TRANSLATION
RNA-Directed Polypeptide Synthesis
In all things of nature there
is something of the
marvelous…
(Aristotle)
Genetic Code
Code that relates genes (DNA) to mRNA and mRNA
to the amino acids of proteins. The genetic code
specifies which amino acids will be used to build a
protein.
~ Universal &
Redundant but not
ambiguous
61 different
codons encode
the 20 amino acids
in proteins
Codon and anticodon
CODON
• Sequence of
nucleotides on
mRNA which
interact with tRNA.
• On mRNA.
ANTICODON
• Sequence of
nucleotides on tRNA
which interact with
mRNA.
• On tRNA.
Deciphering the Genetic Code
tRNA Translates Nucleotide (DNA)
Language into AA (Protein)Language
Carries AA
Associates with mRNA
Binds ribosome
tRNA has 75-80 nucleotides with internal H-bonding.
3’ end of tRNA (CCA) is attached to AA & mid point
has anticodon (antiparallel).
• The codon in mRNA and the amino acid in a protein
are related by way of an adapter—a specific tRNA
with an attached amino acid. For each of the 20
amino acids, there is at least one specific type of
tRNA molecule.
• At the 3′ end of every tRNA molecule is a site to
which its specific amino acid binds covalently. At
about the midpoint of tRNA is a group of three
bases, called the anticodon, that constitutes the site
of complementary base pairing (hydrogen bonding)
with mRNA. Each tRNA species has a unique
anticodon, which is complementary to the mRNA
codon for that tRNAs amino acid. At contact, the
codon and the anticodon are antiparallel to each
other.
tRNA Structure
Aminoacyl-tRNA Synthetase
• The charging of each tRNA with its
correct amino acid is achieved by a
family of activating enzymes, known
more formally as aminoacyl-tRNA
synthetases. Each activating enzyme is
specific for one amino acid and for its
corresponding tRNA
Aminoacyl-tRNA Synthetase
Specific for one AA & its corresponding tRNA.
Enzyme has three active sites for three molecules
(aa, tRNA and ATP)
amino acid + ATP → aminoacyl-AMP + PPi
aminoacyl-AMP + tRNA → aminoacyl-tRNA + AMP
-COOH of AA is attached to 3’ end of tRNA’s free OH
on ribose
This high energy bond provides energy for peptide
bond synthesis
Charging
Aminoacyl-tRNA synthetase
Translation Workbench
Large subunit = 3 rRNAs + 45 poly peptides.
Small subunit = 1 rRNA + 33 poly peptides.
Ionic & hydrophobic forces hold complex together
Complex self assembles.
Large Subunit
T (transfer) site: first site where charged tRNA lands
with the help of transfer factor.
A (AA) site: Anticodon binds to mRNA codon.
P (polypeptide) site: tRNA adds AA to polypeptide.
E (exit) site: after AA transfer tRNA exits from here.
Steps of translation
• Initiation
• Elongation
• Termination
INITIATION
Initiation Complex
Charged tRNA + small subunit +
mRNA
rRNA (small subunit) binds to
complementary sequence at 5’
end on mRNA
rRNA (small subunit) stabilizes
H-bonding between tRNA &
mRNA.
Initiation Complex
Anticodon of methionine
charged tRNA binds the start
codon.
Methionine can be removed
after translation.
Large subunit facilitated by
initiation factors
binds the complex with
methionine charged tRNA at P
site.
ELONGATION
A charged tRNA with anticodon to
next codon binds A site.
Large subunit:
1. breaks bond between tRNA at P
site in its AA.
2. Assisted by elongation factors
large subunit Catalyzes formation
of peptide bond between the AA
and the one attached to tRNA in A
site (peptidyl transferase activity).
TERMINATION
When stop codon enters A
site elongation terminates
Stop codons bind release
factor, which hydrolyze the
bond between polypeptide
chain and tRNA at P site
Protein folding information is
a function of AA sequence
Polyribosome/polysome
Protein Targeting
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