MCAT Biology 4 — Questions and Answers
Question 1: Which of the following structures is responsible for ribosome biogenesis in eukaryotic cells?
- Golgi apparatus
- Smooth endoplasmic reticulum
- Nucleolus (Correct answer)
- Centrosome
Correct answer: Nucleolus
The nucleolus is the site within the nucleus where rRNA genes are transcribed and ribosomal subunits are assembled before being exported to the cytoplasm.
Question 2: A patient has a genetic defect causing dysfunction of cilia in airway epithelial cells. Which cellular process is most likely impaired?
- Phagocytosis of pathogens
- Mucociliary clearance of debris (Correct answer)
- Synthesis of mucus glycoproteins
- Gas exchange across the epithelium
Correct answer: Mucociliary clearance of debris
Ciliary movement in airway epithelium drives mucociliary clearance, propelling mucus and trapped particles toward the throat for removal.
Question 3: In Mendelian genetics, two heterozygous parents (Aa × Aa) produce offspring. What fraction of offspring would display the recessive phenotype?
- 1/4 (Correct answer)
- 1/2
- 3/4
- 1/8
Correct answer: 1/4
A monohybrid cross of Aa × Aa yields a 3:1 phenotypic ratio (3 dominant: 1 recessive), so 1/4 of offspring display the recessive phenotype (aa).
Question 4: Which of the following best describes the role of ubiquitin in cellular protein degradation?
- It directly cleaves peptide bonds in target proteins
- It tags proteins for recognition and degradation by the proteasome (Correct answer)
- It prevents protein aggregation by acting as a chaperone
- It unfolds proteins in the endoplasmic reticulum
Correct answer: It tags proteins for recognition and degradation by the proteasome
Ubiquitin is covalently attached to lysine residues of target proteins, marking them for recognition by the 26S proteasome for degradation.
Question 5: A researcher crosses true-breeding red flowers with true-breeding white flowers and obtains all pink flowers in F1. This result is best explained by:
- Complete dominance of red over white
- Codominance
- Incomplete dominance (Correct answer)
- X-linkage
Correct answer: Incomplete dominance
Incomplete dominance produces a blended intermediate phenotype (pink) in heterozygotes because neither allele is fully dominant over the other.
Question 6: Which of the following correctly describes the orientation of translation?
- 5'→3' reading of mRNA with protein synthesis N-terminus to C-terminus (Correct answer)
- 3'→5' reading of mRNA with protein synthesis C-terminus to N-terminus
- 5'→3' reading of mRNA with protein synthesis C-terminus to N-terminus
- 3'→5' reading of mRNA with protein synthesis N-terminus to C-terminus
Correct answer: 5'→3' reading of mRNA with protein synthesis N-terminus to C-terminus
Ribosomes read mRNA in the 5'→3' direction and synthesize polypeptides from the amino terminus (N) to the carboxyl terminus (C).
Question 7: Which blood vessel has the lowest oxygen content under normal physiological conditions?
- Pulmonary artery (Correct answer)
- Pulmonary vein
- Renal artery
- Hepatic artery
Correct answer: Pulmonary artery
The pulmonary artery carries deoxygenated blood from the right ventricle to the lungs, making it the vessel with the lowest oxygen content in the systemic circulation.
MCAT Biology 4
Which of the following structures is responsible for ribosome biogenesis in eukaryotic cells?