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Translation Continued...... Lecture 16 Dr. Attya Bhatti Translation occurs in three distinct steps; Initiation 2. Elongation 3. termination 1. Initiation: Figure: Initiation of Protein Synthesis. Start signals are required for the initiation of protein synthesis in (A) prokaryotes and (B) eukaryotes. Amino Acids Are First Activated by ATP and then transfer to tRNA to produce aminoacyl-tRNA (aminoacyl-tRNA synthetase). The activated tRNA is bound in the P site on the ribosome. Elongation: Initiation of protein synthesis in E. coli requires initiation factors IF-1, IF-2, & IF-3. Elongation: •First the mRNA attaches itself to a ribosome (to the small subunit). •A complementary tRNA molecule attached on the ribosome in the first position P. •Another tRNA base pairs with the other three mRNA bases in the ribosome at position A. •The enzyme peptidyl transferase forms a peptide bond between the two amino acids. •The first tRNA (without its amino acid) leaves the ribosome. A ribosome on the rough endoplasmic reticulum attaches to the mRNA molecule. ribosome A U G G G C U U AAA G C A G U G C A C G U U Amino acid tRNA molecule A transfer RNA molecule arrives. It brings an amino acid to the first three bases (codon) on the mRNA. anticodon The three unpaired bases (anticodon) on the tRNA link up with the codon. UAC A U G G G C U U AAA G C A G U G C A C G U U Another tRNA molecule comes into place, bringing a second amino acid. Its anticodon links up with the second codon on the mRNA. UAC A U G G G C U U AAA G C A G U G C A C G U U Peptide bond A peptide bond forms between the two amino acids by the enzyme peptidyl transferase.. A U G G G C U U AAA G C A G U G C A C G U U The first tRNA molecule releases its amino acid and moves off into the cytoplasm. A U G G G C U U AAA G C A G U G C A C G U U The ribosome moves along the mRNA to the next codon. A U G G G C U U AAA G C A G U G C A C G U U Another tRNA molecule brings the next amino acid into place. A U G G G C U U AAA G C A G U G C A C G U U A peptide bond joins the second and third amino acids to form a polypeptide chain. A U G G G C U U AAA G C A G U G C A C G U U The process continues. The polypeptide chain gets longer. This continues until a termination (stop) codon is reached. The polypeptide is then complete. A U G G G C U U AAA G C A G U G C A C G U U Termination: •Chain termination requires release factors. •When the peptidyl-tRNA is in the P site, the release factors, in response to the chain terminating codons, bind to the A site. •The polypeptide is then released from the P site, and •The ribosomes dissociate into two subunits.. Termination: Figure : Steps leading to termination of protein synthesis Protein Processing or Post Translational Modifications: •Glycosylation (Sugars are added) •Proteolytic processing (Signal peptidase) •Phosphorylation •Acetylation •Regulated degradation (Ubiquitin) Why are proteins modified? 1. Regulation of activity modification may turn activity on modification may turn activity off modification may generate a different function 2. Protein-protein interaction modification site may be a binding interface 3. Subcellular localization modification site may be a targeting signal modification may be a membrane anchor 4. Aging modification may identify the protein for degradation modification may target a protein to be scavenged