiation
• This N-formylmethionyl-tRNA attaches itself to the P Site of ribosome(Peptidyl Site)
• mRNA have a special “Initiation Codon” (AUG) that specifically binds with the fMet
tRNA anticodon
• Finally, the 50S subunit binds to the 30S subunit mRNA, forming an active ribosome
mRNA complex
• The attachment of two Subunits is controlled by Mg+2 ions.

19. Initiation

20. Elongation
• At the beginning of elongation cycle, the Peptide Site (P Site) is filled with N-
formymethionyl- tRNA and aminoacyl(A Site) with Exit Site(E Site) are empty
• Aminoacyl-tRNA Binding: The next codon is located with A site and is ready to
direct the binding of an aminoacyl-tRNA
• GTP and Elongation factor donate the aminoacyl-tRNA to ribosomes

21. Transpeptidation Reaction:
 Peptidyl transferase, located on 50S
Subunit catalyse the transpeptidation
reaction
 The α-amino group of A site amino acid
attacks α-carboxyl group of C-terminal
amino acid on P site tRNA in this
reaction resulting in peptide bond
formation
 A specific adenine base seems to
participate in catalyzing peptide bond
formation

22. Translocation:
• Movement of Ribosome on mRNA is
called Translocation
• There are three Phases of Translocation
• The peptidyl-tRNA moves from the A
site to P site
• The ribosome moves one codon along
mRNA so that a new codon is
positioned in the A site
• The empty tRNA leaves the P site
• Translocation requires GTP and
elongation factor complex to proceed

23. Termination
 Protein Synthesis stops when the ribosomes reaches one of three special non-
sense codons- UAA, UAG, UGA
 Three release factors(RF-1, RF-2, RF-3) aid the ribosomes in recognizing these
codons
 After the ribosome has stopped, peptidyl transferase hydrolyzes the peptide
free from its tRNA, and the empty tRNA is released GTP hydrolyzes required
for this process
 Last tRNA leaves ribosomes, leaving behind completed peptide.
 Next the ribosome dissociates from its mRNA and separates into 30S and 50S
subunits and will remain this way until another mRNA comes along to restart
the process.

24. Case Study
Isolation and characterization of ribosomes from yeast mitochondria
• In this they describe the isolation of ribosomes from Candida utilis
mitochondria, their sedimentation pattern in a sucrose gradient and their ability
to function in protein synthesis. Polysome-like structures were demonstrated in
thin sections of intact mitochondria by electron microscopy.
• Candida utilis (Strain CBS 1516) was grown for 16-18 hr at 25˚ under forced
aeration in fermentor jars containing 10 1 of a medium made of 1% yeast
extract, 2% peptone and 1.5% glucose.
Vignais et al.,
France

25. Fig. 1. Electron micrograph of a thin
section of a mitochondrial pellet showing
the almost complete absence of
cytoplasmic contamination. The arrows
point to tangential sections of
mitochondria, not be be confused with
contamination. X 24 000.
Fig. 2. Thin section of mitochondria
showing a reasonably well
preserved mitochondrial structure. A
number of mitoribosomes are alined
against the inner side of the inner
membrane.
X 90000.
Fig. 3. Mitochondria in which several
cristae are seen in grazing section.
Most mitoribosomes are lined up in
curved rows, spirals, etc., reminiscent
of polysomes. X 60000.
180 Å.
Vignais et al.,
1969

26. Fig. 4. Sedimentation profile of mitoribosomes and
cytoribosomes from Candida utilis. Centrifugation was for
60 min at 40000 rpm in the Spinco SW 50 rotor at 4˚. The
tracing obtained with mitoribosomes treated with MgCl2
(MTK medium) (seeExperimental) has been superimposed
on the tracing obtained with mitoribosomes treated with
EDTA.
Fig. 5. Sucrose gradient analysis of the ribosome
fraction isolated from Candida utilis mitochondria
which were incubated with [ 14C3 leucine as
described under Experimental. Centrifugation was for
60 min at 40000 r-pm in the Spinco SW 50 rotor at
4˚. Vignais et al.,