PCAT Biological 3 — Questions and Answers
Question 1: Which of the following best describes a competitive inhibitor of an enzyme?
- It permanently binds the active site and denatures the enzyme
- It binds the allosteric site and changes enzyme shape
- It competes with substrate for the active site and its effect can be overcome by excess substrate (Correct answer)
- It covalently modifies the substrate to prevent binding
Correct answer: It competes with substrate for the active site and its effect can be overcome by excess substrate
Competitive inhibitors bind the active site reversibly; increasing substrate concentration displaces the inhibitor and restores enzyme activity.
Question 2: During which stage of cellular respiration is the most ATP produced per glucose molecule?
- Glycolysis
- Pyruvate oxidation
- Krebs cycle
- Oxidative phosphorylation (Correct answer)
Correct answer: Oxidative phosphorylation
Oxidative phosphorylation via the electron transport chain generates approximately 32–34 ATP molecules, far more than earlier stages.
Question 3: Which hormone stimulates the reabsorption of sodium in the distal tubule of the nephron?
- ADH
- Aldosterone (Correct answer)
- PTH
- Cortisol
Correct answer: Aldosterone
Aldosterone, produced by the adrenal cortex, promotes Na⁺ reabsorption and K⁺ secretion in the distal tubule and collecting duct.
Question 4: What is the role of telomerase in cancer cells?
- It repairs double-strand DNA breaks
- It adds telomeric sequences to chromosome ends, enabling indefinite replication (Correct answer)
- It activates tumor suppressor genes
- It degrades damaged mitochondria
Correct answer: It adds telomeric sequences to chromosome ends, enabling indefinite replication
Cancer cells overexpress telomerase, which lengthens telomeres after each division, preventing replicative senescence and allowing unlimited proliferation.
Question 5: Which of the following correctly describes the process of translation?
- DNA is copied to produce a complementary RNA strand
- Pre-mRNA is processed to remove introns and join exons
- mRNA codons are read by tRNA anticodons to assemble a polypeptide (Correct answer)
- A single-stranded RNA template is used to synthesize double-stranded DNA
Correct answer: mRNA codons are read by tRNA anticodons to assemble a polypeptide
Translation occurs at ribosomes where tRNA anticodons recognize mRNA codons and deliver amino acids to build a polypeptide chain.
Question 6: Which cells of the immune system are primarily responsible for antibody production?
- Cytotoxic T cells
- Natural killer cells
- Plasma cells (differentiated B cells) (Correct answer)
- Macrophages
Correct answer: Plasma cells (differentiated B cells)
Plasma cells are terminally differentiated B lymphocytes that secrete large quantities of antigen-specific antibodies.
Question 7: In which phase of mitosis do chromosomes align at the cell's equatorial plate?
- Prophase
- Anaphase
- Telophase
- Metaphase (Correct answer)
Correct answer: Metaphase
During metaphase, spindle fibers attach to kinetochores and align chromosomes along the metaphase plate for equal distribution.
PCAT Biological 3
Which of the following best describes a competitive inhibitor of an enzyme?