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MCAT Translation, Genetic Code and Protein Targeting Practice Questions

By StudyStore · Published 2026-10-20 · 8 original questions · about 12 min

Eight free MCAT-style practice questions on codon reading, tRNA charging, ribosome function, antibiotics, energy cost and protein sorting, with worked examples and full explanations.

Quick concept primer

The genetic code is triplet, nearly universal, degenerate (several codons per amino acid) and unambiguous (each codon specifies one amino acid or a stop). Degeneracy is concentrated at the third codon position, where wobble pairing with the tRNA anticodon is permitted. AUG is the usual start codon and encodes methionine; UAA, UAG and UGA are the stop codons.

Aminoacyl-tRNA synthetases attach each amino acid to its tRNA's 3' end using ATP, and the accuracy of this step determines translational accuracy because the ribosome reads only the anticodon. Ribosomes have an A site (incoming aminoacyl-tRNA), P site (peptidyl-tRNA) and E site (exit). Peptide bond formation is catalyzed by rRNA, so the ribosome is a ribozyme. Proteins destined for secretion or membranes carry a signal sequence that directs the ribosome to the rough ER.

Traps: stop codons have no tRNA, and ribosomal subunit sizes in Svedberg units do not add arithmetically in the way mass does.

8 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

A point mutation in an mRNA changes a codon from GCU to GCC. Both specify alanine. This is best described as

  1. a missense mutation
  2. a silent mutation, illustrating degeneracy of the code at the wobble position
  3. a nonsense mutation
  4. a frameshift mutation
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Answer: B. a silent mutation, illustrating degeneracy of the code at the wobble position

A change that leaves the encoded amino acid unchanged is silent. The third position of a codon is the least constrained because the first two bases usually determine the amino acid family, and wobble pairing allows a single tRNA to read multiple codons. A missense mutation would change the amino acid, and a nonsense mutation would create a stop codon.

Question 2

The mRNA sequence 5'-AUGGCAUUUAAGUGA-3' is translated from its start codon. Using the standard code (AUG Met, GCA Ala, UUU Phe, AAG Lys, UGA stop, UAG stop), what happens if the adenine at position 10 is changed to uracil?

  1. The peptide is unchanged
  2. The peptide has the sequence Met-Ala-Phe-Ser
  3. The peptide becomes longer
  4. The peptide is truncated to three amino acids, Met-Ala-Phe, because AAG becomes UAG
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Answer: D. The peptide is truncated to three amino acids, Met-Ala-Phe, because AAG becomes UAG

Numbering the bases, positions 10 to 12 are A-A-G, the codon for lysine. Changing position 10 from A to U gives UAG, a stop codon. Translation of Met-Ala-Phe stops there, producing a three-amino-acid peptide. A change that converts an amino acid codon into a stop codon is called a nonsense mutation.

Question 3

An aminoacyl-tRNA synthetase mistakenly attaches serine to the tRNA whose anticodon pairs with codons for alanine. When this charged tRNA reaches the ribosome, what happens?

  1. The tRNA is converted to a tRNA for serine by the ribosome
  2. The ribosome detects the wrong amino acid and rejects the tRNA
  3. Serine is incorporated into the protein at alanine codons, because the ribosome reads only the anticodon-codon pairing
  4. The ribosome stops translation
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Answer: C. Serine is incorporated into the protein at alanine codons, because the ribosome reads only the anticodon-codon pairing

The ribosome decodes by base pairing between the codon and the anticodon, not by checking the attached amino acid. This is why the accuracy of charging, with its proofreading by the synthetases, is critical to the fidelity of protein synthesis.

Question 4

In a eukaryotic cell, which feature of a mature mRNA is recognized by initiation factors to help recruit the small ribosomal subunit?

  1. The Shine-Dalgarno sequence
  2. The TATA box
  3. The 3' poly-A signal sequence alone
  4. The 5' 7-methylguanosine cap
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Answer: D. The 5' 7-methylguanosine cap

Eukaryotic initiation factors bind the 5' cap and recruit the 40S subunit, which scans along the mRNA until it finds the start codon in a favorable context. The poly-A tail also contributes to translation efficiency and mRNA stability but through proteins that interact with the cap-binding complex. The Shine-Dalgarno sequence is a bacterial feature, and the TATA box is part of a DNA promoter, not mRNA.

Question 5

Approximately how many high-energy phosphate bonds are consumed for each peptide bond formed during elongation, counting tRNA charging?

  1. 1
  2. 4
  3. 2
  4. 8
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Answer: B. 4

Charging a tRNA converts ATP to AMP and pyrophosphate, which is equivalent to spending two high-energy phosphate bonds. Delivery of the aminoacyl-tRNA by EF-Tu uses one GTP, and translocation by EF-G uses another GTP. The total is 2 + 1 + 1 = 4 high-energy bonds per peptide bond, which is why protein synthesis consumes a large part of a cell's energy budget.

Question 6

The antibiotic puromycin resembles the 3' end of an aminoacyl-tRNA. It enters the ribosomal A site and accepts the growing peptide chain. What is the consequence?

  1. The mRNA is cleaved
  2. Translation proceeds normally
  3. The ribosome moves backward
  4. The chain is released prematurely as a short peptide attached to puromycin
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Answer: D. The chain is released prematurely as a short peptide attached to puromycin

Peptidyl transferase transfers the growing chain to puromycin as if it were an aminoacyl-tRNA, but puromycin cannot be translocated or extended, so the peptide dissociates. This causes premature termination of translation, which is why puromycin is toxic to both bacteria and eukaryotes.

Question 7

Lysosomal enzymes are normally tagged with mannose-6-phosphate in the Golgi and delivered to lysosomes. In a cell lacking the enzyme that adds this tag, what is the most likely finding?

  1. Lysosomal enzymes accumulate in the nucleus
  2. Lysosomal enzymes are degraded in the ER
  3. Lysosomal enzymes are secreted from the cell instead of reaching lysosomes, and lysosomes accumulate undigested material
  4. Lysosomal enzymes are delivered normally
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Answer: C. Lysosomal enzymes are secreted from the cell instead of reaching lysosomes, and lysosomes accumulate undigested material

Mannose-6-phosphate is the signal recognized by receptors in the trans-Golgi network that package enzymes into vesicles bound for lysosomes. Without the tag, the enzymes are sent by default to the cell surface and secreted, and the lysosomes lack hydrolases. This is the basis of I-cell disease.

Question 8

Which pairing of ribosomal subunits and complete ribosome is correct?

  1. Bacteria: 40S + 60S = 80S; eukaryotes: 30S + 50S = 70S
  2. Bacteria: 30S + 50S = 70S; eukaryotes: 40S + 60S = 80S
  3. Both use 60S + 40S = 100S
  4. Bacteria: 30S + 50S = 80S; eukaryotes: 40S + 60S = 100S
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Answer: B. Bacteria: 30S + 50S = 70S; eukaryotes: 40S + 60S = 80S

Svedberg units measure sedimentation rate, which depends on shape as well as mass, so they do not add linearly. Bacterial ribosomes are 70S (30S + 50S) and eukaryotic cytoplasmic ribosomes are 80S (40S + 60S). The difference allows antibiotics such as macrolides to target the bacterial 50S subunit with less effect on the host's cytosolic ribosomes.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.