PCAT Biology 3 — Questions and Answers
Question 1: Which enzyme unwinds the DNA double helix ahead of the replication fork?
- DNA polymerase
- Primase
- Helicase (Correct answer)
- Ligase
Correct answer: Helicase
Helicase breaks hydrogen bonds between base pairs and unwinds the double helix, creating the replication fork for DNA synthesis.
Question 2: The Hardy-Weinberg equilibrium requires which of the following conditions?
- Small population size
- Natural selection acting on the trait
- Random mating within the population (Correct answer)
- High mutation rate
Correct answer: Random mating within the population
Hardy-Weinberg equilibrium requires random mating, no selection, no mutation, no genetic drift, and no gene flow.
Question 3: Which molecule acts as the final electron acceptor in aerobic cellular respiration?
- NAD⁺
- FAD
- Oxygen (Correct answer)
- Carbon dioxide
Correct answer: Oxygen
Oxygen is the terminal electron acceptor in the electron transport chain, combining with electrons and H⁺ to form water.
Question 4: Nondisjunction during meiosis I results in which outcome?
- All daughter cells have the normal chromosome number
- Two cells with extra chromosomes and two normal cells
- All daughter cells receive abnormal chromosome numbers (Correct answer)
- Only one daughter cell is affected
Correct answer: All daughter cells receive abnormal chromosome numbers
When homologous chromosomes fail to separate in meiosis I, all four resulting gametes receive an abnormal number of chromosomes.
Question 5: In an enzyme-substrate interaction, what does the Michaelis constant (Km) represent?
- Maximum reaction velocity
- Substrate concentration at half-maximal velocity (Correct answer)
- Total enzyme concentration
- Product inhibition constant
Correct answer: Substrate concentration at half-maximal velocity
Km is the substrate concentration at which the reaction rate is half of Vmax, indicating the enzyme's affinity for its substrate.
Question 6: Which process allows RNA to be synthesized from a DNA template?
- Translation
- Replication
- Transcription (Correct answer)
- Reverse transcription
Correct answer: Transcription
Transcription uses RNA polymerase to synthesize an RNA strand complementary to the DNA template strand.
Question 7: T lymphocytes mature and become immunocompetent in which organ?
- Bone marrow
- Spleen
- Thymus (Correct answer)
- Lymph nodes
Correct answer: Thymus
T cells originate in bone marrow but migrate to the thymus where they undergo positive and negative selection to become mature, self-tolerant T lymphocytes.
PCAT Biology 3
Which enzyme unwinds the DNA double helix ahead of the replication fork?