IB DP Biology- C3.2 Defense against disease- IB Style Questions For HL Paper 1A -FA 2025
▶️ Answer/Explanation
✅ Answer: (A) Phagocytes
Question
Some strains of the pathogenic bacterium Staphylococcus aureus have developed mechanisms that protect them against foreign DNA. What effect does this have on the evolution of antibiotic resistance in these strains of S. aureus?
A. Slower evolution, as bacteria with the antibiotic resistance gene will not reproduce
B. Slower evolution, as the antibiotic resistance gene from other species will not be accepted
C. Faster evolution, as mutations within a population are less likely to occur
D. Faster evolution, as antibiotic resistance genes can only be passed to individuals of the same species
▶️Answer/Explanation
Answer: B. Slower evolution, as the antibiotic resistance gene from other species will not be accepted
Explanation:
Some strains of Staphylococcus aureus have developed mechanisms to protect themselves from foreign DNA, such as restriction-modification systems or CRISPR-Cas systems. These mechanisms prevent the uptake and incorporation of foreign genetic material, including plasmids or transposons carrying antibiotic resistance genes.
This leads to slower evolution of antibiotic resistance because horizontal gene transfer (HGT) is a major way bacteria acquire resistance genes from other species. If S. aureus rejects foreign DNA, it is less likely to gain resistance genes from other bacteria, reducing the rate at which it evolves resistance.
Why the other options are incorrect:
A (Incorrect) – Bacteria with resistance genes can still reproduce if they inherit the genes through vertical transmission.
C (Incorrect) – Mutations within a population can still occur regardless of foreign DNA protection, so this does not slow evolution.
D (Incorrect) – If resistance genes are only passed within the same species, it does not necessarily speed up evolution; HGT generally accelerates resistance acquisition.
Question
In 1940, Florey and Chain tested penicillin on bacterial infections in eight mice, before treating a sick patient. Currently, what is the correct order for testing the effectiveness of an antibiotic or other drug?
A. Affected patients, animals, healthy people
B. Animals, healthy people, affected patients
C. Healthy people, animals, affected patients
D. Animals, affected patients, healthy people
▶️Answer/Explanation
Answer: B. Animals, healthy people, affected patients
Explanation:
Modern drug testing follows a strict sequence to ensure safety and effectiveness:
- Animals – Tested first to observe general effects and possible toxicity.
- Healthy people (Phase I trials) – To confirm the drug is safe in humans without risking sick individuals.
- Affected patients (Phase II & III trials) – To test how effective the drug is in treating the actual condition.
Why the other options are wrong:
A. Affected patients, animals, healthy people → Unsafe! Testing directly on sick humans without prior animal or safety trials is unethical.
C. Healthy people, animals, affected patients → Illogical order. Animal testing must come first before any human exposure.
D. Animals, affected patients, healthy people → Skips crucial safety step in healthy volunteers — riskier for patients.