TRANSLATION

In all things of nature there is something of the

marvelous…(Aristotle)

RNA-Directed Polypeptide Synthesis

Genetic Code

~ Universal & Redundant but not

ambiguous

61 different codons encode the

20 amino acidsin proteins

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.

Codon and anticodonCODON

• 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 AAAssociates 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

• 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

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 mRNArRNA (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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