Chapter 21 Nucleic Acids and Protein Synthesis

1. Chapter 21 Nucleic Acids and Protein Synthesis 21.7Protein Synthesis: Translation

2. tRNA Activation The activation of tRNA • occurs when a synthetase uses energy from ATP hydrolysis to attach an amino acid to a specific tRNA • prepares each tRNA to use a triplet called an anticodon to complement a codon on mRNA

3. Initiation of Protein Synthesis For the initiationof protein synthesis, • an mRNA attaches to a ribosome • the start codon (AUG) binds to a tRNA with methionine • the second codon attaches to a tRNA with the next amino acid • a peptide bond forms between the adjacent amino acids at the first and second codons

4. Translocation During translocation, • the first tRNA detaches from the ribosome • the ribosome shifts to the adjacent codon on the mRNA • a new tRNA/amino acid attaches to the open binding site • a peptide bond forms, and the empty tRNA detaches • the ribosome shifts down the mRNA to read the next codon

5. Protein Synthesis Translation

6. Termination of Protein Synthesis The terminationof protein synthesis occurs when • a polypeptide with all the amino acids for a protein is synthesized • the ribosome reaches a “stop” codon: UGA, UAA, or UAG • there is no tRNA with an anticodon for the “stop” codons • the polypeptide detaches from the ribosome

7. Learning Check Match the following: 1) activation 2) initiation 3) translocation 4) termination A. Ribosomes move along mRNA, adding amino acids to a growing peptide chain. B. A completed peptide chain is released. C. A tRNA attaches to its specific amino acid. D. A tRNA binds to the AUG codon of the mRNA on the ribosome.

8. Solution Match the following: 1) activation 2) initiation 3) translocation 4) termination 3 A. Ribosomes move along mRNA, adding amino acids to a growing peptide chain. 4 B. A completed peptide chain is released. 1 C. A tRNA attaches to its specific amino acid. 2 D. A tRNA binds to the AUG codon of the mRNA on the ribosome.

9. Summary of Protein Synthesis To summarize protein synthesis: • An mRNA attaches to a ribosome. • Molecules of tRNA bonded to specific amino acids attach to the codons on mRNA. • Peptide bonds form between an amino acid and the peptide chain. • The ribosome shifts to each codon on the mRNA until it reaches the STOP codon. • The polypeptide chain detaches to function as an active protein.

10. Overview of Protein Synthesis

11. Learning Check The following section of DNA is used to build an mRNA for a protein: 3′—GAA—CCC—TTT—5′ A. What is the corresponding mRNA sequence? B. What are the anticodons on the tRNAs? C. What is the amino acid order in the peptide?

12. Solution The following section of DNA is used to build an mRNA for a protein: 3′—GAA—CCC—TTT—5′ A. What is the corresponding mRNA sequence? 5′—CUU—GGG—AAA—3′ mRNA B. What are the anticodons for the tRNAs? mRNA codons: CUU GGG AAA tRNA anticodons: GAA CCC UUU C. What is the amino acid order in the peptide? mRNA:5′—CUU—GGG—AAA—3′ peptide: Leu — Gly — Lys