MCAT (Biological and Biochemical) 3 — Questions and Answers
Question 1: Which cell type in the nervous system forms the myelin sheath in the peripheral nervous system?
- Oligodendrocytes
- Schwann cells (Correct answer)
- Astrocytes
- Microglia
Correct answer: Schwann cells
Schwann cells myelinate axons in the peripheral nervous system, while oligodendrocytes perform the same function in the central nervous system.
Question 2: A substrate analog that binds irreversibly to an enzyme's active site is best described as what kind of inhibitor?
- Competitive inhibitor
- Noncompetitive inhibitor
- Uncompetitive inhibitor
- Suicide (mechanism-based) inhibitor (Correct answer)
Correct answer: Suicide (mechanism-based) inhibitor
Suicide inhibitors (mechanism-based inhibitors) bind covalently and irreversibly to the active site, permanently inactivating the enzyme.
Question 3: Which phase of meiosis is responsible for genetic recombination through crossing over?
- Prophase I (Correct answer)
- Metaphase II
- Anaphase I
- Telophase II
Correct answer: Prophase I
Crossing over between homologous chromosomes occurs during Prophase I of meiosis, at the tetrad stage (synapsis), increasing genetic diversity.
Question 4: Which hormone stimulates the reabsorption of sodium in the collecting duct of the kidney?
- ADH (vasopressin)
- Aldosterone (Correct answer)
- Atrial natriuretic peptide
- Parathyroid hormone
Correct answer: Aldosterone
Aldosterone, a mineralocorticoid secreted by the adrenal cortex, acts on the collecting duct to increase Na+ reabsorption and K+ secretion.
Question 5: What is the Michaelis constant (Km) a measure of?
- Maximum reaction velocity
- Enzyme concentration
- Substrate affinity for the enzyme (Correct answer)
- Product inhibition strength
Correct answer: Substrate affinity for the enzyme
Km is the substrate concentration at which reaction velocity is half of Vmax; a lower Km indicates higher affinity of the enzyme for its substrate.
Question 6: Which lipid modification is required to anchor a protein to the outer leaflet of the plasma membrane?
- Myristoylation
- GPI anchor (Correct answer)
- Palmitoylation
- Prenylation
Correct answer: GPI anchor
GPI (glycosylphosphatidylinositol) anchors attach proteins to the extracellular leaflet of the plasma membrane via a lipid-linked glycan bridge.
Question 7: In a feedback inhibition pathway, the end product of a biosynthetic pathway typically inhibits which enzyme?
- The last enzyme in the pathway
- The enzyme that produced it directly
- The first (committed) step enzyme (Correct answer)
- A downstream enzyme in a parallel pathway
Correct answer: The first (committed) step enzyme
End-product (feedback) inhibition classically targets the first committed step of the pathway, preventing wasteful synthesis when the product is abundant.
MCAT (Biological and Biochemical) 3
Which cell type in the nervous system forms the myelin sheath in the peripheral nervous system?