Application Questions And Problems

16. Sydney Brenner isolated Salmonella typhimurium mutants that were implicated in the biosynthesis of tryptophan and would not grow on minimal medium. When these mutants were tested on minimal medium to which one of four compounds (indole glycerol phosphate, indole, anthranilic acid, and tryptophan) had been added, the growth responses shown in the following table were obtained.

Indole

Minimal Anthranilic glycerol Trypto-

Mutant medium acid phosphate Indole phan trp-1 - - - - +

Give the order of indole glycerol phosphate, indole, anthranilic acid, and tryptophan in a biochemical pathway leading to the synthesis of tryptophan. Indicate which step in the pathway is affected by each of the mutations.

17. The addition of a series of compounds yielded in the following biochemical pathway:

precursor-* compound I-*

L enzyme L enzyme

compound II-* compound III

enzyme C

Mutation a inactivates enzyme A, mutation b inactivates enzyme B, and mutation c inactivates enzyme C. Mutants, each having one of these defects, were tested on minimal medium to which compound I, II, or III was added. Fill in the results expected of these tests by placing a plus sign ( + ) for growth or a minus sign ( — ) for no growth in the following table:

Minimal medium to which is added

Strain with Compound Compound Compound mutation I II III

18. Assume that the number of different types of bases in RNA is four. What would be the minimum codon size (number of nucleotides) required if the number of different types of amino acids in proteins were: (a) 2, (b) 8, (c) 17, (d) 45, (e) 75.

19. How many codons would be possible in a triplet code if only three bases (A, C, and U) were used?

20. Using the genetic code given in Figure 15.14, give the amino acids specified by the following bacterial mRNA sequences, and indicate the amino and carboxyl ends of the polypeptide produced.

(a) 5'-AUGUUUAAAUUUAAAUUUUGA-3'

(b) 5'-AUGUAUAUAUAUAUAUGA-3'

(c) 5'-AUGGAUGAAAGAUUUCUCGCUUGA-3'

(d) 5'-AUGGGUUAGGGGACAUCAUUUUGA-3'

21. A nontemplate strand on DNA has the following base sequence. What amino acid sequence would be encoded by this sequence?

22. The following amino acid sequence is found in a tripeptide: Met-Trp-His. Give all possible nucleotide sequences on the mRNA, on the template strand of DNA, and on the nontemplate strand of DNA that could encode this tripeptide.

23. How many different mRNA sequences can code for a polypeptide chain with the amino acid sequence Met-Leu-Arg? (Be sure to include the stop codon.)

24. A series of tRNAs have the following anticodons. Consider the wobble rules given in Table 14.2 and give all possible codons with which each tRNA can pair.

25. An anticodon on a tRNA has the sequence 5'-GCA-3'.

(a) What amino acid is carried by this tRNA?

(b) What would be the effect if the G in the anticodon were mutated to a U?

26. Which of the following amino acid changes could result from a mutation that changed a single base? For each change that could result from the alteration of a single base, determine which position of the codon (first, second, or third nucleotide) in the mRNA must be altered for the change to result.

27. A synthetic mRNA added to a cell-free protein-synthesizing system produces a peptide with the following amino acid sequence: Met-Pro-Ile-Ser-Ala. What would be the effect on translation if the following components were omitted from the cell-free protein-synthesizing system? What, it any, type of protein would be produced? Explain your reasoning.

(a) initiation factor 1

(b) initiation factor 2

(a) 5'-GGC-3'

(c) elongation factor Tu

(b) 5'-AAG-3'

(d) elongation factor G

(c) 5'-IAA-3'

(f) release factors Rj, R2, and R3

(d) 5'-UGG-3'

(g) ATP

(e) 5'-CAG-3'

(h) GTP

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