Chapter 30

Review the evidence establishing that aminoacyl-tRNA synthetases bridge the information gap between amino acids and codons. Indicate the various levels of specificity possessed by aminoacyl-tRNA synthetases that are essential for high-fidelity translation of messenger RNA molecules.

(Integrates with Chapter \(11 .\) ) Draw base-pair structures for \((\mathrm{a})\) \(G: C\) base pair, \((b)\) a \(C: G\) base pair, \((c)\) a \(G: U\) base pair, and \((d)\) a U:G base pair. Note how these various base pairs differ in the potential hydrogen-bonding patterns they present within the major groove and minor groove of a double-helical nucleic acid.

Point out why Crick's wobble hypothesis would allow fewer than 61 anticodons to be used to translate the 61 sense codons. How might "wobble" tend to accelerate the rate of translation?

Why do you suppose eukaryotic protein synthesis is only \(10 \%\) as fast as prokaryotic protein synthesis?

Eukaryotic ribosomes are larger and more complex than prokaryotic ribosomes. What advantages and disadvantages might this greater ribosomal complexity bring to a eukaryotic cell?

What ideas can you suggest to explain why ribosomes invariably exist as two- subunit structures, instead of a larger, single-subunit entity?

How do prokaryotic cells determine whether a particular methionyltRNA \(^{\text {Met }}\) is intended to initiate protein synthesis or to deliver a Met residue for internal incorporation into a polypeptide chain? How do the Met codons for these two different purposes differ? How do eukaryotic cells handle these problems?

What is the Shine-Dalgarno sequence? What does it do? The efficiency of protein synthesis initiation may vary by as much as 100 -fold for different mRNAs. How might the Shine-Dalgarno sequence be responsible for this difference?

In the protein synthesis elongation events described under the section on translocation, which of the following seems the most apt account of the peptidyl transfer reaction: (a) The peptidyl-tRNA delivers its peptide chain to the newly arrived aminoacyl-tRNA situated in the A site, or (b) the aminoacyl end of the aminoacyl-tRNA moves toward the \(P\) site to accept the peptidyl chain? Which of these two scenarios makes more sense to you? Why?

How many ATP equivalents are consumed for each amino acid added to an elongating polypeptide chain during the process of protein synthesis?