2. TRANSLATION
The process involves in converting the nucleic
acid “language,” the genetic code, to protein
“language,” and is therefore known as
translation.
This was first formulated by Sir Francis Crick.
3. AMINO ACID CODING
There are 20 amino acids in proteins but only
four different bases in the mRNA.
During translation,the bases of mRNA are read
off in groups of three, which are known as
codons.
Four different bases gives 64 possible groups of
three bases; that is, 64 different codons in the
genetic code.
4. In addition, three of the codons are used for
punctuation. Those are the stop codons that
signal the end of a polypeptide chain.
To read the codons,a set of adapter molecules that
recognize the codon on the mRNA at one end and
carry the corresponding amino acid attached to
their other end is needed.
These adapters are small RNA molecules, or
transfer RNA (tRNA).
5. This adapter contains anticodon at one end which
are complementary to the three bases of the
codon on the mRNA.
The codon and anticodon recognize each other by
base pairing and are held together by hydrogen
bonds.
6.
7. COMPONENTS OF RIBOSOME & DIFFERENCE
PROKARYOTES EUKARYOTES
70s Ribosome composed of 30s
(small subunit) & 50s (large subunit).
Small subunit - 16s rRNA contains
21 different proteins.
Large subunit – 5s & 23s rRNA
contains 32 different proteins.
The initiator tRNA is first charged
with methionine by methionyl-tRNA
synthetase. The methionine residue is
then converted to N-formylmethionine
by transformylase.
Multiple start sites.
Initiation factors, IF1, IF2 and IF3.
80s Ribosome composed of 60s (large
subunit) & 40s (small subunit).
5’ r cap instead of Shine-Dalgarno
sequence.
In eukaryotes, the methionine on the
initiator tRNA is not modified.
The initiating codon in eukaryotes is
always AUG.
Initiation factors, eIF1, eIF2, eIF3,
eIF4, eIF5 and eIF6.
9. TRANSLATION: THE PROCESS OF PROTEIN
SYNTHESIS
1. Ribosomes translate the genetic message of mRNA into
proteins.
2. The mRNA is translated 5’3’, producing a corresponding
N-terminal C-terminal polypeptide.
3. Amino acids bound to tRNAs are inserted in the proper
sequence due to:
a. Specific binding of each amino acid to its tRNA.
b.Specific base pairing between the mRNA codon and tRNA anticodon.
10. CHARGING tRNA
1. Aminoacyl-tRNA synthetase attaches amino acids to their
specific tRNA molecules. The charging process
(aminoacylation) produces a charged tRNA (aminoacyl-
tRNA), using energy from ATP hydrolysis.
2. There are 20 different aminoacyl-tRNA synthetase enzymes,
one for each amino acid. Some of these enzymes recognize
tRNAs by their anticodon regions.
3. The amino acid and ATP bind to the specific aminoacyl-
tRNA synthetase enzyme. ATP loses two phosphates and the
resulting AMP is bound to the amino acid, forming
aminoacyl-AMP.
4. The tRNA binds to the enzyme, and the amino acid is
transferred onto it, displacing the AMP. The aminoacyl-tRNA
is released from the enzyme.
11. CHARGING OF A tRNA MOLECULE BY AMINOACYL-
tRNA SYNTHETASE
12. 5. The amino acid is now covalently attached by its carboxyl
group to the 3’r end of the tRNA. Every tRNA has a 3’r
adenine, and the amino acid is attached to the 3’r–OH or 2’r–
OH of this nucleotide.
15. INITIATION
In prokaryotes, initiation requires the large and small ribosome
subunits, the mRNA, the initiator tRNA, three initiation factor(IFs)
and GTP.
IF1 and IF3 bind to the 30S subunit and prevent the large subunit
binding.
IF2+GTP can then bind and will help the initiator tRNA to bind
later.
This small subunit complex can now attach to an mRNA via its
ribosome-binding site.
The initiator tRNA can then base-pair with the AUG initiation
codon which releases IF3 thus creating the 30S initiation complex.
The large subunit then binds, displacing IF1 and IF2+GTP, giving
the 70S initiation comple which is the fully assembled ribosome at
the correct position on the mRNA.
16.
17. SHINE-DALGARNO SEQUENCE
Shine-Dalgarno sequence is an ribosomal binding site in
prokaryotic mRNA, generally located around 8 bases
upstream of the start codon AUG.
Shine-Dalgarno sequence exists both in bacteria &
archaea and also in some chloroplast & mitochondria.
Shine-Dalgarno sequence helps to make ribosome
available to the mRNA to initiate protein synthesis by
aligning it with the start codon.
18.
19. ELONGATION
Elongation involves the three factors(Efs), EF-Tu, EF-Ts and EF -
G, GTP, charged tRNA and the 70S initiation complex(or its
equivalent). It takes place in three steps.
A charged tRNA is delivered as a complex with EF-Tu and GTP.
The GTP is hydrolyzed and EF-Tu.GTP is released which can be
re-used with the help of EF-Ts and GTP(via the EF-Tu-EF-Ts
exchange cycle).
Peptidyl transferase makes a peptide bond by joining the two
adjacent amino acid without the input of more energy.
Translaocase(EF-G), with energy from GTP, move the ribosome
one codon along the mRNA, ejecting the uncharged tRNA and
transferring the growing peptide chain to the P-site.
20.
21. ACTION OF PEPTIDYL TRANSFERASE
The two aminoacyl-tRNAs are positioned by the ribosome
for peptide bond formation, which occurs in two steps:
a.In the P site, the bond between the amino acid and its
tRNA is cleaved.
b.Peptidyl transferase forms a peptide bond between the
now-free amino acid in the P site and the amino acid
attached to the tRNA in the A site. Experiments indicate
that the 23S rRNA is most likely the catalyst for peptide
bond formation.
c.The tRNA in the A site now has the growing
polypeptide chain attached to it.
22.
23. TERMINATION
Release factors(RF1 or RF2) recognize the stop codon
and, helped by RF3, make peptidyl transferase join the
polypeptide chain to a water molecule, thus releasing it.
Ribosome release factor helps to dissociate the ribosome
subunit from the mRNA.
27. INITIATION
This is the major point of difference between prokaryotic
and eukaryotic protein synthesis, there being at least nine
eIF involved.
Functionally, these factors can be grouped. They either
bind to the ribosome subunit or to the mRNA, deliver the
initiator tRNA or displace other factors.
In contrast to the events in prokaryotes, initiation
involves the initiator tRNA binding to the 40S subunit
before it can bind to the mRNA.
Phosphorylation of eIF2, which delivers the initiator
tRNA, is an important control point.
28.
29. ELONGATION & TERMINATION
This stage of protein synthesis is essentially identical to
that described for prokaryotes.
The factors EF-Tu, EF-Ts and equivalents called eEF1α,
eEF1βγ and eEF2 respectively, which carry out the same
roles. EF-G have direct eukaryotic activity.
Eukaryotes use only one release factor (eRF), which
requires GTP, for termination of protein synthesis. It can
recognize all three codons.
