second step in gene expression second step of gene
TRANSCRIPT
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Second step of Gene expression (Protein synthesis)
Translation: mRNA to ProteinSecond step in Gene Expression
Dr Mahmood S Choudhery, PhD, POSTDOC (USA)
Assistant Professor,
King Edward Medical University/
Mayo Hospital Lahore
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Codon
Export of mRNA into Cytoplasm
After processing in nucleus
mRNA moves to cytoplasm
through nuclear pore
Codon
Information in
the mRNA
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Genetic Code Table
How to read genetic code?
CAG AGG GUC
Total codons 64
Start codon 01
Stop codons 03
61 codons for 20
amino acids
Each codon consists of
three bases (triplet)
41 = 4
42 = 16
43 = 64
5' 3’
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Characteristics of Genetic
Code
1. Specificity
2. Universality
3. Non-overlapping
4. Non-punctuated
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Genetic code is redundant but not
ambiguous
Examples
UCU always codes for serine
CCU always codes for Proline
1. Specificity
First 2 bases determine
amino acid
Third Base is usually less
specific than first two
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What is the advantage of a degenerate
code?
Reduces rate of possible mutations
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2. Genetic code is (nearly)
universal In all living organism genetic code is the same
7
Codon Cytoplasm Mitochondria
AUA Isolucine Methionine
UGA Stop Tryptophan
AGA, AGG Arginine Stop
Profound implications in genetic engineering
Exceptions are in mitochondria and chloroplasts
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3. Genetic Code is Non-overlapping
A U G U A G G A G U C G U U
A U G U A G G A G U C G U U
All codons in mRNA are independent sets of 3
bases
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4. Genetic Code is Non-Punctuated
Codon is read from a fixed starting point as a
continuous sequence of bases, taken three at a
time
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Biologists deciphered genetic code
by using artificial mRNA
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Second step of Gene expression (Protein synthesis)
Translation: mRNA to Protein
Process by which genetic information in mRNA is translated into
amino acid sequence of proteins
First step Second step 1st step 2nd stepGene
expression
Molecular
Components of
Translation
1. tRNA
2. Ribosomes
3. Aminoacyl-tRNA synthetase
4. mRNA
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Transfer RNATranslates Nucleotide (mRNA) Language into amino
acid (Protein) Language
tRNA StructuretRNA has 3 important functions
Carries an amino acid
Associates with mRNA
Interacts with ribosomes
tRNAs are adapters between amino acids and
codons in mRNA molecules
Molecular structure of tRNA is
related to its functions
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Aminoacyl-tRNA SynthetaseThis enzyme link right amino acid to tRNA (Charging of
tRNA)
Enzyme has a 3-part active site
Charging of tRNA
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Ribosomes are the translation
workbench
Each ribosome consists of 2
subunits
Four important sites on ribosome
1. T (transfer) site
2. A (amino acid) site
3. P (polypeptide) site
4. E (exit) site
Ribosomes
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Translation proceeds in three phases
Steps
1. Initiation
2. Elongation
3. Termination
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1. InitiationTranslation of mRNA begins
with the formation initiation
complex
Every protein begins with
the Methionine
But
not all proteins will
ultimately have methionine
at one end ?
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Elongation
A charged tRNA with
anticodon to next codon
binds A site.
Large subunit breaks bond
between tRNA and its AA
(at P site)
Assisted by elongation
factors large subunit
Catalyzes formation of
peptide bond between AA
(at P site) and one attached
to tRNA in A site (peptidyl
transferase activity).
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Elongation continues
and the polypeptide
grows
Next charged tRNA enters
the open A site
Its amino acid forms a
peptide bond with the amino
acid chain in the P site
tRNA in the E site is
released.
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Termination
When stop codon enters A site
elongation is terminates
Stop codons at A site is recognized
by release factor instead of tRNA
Release factor, which hydrolysis
bond between polypeptide chain
and tRNA at P site.
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Ribosomes
P
Site
A
Site
Largesubunit
Small subunit
mRNA
A U G C U A C U U C G
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Initiation
mRNA
A U G C U A C U U C G
2-tRNA
G
aa2
A U
A
1-tRNA
U A C
aa1
anticodon
hydrogenbonds codon
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mRNA
A U G C U A C U U C G
1-tRNA 2-tRNA
U A C G
aa1 aa2
A U
A
anticodon
hydrogen
bonds codon
peptide bond
3-tRNA
G A A
aa3
Elongation
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mRNA
A U G C U A C U U C G
1-tRNA
2-tRNA
U A C
G
aa1
aa2
A U
A
peptide bond
3-tRNA
G A A
aa3
Ribosomes move over one codon
(leaves)
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mRNA
A U G C U A C U U C G
2-tRNA
G
aa1
aa2
A U
A
peptide bonds
3-tRNA
G A A
aa3
4-tRNA
G C U
aa4
A C U
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mRNA
A U G C U A C U U C G
2-tRNA
G
aa1aa2
A U
A
peptide bonds
3-tRNA
G A A
aa3
4-tRNA
G C U
aa4
A C U
(leaves)
Ribosomes move over one codoncopyright cmassengale
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mRNA
G C U A C U U C G
aa1aa2
A
peptide bonds
3-tRNA
G A A
aa3
4-tRNA
G C U
aa4
A C U
U G A
5-tRNA
aa5
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mRNA
G C U A C U U C G
aa1
aa2
A
peptide bonds
3-tRNA
G A A
aa3
4-tRNA
G C U
aa4
A C U
U G A
5-tRNA
aa5
Ribosomes move over one codoncopyright cmassengale
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mRNA
A C A U G U
aa1
aa2
U
primary
structure
of a protein
aa3
200-tRNA
aa4
U A G
aa5
C U
aa200
aa199
terminator
or stop
codon
Termination
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End Product –The Protein!
• The end products of protein synthesis is a primary structureof a protein
• A sequence of amino acidbonded together by peptide bonds
aa1
aa2 aa3 aa4aa5
aa200
aa199
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Post translational Modifications
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Wobbling Phenomenon
Total 64 possible codons
3 are stop codons
For remaining 61 codons, there must be 61 tRNA
However
Most organisms have only 45 tRNA
So
Some tRNA species must pair with more than one codons
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Wobbling Phenomenon5’ base on anticodon which binds to 3’ base on mRNA
could have non-standard base pairing
32
tRNA 5' anti-codon
base
mRNA 3' codon
base
A U
C G
G C or U
U A or G
I A or C or U
tRNA base pairing schemes
3’ 5’
3’5’
3’ 5’
3’5’
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