Chapter 11

12. (a) 1; (b) 2; (c) 3; (d) 3; (e) 4.

13. 3³, or 27 possible codons.

15. 1. amino—fMet-Phe-Lys-Phe-Lys-Phe—carboxyl

    2. amino—fMet-Tyr-Ile-Tyr-Ile—carboxyl

    3. amino—fMet-Asp-Glu-Arg-Phe-Leu-Ala—carboxyl

    4. amino—fMet-Gly—carboxyl (The stop codon UAG follows the codon for glycine.)

17. There are two possible sequences:

    mRNA:	5′—AUGUGGCAU—3′
    DNA template:	3′—TACACCGTA—5′
    DNA nontemplate:	5′—ATGTGGCAT—3′
    mRNA:	5′—AUGUGGCAC—3′
    DNA template:	3′—TACACCGTG—5′
    DNA nontemplate:	5′—ATGTGGCAC—3′

21. Initiation factor 3

    fMet-tRNA^fMet

    30S initiation complex

    70S initiation complex

    Elongation factor Tu

    Elongation factor G

    Release factor 1

24. 1. The lack of IF-3 would prevent protein synthesis. IF-3 separates the large and small ribosomal subunits, which is required for the initiation of translation. The absence of IF-3 would mean that translation would not be initiated and no proteins would be synthesized.

    2. No translation would take place. IF-2 is necessary for the initiation of translation. The lack of IF-2 would prevent fMet-tRNA^fMet from being delivered to the small ribosomal subunit, thus blocking translation.

    3. Although translation would be initiated by the delivery of methionine to the ribosome–mRNA complex, no other amino acids would be delivered to the ribosome. EF-Tu, which binds to GTP and the charged tRNA, is necessary for elongation. This three-part complex enters the A site of the ribosome. If EF-Tu is not present, the charged tRNA will not enter the A site, thus stopping translation.

    4. EF-G is necessary for the translocation (movement) of the ribosome along the mRNA in the 5′→3′ direction. Once a peptide bond had formed between Met and Pro, the lack of EF-G would prevent the movement of the ribosome along the mRNA, and so no new codons would be read. The formation of the dipeptide Met-Pro does not require EF-G.

    5. Release factors RF-1, RF-2, and RF-3 bring about termination of translation. The absence of the release factors would prevent termination at the stop codon and result in a longer protein produced.

    6. ATP is required for tRNAs to be charged with amino acids by aminoacyl-tRNA synthetases. Without ATP, the charging would not take place, and no amino acids would be available for protein synthesis.

    7. GTP is required for the initiation, elongation, and termination of translation. If GTP is absent, protein synthesis will not take place.

28. 1. The results suggest that, to initiate translation, the ribosome scans the mRNA to find the first start codon (AUG) that it encounters. The ribosome finds the appropriate start codon by the anticodon on the initiator tRNA (3′—UAC—5′) pairing with 5′—AUG—3′. If the anticodon on the initiator tRNA is mutated, so that it is now 3′—ACC—5′, the initiator tRNA will start protein synthesis when it pairs with its complement (5′—UGG—3′) in the mRNA.

    2. The initiation of translation in bacteria occurs in a different way—it requires the 16S rRNA of the small ribosomal subunit to interact with the Shine–Dalgarno sequence. This interaction serves to line up the ribosome over the start codon. If the anticodon had been changed such that the start codon could not be recognized, then protein synthesis would be unlikely to take place.

    3. After the anticodon on the initiator tRNA is mutated to 3′—ACC—5′, protein synthesis will be initiated whenever the ribosome encounters 5′—UGG—3′ in the mRNA. If the first UGG codon occurs before the normal AUG start codon, then extra amino acids will be added to the protein. If the first UGG codon occurs after the AUG start codon, then fewer amino acids will be added.