second step in gene expression second step of gene...

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

Codon

Export of mRNA into Cytoplasm

After processing in nucleus

mRNA moves to cytoplasm

through nuclear pore

Codon

Information in

the mRNA

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’

Characteristics of Genetic

Code

1. Specificity

2. Universality

3. Non-overlapping

4. Non-punctuated

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

What is the advantage of a degenerate

code?

Reduces rate of possible mutations

2. Genetic code is (nearly)

universal In all living organism genetic code is the same

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Codon Cytoplasm Mitochondria

AUA Isolucine Methionine

UGA Stop Tryptophan

AGA, AGG Arginine Stop

Profound implications in genetic engineering

Exceptions are in mitochondria and chloroplasts

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

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

Biologists deciphered genetic code

by using artificial mRNA

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

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

Aminoacyl-tRNA SynthetaseThis enzyme link right amino acid to tRNA (Charging of

tRNA)

Enzyme has a 3-part active site

Charging of tRNA

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

Translation proceeds in three phases

Steps

1. Initiation

2. Elongation

3. Termination

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 ?

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).

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.

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


2-tRNA

G

aa2

A U

A

1-tRNA

U A C

aa1

anticodon

hydrogenbonds codon

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mRNA


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

copyright cmassengale

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mRNA


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


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


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

Post translational Modifications

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

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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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