On average, how many phosphoanhydride bonds are hydrolyzed in the course of synthesizing a 400 amino acid protein? Assume that you begin with the mature mRNA, ribosomal subunits, tRNAs, free amino acids, and all necessary factors.

Name the type of chemical bonds that link (a) adjacent amino acids in a protein; (b) an amino acid to tRNA; (c) adjacent nucleotides in RNA; (d) a codon in mRNA to an anticodon in tRNA; (e) the two subunits of a ribosome.

A bacterial ribosome can synthesize about 20 peptide bonds per minute. If the average bacterial protein is approximately 260 amino acids in length, how many proteins can the ribosomes in an E. coli cell synthesize in 20 minutes with all ribosomes functioning at maximum rates?

Discuss the advantages to the cell of having multiple ribosomes translating a single mRNA molecule.

On a bacterial ribosome, only one tRNA binds directly to the P site without first interacting with the A site. Identify that tRNA and explain why the P site binding occurs.

The amino acid hydroxyproline, which is critical to the structure of collagen and certain other proteins, has no representative codon in the genetic code. How might it be incorporated into proteins?

The isoleucyl-tRNA synthetase has a proofreading function that improves the fidelity of the aminoacylation reaction, whereas the histidyl-tRNA synthetase lacks such a proofreading function. Explain why.

As described in Chapter 11, some DNA polymerases have proofreading activities. After a nucleotide is added to a growing nucleic acid chain, it can be removed (if incorrectly paired with the template) by hydrolysis of the phosphodiester bond that links it to the growing polymer. Ribosomes do not have similar proofreading activities; they cannot remove the last amino acid added to a growing polypeptide, regardless of whether it was correctly added or not. If ribosomes possessed such a proofreading function, would cleavage of the bond linking the last amino acid to the polymer have any effect on the rest of the polypeptide? Why or why not?

A researcher isolates mutant variants of the bacterial translation factors IF-2, EF-Tu, and EF-G. In each case, the mutation allows proper folding of the protein and binding of GTP, but does not allow GTP hydrolysis. At what stage would translation be blocked by each mutant protein?

Some aminoacyl-tRNA synthetases do not recognize and bind the anticodon of their cognate tRNAs; they use other structural features of the tRNAs to impart binding specificity. The tRNAs for alanine fall into this category.

When a bacterial mRNA is truncated at the 3′ end so that it is missing part of its gene-encoding sequence, the ribosome stalls at the end of that truncated mRNA. What mechanism can be used to recycle the stalled ribosome to synthesize new polypeptides?

The gene for a eukaryotic polypeptide of 300 amino acid residues is altered so that the polypeptide has an N-terminal signal sequence recognized by SRP and an internal nuclear localization signal, beginning at residue 150. Where is the protein likely to be found in the cell?

Chloramphenicol binds to bacterial ribosomes and is a potent inhibitor of bacterial protein synthesis, but it does not inhibit the cytosolic ribosomes in eukaryotes. However, because of its severe toxicity, chloramphenicol is rarely used as a human antibiotic. Suggest a reason for chloramphenicol toxicity in humans.