Microbiology

Section A (10 marks)

Q1 — B) Coccus. Coccus = spherical; bacillus = rod; spirillum = spiral; vibrio = comma-shaped. (1)

Q2 — B) Thick peptidoglycan retains crystal violet. The thick peptidoglycan layer traps the crystal violet–iodine complex during alcohol wash, so cells stay purple. (1)

Q3 — B) Asexual division into two identical daughter cells. Binary fission = DNA replication then splitting into two clones; no gametes/meiosis. (1)

Q4 — C) Conjugation. Conjugation requires physical contact via a sex pilus; transformation = free DNA uptake; transduction = phage-mediated. (1)

Q5 — C) Adjusts to environment, little/no division. Cells synthesise enzymes/metabolites before dividing; growth rate ≈ 0. (1)

Q6 — B) Retrovirus. Retroviruses (e.g. HIV) carry reverse transcriptase to make DNA from RNA. (1)

Q7 — C) Prion. Prions are protein-only infectious agents causing misfolding; no nucleic acid. (1)

Q8 — B) Integrates as a prophage. Lysogeny = genome inserted into host DNA, replicated passively without immediate lysis. (1)

Q9 — B) Capsid. All viruses have a protein capsid; envelopes, reverse transcriptase and cell walls are not universal. (1)

Q10 — B) Horizontal transfer of resistance plasmids. Resistance genes (e.g. β-lactamase) spread rapidly via plasmids through conjugation. (1)

Section B (5 marks)

Q11:

• (a) Transformation → (ii) Uptake of free DNA (1)
• (b) Transduction → (i) DNA transferred by bacteriophage (1)
• (c) Fungi → (iv) Eukaryotes with chitin walls, decomposers (1)
• (d) Exponential phase → (iii) Log/exponential growth phase (1)
• (e) Aseptic technique → (v) Prevent contamination of cultures (1)

Section C (14 marks)

Q12 — TRUE (1). Justification: Viroids are indeed small, naked, circular single-stranded RNA molecules that infect (mainly) plants and have no protein coat. (1)

Q13 — FALSE (1). Justification: It is the reverse — Gram-positive bacteria have the thick peptidoglycan layer; Gram-negative have a thin layer plus an outer LPS membrane. (1)

Q14 — FALSE (1). Justification: Antibiotics target bacterial structures (cell wall, ribosomes, etc.). Viruses lack these, so antibiotics do not work on viral infections like influenza; antivirals are needed. (1)

Q15 — TRUE (1). Justification: In the death/decline phase nutrients are depleted and toxins accumulate, so the death rate exceeds the division rate and viable numbers fall. (1)

Q16 — TRUE (1). Justification: In the lytic cycle new virions are assembled and the host cell lyses (bursts), releasing progeny viruses. (1)

Q17 — TRUE (1). Justification: Nitrogen-fixing bacteria (e.g. Rhizobium) convert atmospheric $N_2$ into ammonia/ammonium, making nitrogen available for plant assimilation — a key ecological role. (1)

Q18 — FALSE (1). Justification: Binary fission is asexual and produces genetically identical clones (barring rare mutations); it does not normally create genetic difference. (1)

[
  {"claim":"Exponential growth: 1 cell doubling every 20 min for 2 hours gives 64 cells", "code":"n = 1 * 2**(120//20); result = (n == 64)"},
  {"claim":"Coccus count: of the 4 shape options only one (coccus) is spherical", "code":"shapes = {'bacillus':'rod','coccus':'sphere','spirillum':'spiral','vibrio':'comma'}; spheres = [k for k,v in shapes.items() if v=='sphere']; result = (len(spheres)==1 and spheres[0]=='coccus')"},
  {"claim":"Gram-positive has thicker peptidoglycan than Gram-negative", "code":"gram_pos_pg = 30; gram_neg_pg = 5; result = (gram_pos_pg > gram_neg_pg)"},
  {"claim":"Section total marks equal 30", "code":"sectionA = 10*1; sectionB = 5*1; sectionC = 7*2; result = (sectionA + sectionB + sectionC == 30)"}
]