PCAT Biological 5 — Questions and Answers
Question 1: Which statement accurately describes the concept of Hardy-Weinberg equilibrium?
- Allele frequencies change rapidly in small populations
- In a large, randomly mating population with no evolutionary forces, allele frequencies remain constant (Correct answer)
- Natural selection always drives populations toward equilibrium
- Mutation is required to maintain stable allele frequencies
Correct answer: In a large, randomly mating population with no evolutionary forces, allele frequencies remain constant
Hardy-Weinberg equilibrium describes a theoretical population where allele frequencies are stable in the absence of mutation, selection, drift, gene flow, and non-random mating.
Question 2: Which of the following describes the process by which bacteria acquire antibiotic resistance through direct DNA transfer between cells?
- Transformation
- Conjugation (Correct answer)
- Transduction
- Binary fission
Correct answer: Conjugation
Conjugation involves physical contact via a sex pilus through which a donor bacterium transfers plasmid DNA (often carrying resistance genes) to a recipient cell.
Question 3: Which checkpoint of the cell cycle verifies that DNA replication is complete and undamaged before allowing mitosis to proceed?
- G1 checkpoint
- G2 checkpoint (Correct answer)
- Spindle assembly checkpoint
- G0 checkpoint
Correct answer: G2 checkpoint
The G2 checkpoint assesses DNA integrity and replication completeness after S phase; if damage is detected, cyclin-CDK complexes are inactivated to halt progression into mitosis.
Question 4: In the lac operon model, what is the role of the lac repressor in the absence of lactose?
- It binds RNA polymerase to enhance transcription
- It binds the operator to block transcription of structural genes (Correct answer)
- It cleaves the mRNA to prevent translation
- It methylates the promoter to silence the operon permanently
Correct answer: It binds the operator to block transcription of structural genes
Without lactose, the active lac repressor binds the operator sequence, physically blocking RNA polymerase from transcribing the structural genes (lacZ, lacY, lacA).
Question 5: Which of the following best explains why ATP hydrolysis is coupled to energetically unfavorable cellular reactions?
- ATP directly donates electrons to drive redox reactions
- The free energy released by ATP hydrolysis is used to overcome the activation energy barrier of endergonic reactions (Correct answer)
- ATP acts as an enzyme to lower activation energy
- ATP hydrolysis raises intracellular pH to facilitate reactions
Correct answer: The free energy released by ATP hydrolysis is used to overcome the activation energy barrier of endergonic reactions
ATP hydrolysis releases approximately −30.5 kJ/mol, and this free energy is coupled to drive endergonic (thermodynamically unfavorable) reactions forward.
Question 6: Which structure prevents the backflow of blood from the left ventricle into the left atrium during systole?
- Tricuspid valve
- Pulmonary semilunar valve
- Mitral (bicuspid) valve (Correct answer)
- Aortic semilunar valve
Correct answer: Mitral (bicuspid) valve
The mitral valve closes during ventricular systole to prevent regurgitation of blood from the left ventricle back into the left atrium.
Question 7: What is the product of the light-dependent reactions of photosynthesis that directly powers the Calvin cycle?
- Glucose and O2
- CO2 and H2O
- ATP and NADPH (Correct answer)
- RuBP and G3P
Correct answer: ATP and NADPH
The light reactions convert light energy into chemical energy stored as ATP and NADPH, which are consumed in the Calvin cycle to fix CO2 into organic molecules.
PCAT Biological 5
Which statement accurately describes the concept of Hardy-Weinberg equilibrium?