Take the NUMS Biology — Biodiversity: Viruses, Bacteria and Kingdoms (Set 4): 30 multiple-choice questions (40 minutes) with instant scoring and a full…
The first 5 questions of this test, with the correct answer and explanation.
Q1. A culture starts with 1000 bacteria that divide every 20 minutes with no lag phase and no limitation. How many cells are present after 3 hours?
- A. 54 000
- B. 512 000 ✓
- C. 1 048 576
- D. 18 000
Answer: B. 512 000
Explanation: 3 h = 180 min = 9 generations; N = 1000 × 2⁹ = 512 000.
Q2. 0.1 mL of a 10⁻⁵ dilution of a bacterial culture is spread on agar and produces 150 colonies. What is the viable count of the original culture per mL?
- A. 1.5 × 10⁶
- B. 1.5 × 10⁷
- C. 1.5 × 10⁹
- D. 1.5 × 10⁸ ✓
Answer: D. 1.5 × 10⁸
Explanation: Colonies ÷ (volume plated × dilution) = 150 ÷ (0.1 × 10⁻⁵) = 1.5 × 10⁸ per mL.
Q3. A bacterial population increases from 4 × 10⁶ to 6.4 × 10⁷ cells in exactly 2 hours of exponential growth. What is its generation time?
- A. 45 minutes
- B. 15 minutes
- C. 30 minutes ✓
- D. 60 minutes
Answer: C. 30 minutes
Explanation: 6.4 × 10⁷ ÷ 4 × 10⁶ = 16 = 2⁴, so 4 generations in 120 min: 120 ÷ 4 = 30 min.
Q4. Two auxotrophic strains cannot grow on minimal medium alone. When separated by a filter allowing only fluid to pass, no prototrophs appear, but when mixed directly, prototrophs arise. DNase has no effect on the mixture. The process is
- A. transformation by free DNA in the medium
- B. conjugation, because direct cell contact is required ✓
- C. binary fission
- D. transduction by bacteriophage
Answer: B. conjugation, because direct cell contact is required
Explanation: Separation by a filter blocked gene transfer, and DNase did not stop it, ruling out transformation; with no phage involved, direct contact points to conjugation via pili.
Q5. In Griffith's experiment, mice injected with heat-killed smooth (S) cells mixed with live rough (R) cells died and live S cells were recovered. In a repeat, the extract of heat-killed S cells was first treated with DNase. What outcome is expected and why?
- A. Mice survive, because the transforming principle (DNA) was destroyed ✓
- B. Mice die, because RNA is the transforming principle
- C. Mice survive, because DNase kills live R cells
- D. Mice die, because proteins carry the information
Answer: A. Mice survive, because the transforming principle (DNA) was destroyed
Explanation: Avery, MacLeod and McCarty showed the transforming principle is DNA; destroying it prevents R cells acquiring the capsule gene, so no virulence is gained.
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Q6. In the Hershey–Chase experiment, T2 phages labelled with ³⁵S protein infected E. coli and the mixture was blended and centrifuged. Where is most radioactivity found?
- A. In both pellet and progeny equally
- B. In the progeny phages
- C. In the supernatant, with the empty phage coats
- D. In the bacterial pellet
Q7. A researcher mixes RNA from tobacco mosaic virus strain A with the protein coat of strain B and infects tobacco plants. The new virus particles that are produced will have
- A. the protein coat of strain B and cause strain B lesions
- B. the protein coat of strain A and cause strain A lesions
- C. no coat, since coat protein is not replicated
- D. a mixed coat of both strains
Q8. In which part of a T-even bacteriophage would a mutation most likely change the range of host bacteria it can infect?
- A. Contractile sheath
- B. DNA inside the head only
- C. Capsid proteins forming the head
- D. Tail fibres
Q9. A temperate phage infects a bacterium and its DNA integrates into the host chromosome. Years later UV light causes the bacteria to lyse and release phages. This sequence illustrates
- A. conjugation between two bacteria
- B. transformation of the host
- C. a prophage being induced to enter the lytic cycle
- D. a lytic cycle without any integration
Q10. AZT is used in HIV infection. It is a nucleoside analogue that terminates DNA chains. Which viral enzyme is the target?
- A. Integrase of the T-cell
- B. RNA replicase
- C. DNA polymerase of the host nucleus
- D. Reverse transcriptase
Q11. A person exposed to HIV two weeks ago tests negative on an antibody ELISA but is shown to be infected by a viral RNA test. The best explanation is
- A. the window period, during which antibodies are not yet at detectable levels
- B. the ELISA detects viral RNA rather than antibodies, so it misses early infection
- C. HIV does not provoke antibodies at all, so antibody tests can never detect an infection
- D. the person is naturally immune to HIV, so the antibody test is correctly negative
Q12. The gradual fall of CD4⁺ T helper cells in HIV infection leads to opportunistic infections mainly because
- A. HIV blocks antibody production in the bone marrow only
- B. helper T cells coordinate both antibody and cytotoxic responses
- C. HIV destroys red blood cells and causes anaemia
- D. helper cells directly kill all bacteria
Q13. A viral genome is single-stranded RNA that is copied into double-stranded DNA and integrated into the host genome. This virus is best described as a
- A. viroid
- B. bacteriophage with DNA
- C. prion
- D. retrovirus
Q14. Which pair correctly distinguishes a viroid from a prion?
- A. A viroid is a short naked circular RNA; a prion is an infectious misfolded protein with no nucleic acid
- B. A viroid has a protein capsid but no genome; a prion has a lipid envelope and a small DNA genome
- C. A viroid is an infectious protein with no nucleic acid; a prion is a short naked circular RNA
- D. Both are DNA molecules wrapped in a protein coat, but a prion has an additional lipid envelope
Q15. A Gram-stained smear shows purple cells. What does this indicate about the cell wall, and which agent would be most effective?
- A. A thin peptidoglycan wall with outer membrane; lysozyme alone is ineffective
- B. A thick peptidoglycan wall; penicillin acting on wall synthesis is effective
- C. An outer lipopolysaccharide membrane; penicillin is ineffective
- D. No cell wall; penicillin is effective
Q16. A growth curve is plotted as log cell number against time. Which phase gives a straight line with the steepest positive slope?
- A. Lag phase
- B. Log (exponential) phase
- C. Stationary phase
- D. Death phase
Q17. An antibiotic that acts only on actively dividing cells is added to a culture at the start of the lag phase and is then compared with addition at mid-log phase. Which result is expected?
- A. It is equally effective in both phases, because the antibiotic acts on every cell
- B. It is more effective in the lag phase, because the cells are most sensitive then
- C. It is more effective in mid-log phase, when cells are dividing rapidly
- D. It is more effective in the stationary phase, when cell numbers are highest
Q18. A medical instrument is exposed to boiling water for 30 minutes, yet a culture from it still grows bacteria. The most likely explanation is
- A. boiling increased the number of ribosomes
- B. bacterial flagella repaired the DNA
- C. endospores survived, being highly heat-resistant
- D. bacterial capsules were activated by heat
Q19. Nitrosomonas oxidises ammonium to nitrite and uses the energy released to fix CO₂. It is best classified as a
- A. chemoheterotroph
- B. chemoautotroph
- C. photoheterotroph
- D. photoautotroph
Q20. Heterocysts in Anabaena are thick-walled cells that lack photosystem II. How does this suit their role?
- A. They store starch reserves for the filament, so that other cells need not photosynthesise
- B. No oxygen is produced there, so oxygen-sensitive nitrogenase can fix nitrogen
- C. They make endospores that survive drought and heat, so the filament can reproduce
- D. They carry out photosynthesis faster than other cells, producing oxygen for nitrogenase
Q21. A hospital finds that a bacterium acquired resistance to three antibiotics after being in the same patient with a resistant strain. The most likely mechanism is
- A. loss of the cell wall
- B. spontaneous mutation of all three genes simultaneously
- C. transfer of an R plasmid by conjugation
- D. rapid binary fission alone
Q22. Mutant E. coli make β-galactosidase constantly, whether or not lactose is present. Which defect explains this?
- A. A missing β-galactosidase gene
- B. A non-functional repressor that cannot bind the operator
- C. A repressor that cannot bind lactose and always binds the operator
- D. A non-functional promoter
Q23. Which feature is common to all fungi but absent in plants?
- A. Cell walls of chitin and absorptive heterotrophic nutrition
- B. Presence of chloroplasts and starch
- C. Motile gametes with 9+2 flagella in all species
- D. Cell walls of cellulose and photosynthesis
Q24. A mushroom's mycelium is found to have cells each containing two genetically distinct nuclei. This stage is called
- A. dikaryotic, typical of basidiomycetes
- B. diploid with fused nuclei, typical of zygomycetes
- C. haploid with a single nucleus, typical of yeasts
- D. polyploid, typical of bacteria
Q25. A lichen is exposed to air polluted with sulphur dioxide and dies, while nearby mosses survive. What does the lichen's death illustrate?
- A. Lichens absorb substances directly from the air so they are sensitive indicators of pollution
- B. Mosses lack photosynthetic cells, so they are unaffected by pollutants that damage photosynthesis
- C. Lichens obtain food only from the soil through roots, so soil acidification by SO₂ killed them
- D. Lichens are parasites of moss, so the sulphur dioxide spreads from the moss and kills the lichen
Q26. A unicellular organism has a pellicle, a contractile vacuole, a flagellum and chloroplasts, and it can survive in darkness by absorbing organic nutrients. It is most likely
- A. Anabaena, a cyanobacterium
- B. Euglena, a mixotrophic protist
- C. Paramecium, a ciliate
- D. Amoeba, a rhizopod
Q27. In the life cycle of Plasmodium, which stage is injected into humans by the mosquito, and into which cells does it first go?
- A. Sporozoites, entering liver cells
- B. Merozoites, entering red blood cells
- C. Gametocytes, entering liver cells
- D. Zygotes, entering white blood cells
Q28. Which characteristic best distinguishes Archaea from Bacteria?
- A. Archaea are always pathogenic to humans, whereas bacteria are never harmful to their hosts
- B. Archaeal cell walls lack peptidoglycan and membranes contain ether-linked lipids
- C. Archaea have a nuclear envelope around their DNA, whereas bacteria have a free nucleoid
- D. Archaea possess membrane-bound mitochondria, whereas bacteria make ATP at the cell membrane
Q29. Yeast cells in a closed jar of fruit juice multiply by budding and make alcohol. The same yeast in an open, aerated flask makes more cells and little alcohol. This is explained because yeast is
- A. a photosynthetic autotroph, making its own sugar from light and carbon dioxide
- B. a facultative anaerobe, using aerobic respiration when oxygen is present
- C. an obligate anaerobe, so it grows only when oxygen is absent from the medium
- D. an obligate aerobe, so it can grow only by aerobic respiration with oxygen present
Q30. A patient in a city outbreak has high fever, severe joint pain and a falling platelet count. Which causative agent and vector pairing is correct?
- A. A virus transmitted by the Aedes mosquito
- B. A fungus transmitted by air
- C. A protozoan transmitted by the Anopheles mosquito
- D. A bacterium transmitted by contaminated water