MCAT Biology 5 — Questions and Answers
Question 1: Which of the following correctly describes the concept of epistasis?
- One gene masks the expression of an allele at the same locus
- One gene modifies or masks the expression of a gene at a different locus (Correct answer)
- Two alleles at the same locus are both expressed
- Multiple genes contribute additively to a single phenotype
Correct answer: One gene modifies or masks the expression of a gene at a different locus
Epistasis occurs when the expression of one gene (the epistatic gene) suppresses or modifies the phenotypic effect of another gene at a different locus.
Question 2: A muscle cell at rest has a resting membrane potential of -90 mV. This value is primarily determined by:
- The equilibrium potential for sodium
- The equilibrium potential for potassium (Correct answer)
- Equal contributions of Na+ and K+ gradients
- The activity of the sodium-calcium exchanger
Correct answer: The equilibrium potential for potassium
The resting membrane potential of muscle cells is closest to the Nernst equilibrium potential for K+ because membranes are most permeable to potassium at rest.
Question 3: Which of the following is NOT a characteristic of stem cells?
- Self-renewal capacity
- Ability to differentiate into specialized cell types
- Permanent withdrawal from the cell cycle (Correct answer)
- Pluripotency in embryonic stem cells
Correct answer: Permanent withdrawal from the cell cycle
Stem cells are defined by their ability to self-renew and differentiate; permanent cell cycle withdrawal (terminal differentiation) is the opposite of a stem cell property.
Question 4: During meiosis I, crossing over between homologous chromosomes occurs at which structure?
- Kinetochore
- Synaptonemal complex/chiasmata (Correct answer)
- Centromere
- Telomere
Correct answer: Synaptonemal complex/chiasmata
Crossing over (recombination) occurs at chiasmata within the synaptonemal complex during prophase I of meiosis, exchanging segments between homologous chromosomes.
Question 5: Which of the following enzymes would be inhibited by a product of the citric acid cycle in a classic example of feedback inhibition?
- Succinate dehydrogenase inhibited by fumarate
- Isocitrate dehydrogenase inhibited by NADH (Correct answer)
- Malate dehydrogenase inhibited by pyruvate
- Citrate synthase inhibited by acetyl-CoA
Correct answer: Isocitrate dehydrogenase inhibited by NADH
Isocitrate dehydrogenase is allosterically inhibited by NADH (a product of the cycle), slowing the TCA cycle when the cell has sufficient reducing equivalents.
Question 6: A researcher discovers a new virus that infects bacteria and integrates its genome into the host chromosome without immediately causing cell lysis. This virus is best classified as a:
- Lytic phage
- Temperate phage (Correct answer)
- Retrovirus
- Viroid
Correct answer: Temperate phage
A temperate phage can enter lysogeny, integrating its genome as a prophage into the bacterial chromosome and replicating with the host without causing immediate lysis.
Question 7: Which of the following correctly describes the role of the hypothalamus in body temperature regulation?
- It generates heat through shivering thermogenesis directly
- It acts as a thermostat, integrating temperature signals and initiating effector responses (Correct answer)
- It secretes thyroid hormone to increase metabolic rate
- It directly controls sweat gland activity through hormonal signals
Correct answer: It acts as a thermostat, integrating temperature signals and initiating effector responses
The hypothalamus serves as the body's thermostat, receiving afferent temperature information and coordinating efferent responses (vasodilation, sweating, shivering) through the autonomic nervous system.
MCAT Biology 5
Which of the following correctly describes the concept of epistasis?