AP - Advanced Placement Biology MCQ 1 — Questions and Answers
Question 1: In a signal transduction pathway, cAMP (cyclic AMP) functions as a:
- Primary messenger that binds to cell-surface receptors
- Second messenger that relays the signal inside the cell (Correct answer)
- Transcription factor that directly activates gene expression
- Receptor protein embedded in the plasma membrane
Correct answer: Second messenger that relays the signal inside the cell
cAMP is synthesized inside the cell after a primary messenger (e.g., a hormone) binds to a G-protein-coupled receptor. It relays the extracellular signal to intracellular targets, classifying it as a second messenger.
Question 2: Competitive inhibition of an enzyme differs from noncompetitive inhibition in that competitive inhibition:
- Permanently alters the enzyme's active site
- Can be overcome by increasing substrate concentration (Correct answer)
- Lowers the maximum reaction rate (Vmax) even at high substrate levels
- Binds only to the enzyme-substrate complex
Correct answer: Can be overcome by increasing substrate concentration
A competitive inhibitor occupies the active site, but flooding the reaction with excess substrate out-competes the inhibitor for binding, restoring activity. Noncompetitive inhibition lowers Vmax regardless of substrate concentration because the inhibitor binds elsewhere.
Question 3: Crossing over between homologous chromosomes occurs during which phase of meiosis?
- Metaphase I
- Prophase I (Correct answer)
- Anaphase II
- Prophase II
Correct answer: Prophase I
During Prophase I, homologous chromosomes synapse to form bivalents. Crossing over (exchange of segments at chiasmata) occurs at this stage, generating genetic recombination before chromosomes align on the metaphase plate.
Question 4: Which of the following conditions would violate Hardy-Weinberg equilibrium and cause allele frequencies to change over generations?
- Random mating among all individuals
- A very large, isolated population
- Gene flow due to migration into the population (Correct answer)
- Absence of new mutations
Correct answer: Gene flow due to migration into the population
Gene flow (migration of individuals carrying alleles into or out of a population) directly changes allele frequencies, violating one of the five Hardy-Weinberg assumptions and causing the population to deviate from equilibrium.
Question 5: In the E. coli lac operon, maximal transcription occurs when:
- Glucose is abundant and lactose is absent
- The lac repressor is bound to the operator sequence
- Lactose is present and glucose is absent (Correct answer)
- Both glucose and lactose are present in equal amounts
Correct answer: Lactose is present and glucose is absent
Lactose (via allolactose) inactivates the repressor, freeing the operator. Simultaneously, low glucose raises cAMP levels, activating the CAP-cAMP complex that greatly stimulates RNA polymerase binding. Both signals together produce maximal transcription.
Question 6: Facilitated diffusion differs from active transport in that facilitated diffusion:
- Moves solutes against their concentration gradient
- Requires ATP hydrolysis to power movement
- Uses membrane proteins but moves solutes down their concentration gradient (Correct answer)
- Only transports water molecules across the membrane
Correct answer: Uses membrane proteins but moves solutes down their concentration gradient
Facilitated diffusion is passive: specific channel or carrier proteins provide a pathway, but movement is driven by the existing concentration gradient — no energy input is needed. Active transport, by contrast, moves solutes against their gradient and requires ATP.
AP - Advanced Placement Biology MCQ 1
In a signal transduction pathway, cAMP (cyclic AMP) functions as a: