Microbiology

Q1. (3 marks) — 1 mark each (definition + example)

• (a) Coccus — spherical/round shaped bacterium; e.g. Staphylococcus / Streptococcus. (1)
• (b) Bacillus — rod-shaped bacterium; e.g. Bacillus / Escherichia. (1)
• (c) Spirillum — spiral/helical (rigid) shaped bacterium; e.g. Spirillum / Campylobacter. (1) Why: morphology is classified by geometric shape, a core diagnostic feature.

Q2. (4 marks)

• Gram-positive: thick peptidoglycan layer, no outer membrane (1); retains crystal violet → purple/violet (1).
• Gram-negative: thin peptidoglycan layer with an outer lipopolysaccharide membrane (1); loses crystal violet, takes up safranin → pink/red (1). Why: thick peptidoglycan traps the crystal violet–iodine complex during decolourisation.

Q3. (4 marks)

• (a) $N = N_0 \times 2^n$ (1)
• (b) In 2 hours (120 min) with 20-min generations: $n = 120/20 = 6$ generations. $N = 1 \times 2^6 = 64$ cells. (2) (1 for $n=6$, 1 for answer 64)
• (c) Exponential (geometric/logarithmic) growth (1) Why: each division doubles the population, giving a power-of-two increase.

Q4. (6 marks) — 2 marks each (name + description)

• Conjugation: direct transfer of DNA (often a plasmid) from a donor to a recipient cell through a pilus (cell-to-cell contact). (2)
• Transformation: uptake of free/naked DNA from the surrounding environment (from lysed cells) by a competent bacterium. (2)
• Transduction: transfer of bacterial DNA from one cell to another by a bacteriophage (virus) vector. (2)

Q5. (4 marks)

• (a) Lag → Log (exponential) → Stationary → Death (decline) — 0.5 each, (2) total.
• (b) In stationary phase the rate of cell division equals the rate of cell death (1), because nutrients become limited and toxic waste accumulates, so the population size stays roughly constant (1).

Q6. (4 marks)

• (a) Fate of host cell: Lytic — host cell is lysed/killed and releases new virions (1); Lysogenic — host cell survives and continues to divide (1).
• (b) Integration: Lytic — viral DNA is not integrated, replicated immediately (1); Lysogenic — viral DNA integrates into host chromosome as a prophage (1).

Q7. (4 marks)

• (a) A retrovirus is an RNA virus that copies its genome into DNA (1); uses the enzyme reverse transcriptase (1).
• (b) Information flows from RNA → DNA (a DNA copy is synthesised using the RNA genome as template) (2), reversing the normal DNA→RNA direction.

Q8. (4 marks) — 2 marks each

• (a) Prion: an infectious misfolded protein (no nucleic acid) that induces normal proteins to misfold, causing disease (e.g. CJD/BSE). (2)
• (b) Viroid: a small, naked, circular single-stranded RNA molecule (no protein coat) that infects plants. (2)

Q9. (3 marks)

• (a) An antibiotic is a chemical substance (produced by/derived from microorganisms) that kills or inhibits the growth of bacteria. (1)
• (b) Any two of: spontaneous mutation of target genes; horizontal gene transfer (plasmids carrying resistance genes via conjugation); production of enzymes that break down the antibiotic (e.g. β-lactamase); efflux pumps; altered/modified target site. (2)

Q10. (4 marks)

• (a) Aseptic technique: a set of procedures that prevent contamination of cultures by unwanted microorganisms and protect the operator. (1)
• (b) Any three of: flaming the inoculating loop / neck of bottle; working near a Bunsen flame (updraught); keeping lids/Petri dishes closed as much as possible; sterilising media and equipment (autoclave); disinfecting the work surface; not fully opening the lid of the Petri dish. (3)

[
  {"claim":"After 6 generations from 1 cell, N=64","code":"N0=1; n=6; N=N0*2**n; result=(N==64)"},
  {"claim":"n = 120/20 = 6 generations in 2 hours","code":"n=120/20; result=(n==6)"},
  {"claim":"Binary fission formula gives 8 cells after 3 generations","code":"result=(1*2**3==8)"},
  {"claim":"Growth is exponential: ratio between successive generations is constant (=2)","code":"vals=[2**k for k in range(5)]; ratios=[vals[i+1]/vals[i] for i in range(4)]; result=(all(r==2 for r in ratios))"}
]