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(Comparethe enzymatic machinery used to synthesize proteins and glycogen.)9. Predicting Anticodons from Codons Mostamino acids have more than one codon and will beattached to more than one tRNA, each with a different anticodon. Write all possible anticodons forthe four codons for glycine: (5')GGU, GGC, GGA,and GGG.(a) From your answer, which of the positions inthe anticodons are primary determinants of theircodon specificity in the case of glycine?(b) Which of these anticodon-codon pairingshave a wobbly base pair?(c) In which of the anticodon-codon pairings doall three positions exhibit strong Watson-Crickhydrogen bonding?10. The Effect of Single-Base Changes on AminoAcid Sequence Much important confirmatory evidence on the genetic code has come from the natureof single-residue changes in the amino acid sequence of mutant proteins.

Which of the followingsingle-residue amino acid replacements would beconsistent with the genetic code? Which cannot bethe result of single-base mutations? Why?(a) Phe -» Leu(e) He -> Leu(b) Lys -> Ala(f) His -* Glu(c) Ala -> Thr(g) Pro -» Ser(d) Phe ^ Lys11. The Basis of the Sickle-Cell Mutation In sicklecell hemoglobin there is a Val residue at position 6of the /3-globin chain, instead of the Glu residuefound in this position in normal hemoglobin A.

Canyou predict what change took place in the DNAcodon for glutamate to account for its replacementby valine?12. Importance of the "Second Genetic Code" Someaminoacyl-tRNA synthetases do not bind the anticodon of their cognate tRNAs but instead use otherstructural features of the tRNAs to impart bindingspecificity. The tRNAs for alanine apparently fallinto this category.

Describe the consequences of aC -> G mutation in the third position of the anticodon of tRNA^*. What other kinds of mutationsmight have similar effects? Mutations of thesekinds are never found in natural populations ofany organism. Why? (Hint: Consider what mighthappen both to individual proteins and to the organism as a whole.)13. Maintaining the Fidelity of Protein Synthesis The chemical mechanisms used to avoid errors in protein synthesis are different from thoseused during DNA replication.

DNA polymerasesutilize a 3'—»5' exonuclease proofreading activityto remove mispaired nucleotides incorrectly inserted into a growing DNA strand. There is noanalogous proofreading function on ribosomes;and, in fact, the identity of amino acids attached toincoming tRNAs and added to the growing polypeptide is never checked. A proofreading step thathydrolyzed the last peptide bond formed when anincorrect amino acid was inserted into a growingpolypeptide (analogous to the proofreading step ofDNA polymerases) would actually be chemicallyimpractical. Why? (Hint: Consider how the linkbetween the growing polypeptide and the mRNA ismaintained during the elongation phase of proteinsynthesis; see Figs. 26-27 and 26-28.).

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