Answers To Review Questions

1. A. Synthesis of mRNA is not part of translation. All the others are, in particular, "charging" means attaching an amino acid to a tRNA ready for use in protein synthesis.

2. C. AUG, which codes for the amino acid methionine.

3. B. The enzyme is called aminoacyl tRNA synthase. Concerning the other answers: (A) Because amino acids are not attached to mRNA, there is no such process in cells. (C) Amino acid attachment enzyme is a plausible name, albeit less specific since it does not say to what the amino acid is attached. However, the name is not used. (D) Elongation factor tu plays a role later in the process of protein synthesis, not during tRNA charging. (E) tRNA polymerase is not a name that is ever used. If it did mean anything, it would mean the enzyme that makes tRNA (there is such an enzyme in eukaryotes: it is called RNA polymerase III), not the one that attaches an amino acid.

4. C. The rRNA molecule within the 30S ribosomal subunit has a short stretch of sequence, 3' CCUCC 5', at its 3' end that is complementary in sequence to the 5' GGAGG 3' of the mRNA. Hydrogen bonding between these two sequences aligns the ribosome for protein synthesis. Concerning answer D: the 40S ribosomal subunit plays a similar role in eukaryotes as the 30S unit does in prokaryotes. However, eukaryotic mRNAs do not have the prokaryotic GGAGG Shine-Dalgarno sequence.

5. C. The enzyme is called peptidyl transferase. Concerning the other answers: A, B, and D are plausible answers, though D is the least plausible since most enzymes have names ending in ase; however, they are not the names used. Puromycin is an antibiotic, not a protein (and therefore certainly not an enzyme).

6. A. These are codons directing the incorporation of methionine, arginine, and valine, respectively. All the others do play a part in the termination of protein synthesis, in particular, UAA, UAG, and UGA are stop codons, while if the A site is occupied by a release factor, peptidyl transferase catalyzes the hydrolysis of the bond joining the polypeptide chain to the tRNA. The carboxyl (COOH) end of the protein is therefore freed from the tRNA and the completed protein is released.

7. D. Hypoxanthine can bond with any of U, C, or A.

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