Choose the ONE best answer.
2.1. A 20-year-old man with a microcytic anemia is found to have an abnormal form of β-globin (Hemoglobin Constant Spring) that is 172 amino acids long, rather than the 141 found in the normal protein. Which of the following point mutations is consistent with this abnormality? Use Figure 32.2 to answer the question.
A. CGA→UGA
B. GAU→GAC
C. GCA→GAA
D. UAA→CAA
D. UAA→UAG
Correct answer = D. Mutating the normal termination (stop) codon from UAA to CAA in β-globin messenger RNA causes the ribosome to insert a glutamine at that point. It will continue extending the protein chain until it comes upon the next stop codon farther down the message, resulting in an abnormally long protein. The replacement of CGA (arginine) with UGA (stop) would cause the protein to be too short. GAU and GAC both code for aspartate and would cause no change in the protein. Changing GCA (alanine) to GAA (glutamate) would not change the size of the protein product. A change from UAA to UAG would simply change one termination codon for another and would have no effect on the protein.
2.2. A pharmaceutical company is studying a new antibiotic that inhibits bacterial protein synthesis. When this antibiotic is added to an in vitro protein synthesis system that is translating the messenger RNA sequence AUGUUUUUUUAG, the only product formed is the dipeptide fMet-Phe. What step in protein synthesis is most likely inhibited by the antibiotic?
A. Initiation
B. Binding of a charged transfer RNA to the ribosomal A site
C. Peptidyltransferase activity
D. Ribosomal translocation
E. Termination
Correct answer = D. Because fMet-Phe (formylated methionyl-phenylalanine) is made, the ribosomes must be able to complete initiation, bind Phe-tRNA to the A site, and use peptidyltransferase activity to form the first peptide bond. Because the ribosome is not able to proceed any further, ribosomal movement (translocation) is most likely the inhibited step. Therefore, the ribosome is stopped before it reaches the termination codon of this message.
2.3. A transfer RNA (tRNA) molecule that is supposed to carry cysteine (tRNAcys) is mischarged, so that it actually carries alanine (ala-tRNAcys). Assuming no correction occurs, what will be the fate of this alanine residue during protein synthesis? It will:
A. be incorporated into a protein in response to a codon for alanine.
B. be incorporated into a protein in response to a codon for cysteine.
C. be incorporated randomly at any codon.
D. remain attached to the tRNA because it cannot be used for protein synthesis.
E. be chemically converted to cysteine by cellular enzymes.
Correct answer = B. Once an amino acid is attached to a tRNA molecule, only the anticodon of that tRNA determines the specificity of incorporation. Therefore, the incorrectly activated alanine will be incorporated into the protein at a position determined by a cysteine codon.
2.4. In a patient with cystic fibrosis (CF) caused by the ∆F508 mutation, the mutant CF transmembrane conductance regulator (CFTR) protein folds incorrectly. The patient’s cells modify this abnormal protein by attaching ubiquitin molecules to it. What is the fate of this modified CFTR protein?
A. It performs its normal function because the ubiquitin largely corrects for the effect of the mutation.
B. It is degraded by the proteasome.
C. It is placed into storage vesicles.
D. It is repaired by cellular enzymes.
E. It is secreted from the cell.