NUMS — Medical & Dental Entry Test

NUMS Biology — Biodiversity: Viruses, Bacteria and Kingdoms (Set 6)

Take the NUMS Biology — Biodiversity: Viruses, Bacteria and Kingdoms (Set 6): 30 multiple-choice questions (40 minutes) with instant scoring and a full…

What this test covers

30 NUMS Biology

Sample questions with answers

The first 5 questions of this test, with the correct answer and explanation.

Q1. A phage lysate is diluted 10⁻⁶-fold and 0.1 mL of the dilution produces 45 plaques on a bacterial lawn. What is the titre of the original lysate?
- A. 4.5 × 10⁶ plaque-forming units per mL
- B. 4.5 × 10⁸ plaque-forming units per mL ✓
- C. 4.5 × 10⁹ plaque-forming units per mL
- D. 4.5 × 10⁷ plaque-forming units per mL
Answer: B. 4.5 × 10⁸ plaque-forming units per mL
Explanation: Titre = plaques ÷ volume × dilution factor = 45 ÷ 0.1 × 10⁶ = 4.5 × 10⁸ per mL.
Q2. A bacterial culture grows from 500 cells to 4,000 cells in 90 minutes during exponential growth. What is the generation time?
- A. 90 minutes
- B. 30 minutes ✓
- C. 15 minutes
- D. 45 minutes
Answer: B. 30 minutes
Explanation: 4,000/500 = 8 = 2³, so there were 3 doublings in 90 minutes: 90 ÷ 3 = 30 minutes.
Q3. In a Gram stain, a student leaves out the alcohol decolourising step. What colour will Gram-negative cells appear?
- A. Green, since iodine reacts with their wall
- B. Pink, since they absorb safranin
- C. Purple, like Gram-positive cells ✓
- D. Colourless, since they cannot hold any dye
Answer: C. Purple, like Gram-positive cells
Explanation: The decolouriser washes crystal violet out of Gram-negative cells. If it is omitted, the stain stays in all cells and they look purple.
Q4. Penicillin kills growing bacteria but does not harm Mycoplasma. The most likely reason is that Mycoplasma:
- A. has no ribosomes at all
- B. lacks a cell wall ✓
- C. has a thick capsule that blocks penicillin
- D. is a eukaryote with a nuclear membrane
Answer: B. lacks a cell wall
Explanation: Penicillin blocks cross-linking of peptidoglycan. Organisms without a wall are not affected.
Q5. A bacterial culture lyses after a short exposure to UV light, releasing phage particles, although no phages were added. The best explanation is that the bacteria were:
- A. mutated so that they could no longer make cell walls
- B. infected with a virulent phage that was dormant in the medium
- C. lysogenic, carrying a prophage induced into the lytic cycle ✓
- D. carrying a plasmid that makes lysozyme
Answer: C. lysogenic, carrying a prophage induced into the lytic cycle
Explanation: Stress such as UV can excise a prophage from the bacterial chromosome and start the lytic cycle.

The other 25 questions

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Q6. In a Hershey–Chase type experiment, phage T2 with ³²P-labelled DNA infects bacteria which are then spun in a blender and centrifuged. Where is most of the radioactivity found?
- A. In the newly formed phage protein coats only
- B. Equally in pellet and supernatant
- C. In the supernatant containing empty phage coats
- D. In the bacterial pellet
Q7. A patient's blood shows 150 CD4⁺ T cells per µL (normal is 500–1,200). Taking AIDS to be diagnosed at counts below 200 per µL, which is the most appropriate statement?
- A. HIV has depleted T cells enough to meet the AIDS criterion
- B. The count shows only that antibody levels are high
- C. The patient has a bacterial infection, since T cells are low
- D. The patient is immune to HIV because few target cells remain
Q8. An infectious agent keeps its infectivity after nuclease treatment but loses it after protease treatment. It is most likely:
- A. a viroid
- B. a retrovirus
- C. a prion
- D. a bacteriophage
Q9. Strain A and strain B of bacteria are grown in the two arms of a U-tube separated by a filter with pores that stop cells but allow DNA and viruses through. No gene transfer is seen. Gene transfer does occur if the filter is removed. This indicates transfer by:
- A. binary fission, which needs no contact
- B. transformation, which needs only free DNA in the medium
- C. transduction, which needs phage particles
- D. conjugation, which needs direct cell-to-cell contact
Q10. In Griffith's experiment, mice were injected with heat-killed smooth (S) bacteria mixed with living rough (R) bacteria. What was found in the blood of dead mice?
- A. Only dead S-type bacteria, killed by heat treatment
- B. Living S bacteria produced by mutation of R bacteria
- C. Only living R-type bacteria, unchanged by the dead S cells
- D. Living S-type bacteria, showing R cells had been transformed
Q11. A farmer's field becomes waterlogged for weeks and the crop turns yellow, showing nitrogen deficiency. Which process in the soil best explains the loss of nitrate?
- A. Nitrogen fixation by Rhizobium, forming nitrate
- B. Denitrification by anaerobic bacteria, releasing N₂ gas
- C. Ammonification by saprophytic fungi, forming N₂
- D. Nitrification by Nitrobacter, giving more ammonia in the soil
Q12. Cyanobacteria such as Anabaena form heterocysts when combined nitrogen is scarce. What is the function of these thick-walled cells?
- A. Storing glycogen for use during the night
- B. Carrying out faster photosynthesis than ordinary cells
- C. Forming resistant spores for surviving dry conditions
- D. Fixing atmospheric nitrogen in an oxygen-poor cell
Q13. A strict anaerobe isolated from a cow's rumen produces methane from carbon dioxide and hydrogen. It is most likely a member of:
- A. the fungi
- B. the cyanobacteria
- C. the Gram-negative proteobacteria
- D. the archaebacteria (methanogens)
Q14. An unknown organism has no chlorophyll, a cell wall containing chitin, stores glycogen and feeds by absorbing digested food. Which kingdom is it?
- A. Animalia
- B. Fungi
- C. Plantae
- D. Protista
Q15. Rhizopus (bread mould) produces a zygospore by fusion of two compatible hyphae. Which statement about ploidy is correct?
- A. The vegetative hyphae are diploid, and the zygospore is haploid
- B. The zygospore is haploid, and mitosis gives diploid spores
- C. The zygospore is diploid, and meiosis gives haploid spores
- D. The zygospore is diploid and the spores it gives rise to are diploid
Q16. Pine seedlings grow poorly in sterilised, phosphate-poor soil but well in soil with mycorrhizal fungi. The fungus helps mainly by:
- A. fixing atmospheric nitrogen into nitrate in its own cells
- B. absorbing extra phosphate and receiving sugars in return
- C. killing all the competing roots in the soil nearby
- D. carrying out photosynthesis on behalf of the plant
Q17. Lichens disappear from tree trunks as a city develops near a coal-burning factory. Lichens are used as indicators because they:
- A. need high nitrate levels in the air to survive
- B. are very sensitive to sulphur dioxide pollution
- C. only grow on healthy living bark with no pollution
- D. are killed by the direct sunlight reaching town trunks
Q18. Female Anopheles mosquitoes pick up Plasmodium gametocytes while feeding. Which statement about the parasite's life cycle is correct?
- A. Sexual reproduction occurs in red blood cells of humans
- B. Sexual reproduction occurs in the human liver, so humans are the definitive host
- C. Sexual reproduction occurs in the mosquito, so it is the definitive host
- D. The mosquito only carries the parasite and has no role in its reproduction
Q19. Euglena photosynthesises in light. A culture kept in continuous darkness with a rich organic medium survives and divides. Taken together, Euglena is best described as:
- A. strictly autotrophic, unable to feed on organic matter at all
- B. mixotrophic, using both autotrophic and heterotrophic nutrition
- C. chemoautotrophic, using inorganic substances as its energy source
- D. strictly heterotrophic and incapable of photosynthesis in any light
Q20. During conjugation in Paramecium, two cells come together. What is exchanged between them?
- A. Macronuclei, which are kept completely unchanged
- B. Cytoplasm only, with no exchange of any nuclei
- C. Haploid micronuclei, which then fuse in each cell
- D. Entire cells, which fuse permanently into one cell
Q21. Under the microscope, a Gram-positive coccus is seen in irregular grape-like clusters. Which genus is most likely?
- A. Streptococcus
- B. Bacillus
- C. Staphylococcus
- D. Vibrio
Q22. A food technician finds that boiling a broth for 10 minutes did not make it sterile. The most probable cause is that:
- A. bacterial plasmids became active at high temperature
- B. capsules of the bacteria protected them from heat
- C. viruses grew in the broth while it cooled
- D. heat-resistant endospores survived the boiling
Q23. Resistance to an antibiotic is found within a short time in several unrelated bacterial species in a hospital. Which mechanism most readily accounts for this?
- A. Mutation in all species at once at the same gene
- B. Direct induction of mutations by the antibiotic
- C. Plasmid transfer carrying resistance genes by conjugation
- D. Binary fission copying the resistance gene between the species
Q24. A sputum smear stained with carbol fuchsin retains the red colour even after treatment with acid-alcohol. Which pathogen is suggested?
- A. Corynebacterium diphtheriae
- B. Streptococcus pneumoniae
- C. Mycobacterium tuberculosis
- D. Salmonella typhi
Q25. A patient has high fever and joint pain after a daytime mosquito bite. Dengue virus is suspected. Which description is correct?
- A. A DNA virus spread by the night-biting Anopheles mosquito
- B. A protozoan spread by the Aedes mosquito
- C. An RNA virus spread by the day-biting Aedes mosquito
- D. A bacterium spread by the Culex mosquito
Q26. Alcohol-based hand sanitisers inactivate enveloped viruses such as influenza more readily than some non-enveloped viruses. The reason is that alcohol:
- A. destroys the capsid of all viruses equally
- B. denatures the viral DNA inside the capsid
- C. prevents virus entry by blocking host receptors
- D. dissolves the lipid envelope of the virus
Q27. Two organisms belong to the same order but different families. Which statement is certainly correct?
- A. They can interbreed freely with each other
- B. They belong to the same genus as each other
- C. They belong to the same family as each other
- D. They belong to the same class and phylum
Q28. In which growth phase are bacteria most susceptible to penicillin, and why?
- A. Stationary phase, because cells are dividing at their fastest
- B. Lag phase, because cells are adapting and growing quickly
- C. Log phase, because cells are actively building new cell wall
- D. Death phase, because cells are producing the most peptidoglycan
Q29. The Whittaker five-kingdom system separated fungi from plants chiefly on the basis of:
- A. ability to move and to reproduce
- B. mode of nutrition and cell wall composition
- C. presence of ribosomes and a plasma membrane
- D. presence of a nucleus and a cellulose cell wall
Q30. Cut root nodules of a healthy legume are pink inside. The pink pigment, leghaemoglobin, is important because it:
- A. makes the nodule wall impermeable to nitrogen gas
- B. binds oxygen, protecting nitrogenase from damage
- C. traps light for photosynthesis inside the root nodule
- D. carries nitrate from the nodule up to the shoot