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Multiple Choice Questions
1. The genetic code Page: 1069 Difficulty: 2 Ans: C A certain bacterial mRNA is known to represent only one gene and to contain about 800 nucleotides. If you assume that the average amino acid residue contributes 110 to the peptide molecular weight, the largest polypeptide that this mRNA could code for would have a molecular weight of about: A) 800. B) 5,000. C) 30,000. D) 80,000. E) An upper limit cannot be determined from the data given. 2. The genetic code Page: 1038 Difficulty: 2 Ans: C Assuming that the average amino acid residue contributes 110 to the peptide molecular weight, what will be the minimum length of the mRNA encoding a protein of molecular weight 50,000? A) 133 nucleotides B) 460 nucleotides C) 1,400 nucleotides D) 5,000 nucleotides E) A minimum length cannot be determined from the data given. 3. The genetic code Pages: 1070-1074 Difficulty: 3 Ans: D Which of the following are features of the wobble hypothesis? A) A naturally occurring tRNA exists in yeast that can read both arginine and lysine codons. B) A tRNA can recognize only one codon. C) Some tRNAs can recognize codons that specify two different amino acids, if both are nonpolar. D) The “wobble” occurs only in the first base of the anticodon. E) The third base in a codon always forms a normal Watson-Crick base pair. 4. The genetic code Page: 1069 Difficulty: 2 Ans: C Which one of the following is true about the genetic code?
A) All codons recognized by a given tRNA encode different amino acids. B) It is absolutely identical in all living things. C) Several different codons may encode the same amino acid. D) The base in the middle position of the tRNA anticodon sometimes permits “wobble” base pairing with 2 or 3 different codons. E) The first position of the tRNA anticodon is always adenosine.
B) The enzyme attaches an amino acid to the 3 ' end of a tRNA. C) The enzyme splits ATP to AMP + PPi. D) The enzyme will use any tRNA species, but is highly specific for a given amino acid. E) There is a different synthetase for every amino acid.
10. Protein synthesis Pages: 1081-1082 Difficulty: 2 Ans: D The enzyme that attaches an amino acid to a tRNA (aminoacyl-tRNA synthetase): 11. Protein synthesis Page: 1088 Difficulty: 1 Ans: B Which of the following is(are) true for protein synthesis in eukaryotes? A) All proteins are initially synthesized with methionine at their C-terminus. B) All proteins are initially synthesized with methionine at their N-terminus. C) All proteins are initially synthesized with tryptophan at their C-terminus. D) All proteins are initially synthesized with a multiple of 3 amino acids in their sequence. E) None of the above. 12. Protein synthesis Pages: 1088-1089 Difficulty: 2 Ans: A Formation of the ribosomal initiation complex for bacterial protein synthesis does not require: A) EF-Tu. B) formylmethionyl tRNAfMet. C) GTP. D) initiation factor 2 (IF-2). E) mRNA. 13. Protein synthesis Page: 1091 Difficulty: 2 Ans: D In bacteria the elongation stage of protein synthesis does not involve: A) aminoacyl-tRNAs. B) EF-Tu. C) GTP. D) IF-2. E) peptidyl transferase. 14. Protein synthesis Page: 1091 Difficulty: 2 Ans: E Which one of the following statements about the elongation phase of protein synthesis is true? A) At least five high-energy phosphoryl groups are expended for each peptide bond formed.
B) During elongation, incoming aminoacylated tRNAs are first bound in the P site. C) Elongation factor EF-Tu facilitates translocation. D) Peptidyl transferase catalyzes the attack of the carboxyl group of the incoming amino acid on an ester linkage in the nascent polypeptide. E) Peptidyl transferase is a ribozyme.
15. Protein synthesis Pages: 1092-1093 Difficulty: 2 Ans: C Which of the following statements about bacterial mRNA is true? A) A ribosome usually initiates translation near the end of the mRNA that is synthesized last. B) An mRNA is never degraded but is passed on to the daughter cells at cell division. C) During polypeptide synthesis, ribosomes move along the mRNA in the direction 5 ' → 3 '. D) The codon signaling peptide termination is located in the mRNA near its 5 ' end. 16. Protein synthesis Pages: 1092-1093 Difficulty: 2 Ans: B Bacterial ribosomes: A) bind tightly to specific regions of DNA, forming polysomes. B) contain at least one catalytic RNA molecule (ribozyme). C) have specific, different binding sites for each of the 20 tRNAs. D) require puromycin for normal function. 17. Protein synthesis Page: 1096 Difficulty: 1 Ans: B The large structure consisting of a mRNA molecule being translated by multiple copies of the macromolecular complexes that carry out protein synthesis is called a: A) lysosome. B) polysome. C) proteosome. D) ribosome. E) synthosome. 18. Protein synthesis Page: 1093 Difficulty: 3 Ans: C It is possible to convert the Cys that is a part of Cys-tRNACys^ to Ala by a catalytic reduction. If the resulting Ala-tRNACys^ were added to a mixture of (1) ribosomes, (2) all the other tRNAs and amino acids, (3) all of the cofactors and enzymes needed to make protein in vitro, and (4) mRNA for hemoglobin, where in the newly synthesized hemoglobin would the Ala from Ala-tRNACys^ be incorporated? A) Nowhere; this is the equivalent of a nonsense mutation B) Wherever Ala normally occurs
there would be 80 amino acid residues of identical sequence in the two proteins. One possible explanation is that the two genes coding for these proteins overlap and/or are read in different reading frames.
- The genetic code Page: 1069 Difficulty: 2 The template strand of a segment of double-stranded DNA contains the sequence: (5 ' )CTT TGA TAA GGA TAG CCC TTC (a) What is the base sequence of the mRNA that can be transcribed from this strand? (b) What amino acid sequence could be coded by the mRNA base sequence in (a), using only the first reading frame starting at the 5 ' end? (Refer to Fig. 27-7, p. 1069.) (c) Suppose the other (complementary) strand is used as a template for transcription. What is the amino acid sequence of the resulting peptide, again starting from the 5 ' end and using only the first reading frame? Ans: (a) (5 ' )GAA GGG CUA UCC UUA UCA AAG(3’) (b) Glu-Gly-Leu-Ser-Leu-Ser-Lys (c) The codons translate to Leu-Stop-Stop. No peptide would be produced because of the stop codons. (See also Fig. 27-6, p. 1069.) 23. The genetic code Page: 1069 Difficulty: 3 Describe the possible outcomes that could occur because of a single base change in an mRNA Ans: The most likely result is a single amino acid change in the encoded protein, when a codon is altered to one of another amino acid. However, some nucleotide changes will be “silent” and not change the protein, if the altered codon still specifies the original amino acid. Conversion to a nonsense codon will result in a truncated polypeptide, while alteration of the normal stop codon to a “sense” codon will result in a lengthened protein. Alternation of the initiaton codon may result in the total failure to translate, and result in no protein product. Changes in mRNA sequence outside the protein-coding region may affect translational efficiency, splicing, or mRNA turnover rates. 24. The genetic code Page: 1069 Difficulty: 3 The following sequence of four amino acids occurred in the structure of a polypeptide found in a wild-type organism: Leu-Ser-Ile-Arg. Several mutants were isolated, each of which carried a single base pair change in the region of DNA that coded for this amino acid sequence. Their corresponding amino acid sequences are: Mutant 1 MET-Ser-Ile-Arg
2 Leu-TRP-Ile-Arg 3 Leu-Ser-ARG-Arg 4 Leu-Ser-Ile-PRO 5 Leu-Ser-Ile-TRP What was the nucleotide sequence of the region of mRNA that coded for the amino acid sequence in the wild-type organism? (Refer to Fig. 27-6, p. 1069.) Ans: (5 ' )C or (5 ' )U UG UCG AUA CGG
codon (AUG) on the 30S ribosomal subunit. Thus, the initiating AUG is distinguished by its proximity to the Shine-Dalgarno sequence.
29. Protein synthesis Pages: 1088-1095 Difficulty: 1 Match the factor or enzyme at the right with the stage(s) of protein synthesis at which it acts. If a factor or enzyme participates in two stages of protein synthesis, indicate both of them. ___ Amino acid activation (a) RF 1 ___ Initiation (b) EF-Tu ___ Elongation (c) aminoacyl-tRNA ___ Termination (d) Shine-Dalgarno sequence Ans: c; d; b; a 30. Protein synthesis Pages: 1088-1095 Difficulty: 2 Indicate whether each of the following statements is true (T) or false (F). ___Assembly of a complete ribosome onto an mRNA requires ATP hydrolysis. ___Aminoacylation or “charging” of tRNA requires the formation of an aminoacyl-AMP intermediate. ___Aminoacyl-tRNA binding to the A site of the ribosome requires the accessory factor EF-G and GTP hydrolysis. ___Translocation of a growing polypeptide from the A to the P site on the ribosome requires EF-G and GTP hydrolysis. ___Termination of translation requires release factors, but no NTP hydrolysis. Ans: F; T; F; T; T 31. Protein synthesis Pages: 1088-1096 Difficulty: 2 Indicate whether each of the following statements is true (T) or false (F). ___ Bacterial mRNA is broken down within a few minutes of its formation in E. coli. ___ Bacterial mRNA consists only of the bases that code for amino acids. ___ Polysomes do not necessarily contain mRNA. ___ Bacterial mRNA normally occurs as a double-stranded structure, with one strand containing codons, the other containing anticodons. ___ Bacterial mRNA can be translated while it is still being synthesized. Ans: T; F; F; F; T 32. Protein synthesis Pages: 1091-1094 Difficulty: 2 Polypeptide chain elongation in E. coli occurs by the cyclical repetition of three steps. What
are these steps and what cellular components are necessary for each of them to occur? Ans: The three steps are: (1) An aminoacyl-tRNA is brought to the A site by EF-Tu with bound GTP; (2) peptidyl transferase (a ribozyme) catalyzes peptide-bond formation; (3) the ribosome translocates three nucleotides down the mRNA, helped by EF-G (translocase). This shifts the peptidyl-tRNA to the A site and the deacylated tRNA to the E site.
33. Protein synthesis Pages: 1091-1092 Difficulty: 2 A new antibiotic was recently discovered that inhibits prokaryotic protein synthesis. In the presence of the antibiotic, protein synthesis can be initiated, but only dipeptides that remain bound to the ribosome are formed. What specific step of protein synthesis is likely to be blocked by this antibiotic? Ans: The antibiotic probably blocks translocation.
