MCAT Biochemistry Practice Test 7: RNA and the Genetic Code

1. What role does peptidyl transferase play in protein synthesis?

• A. It transports the initiator aminoacyl-tRNA complex.
• B. It helps the ribosome to advance three nucleotides along the mRNA in the 5′ to 3′ direction.
• C. It holds the protein in its tertiary structure.
• D. It catalyzes the formation of a peptide bond.

2. Which stage of protein synthesis does NOT require energy?

• A. Initiation
• B. Elongation
• C. Termination
• D. All stages of protein synthesis require energy.

3. Topoisomerases are enzymes involved in:

• A. DNA replication and transcription.
• B. posttranscriptional processing.
• C. RNA synthesis and translation.
• D. posttranslational processing.

4. Val-tRNA^Val is the tRNA that carries valine to the ribosome during translation. Which of the following sequences gives an appropriate anticodon for this tRNA? (Note: Refer back to Figure 7.5 for a genetic code table.)

• A. CAU
• B. AUC
• C. UAC
• D. GUG

5. Enhancers are transcriptional regulatory sequences that function by enhancing the activity of:

• A. RNA polymerase at a single promoter site.
• B. RNA polymerase at multiple promoter sites.
• C. spliceosomes and lariat formation in the ribosome.
• D. transcription factors that bind to the promoter but not to RNA polymerase.

6. In the genetic code of human nuclear DNA, one of the codons specifying the amino acid tyrosine is UAC. If one nucleotide is changed, and the codon is mutated to UAG, what type of mutation will occur?

• A. Silent mutation
• B. Missense mutation
• C. Nonsense mutation
• D. Frameshift mutation

7. A double-stranded RNA genome isolated from a virus was found to contain 15% uracil. What percentage of guanine should exist in this virus's genome?

• A. 15%
• B. 35%
• C. 70%
• D. 85%

8. When trypsin converts chymotrypsinogen to chymotrypsin, some molecules of chymotrypsin bind to a repressor, which in turn binds to the operator and prevents further transcription of trypsin. This is most similar to which of the following operons?

• A. trp operon during lack of tryptophan
• B. trp operon during abundance of tryptophan
• C. lac operon during lack of lactose
• D. lac operon during abundance of lactose

9. Which of the following RNA molecules or proteins is NOT found in the spliceosome during intron excision?

• A. snRNA
• B. hnRNA
• C. shRNA
• D. snRNPs

10. A 4-year old toddler with cystic fibrosis (CF) is seen by his physician for an upper respiratory infection. Prior genetic testing has shown that there has been a deletion of three base pairs in exon 10 of the CFTR gene that affects codons 507 and 508. The nucleotide sequence in this region for normal and mutant alleles is shown below (X denotes the missing nucleotide):

What effect will this mutation have on the amino acid sequence of the protein encoded by the CFTR gene?

• A. Deletion of a phenylalanine residue with no change in the C-terminus sequence.
• B. Deletion of a leucine residue with no change in the C-terminus sequence.
• C. Deletion of a phenylalanine residue with a change in the C-terminus sequence.
• D. Deletion of a leucine residue with a change in the C-terminus sequence.

11. A gene encodes a protein with 150 amino acids. There is one intron of 1000 base pairs (bp), a 5′-untranslated region of 100 bp, and a 3′-untranslated region of 200 bp. In the final mRNA, about how many bases lie between the start AUG codon and final termination codon?

• A. 150
• B. 450
• C. 650
• D. 1750

12. Peptidyl transferase connects the carboxylate group of the one amino acid to the amino group of an incoming amino acid. What type of linkage is created in this peptide bond?

• A. Ester
• B. Amide
• C. Anhydride
• D. Ether

13. A eukaryotic cell has been found to exhibit a truncation mutation that creates an inactive RNA polymerase I enzyme. Which type of RNA will be affected by this inactivation?

• A. rRNA
• B. tRNA
• C. snRNA
• D. hnRNA

14. You have just sequenced a piece of DNA that reads as follows:

5′—TCTTTGAGACATCC—3′

What would the base sequence of the mRNA transcribed from this DNA be?

• A. 5′—AGAAACUCUGUAGG—3′
• B. 5′—GGAUGUCUCAAAGA—3′
• C. 5′—AGAAACTCTGTAGG—3′
• D. 5′—GGATCTCTCAAAGA—3′

15. Double-stranded RNA cannot be translated by the ribosome and is marked for degradation in the cell. Which of the following strands of RNA would prevent mature mRNA in the cytoplasm from being transcribed?

• A. Identical mRNA to the one produced
• B. Antisense mRNA to the one produced
• C. mRNA with thymine substituted for uracil
• D. Sense mRNA to the one produced