NUMS — Medical & Dental Entry Test

NUMS Biology — Biodiversity: Viruses, Bacteria and Kingdoms (Set 5)

Take the NUMS Biology — Biodiversity: Viruses, Bacteria and Kingdoms (Set 5): 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 suspension is diluted 10⁻⁷-fold. Plating 0.5 mL of this dilution on a bacterial lawn gives 36 plaques. What is the titre of the original suspension?
- A. 7.2 × 10⁷ plaque-forming units per mL
- B. 1.8 × 10⁸ plaque-forming units per mL
- C. 3.6 × 10⁹ plaque-forming units per mL
- D. 7.2 × 10⁸ plaque-forming units per mL ✓
Answer: D. 7.2 × 10⁸ plaque-forming units per mL
Explanation: Titre = plaques ÷ volume plated × dilution factor = 36 ÷ 0.5 × 10⁷ = 7.2 × 10⁸ PFU/mL.
Q2. A culture starts with 5 bacterial cells and each cell divides every 30 minutes with no deaths. How many cells are present after 3 hours?
- A. 30
- B. 160
- C. 320 ✓
- D. 640
Answer: C. 320
Explanation: 3 hours = 6 generations; 5 × 2⁶ = 320.
Q3. A 0.1 mL sample of a 10⁻⁵ dilution of broth is spread on agar and gives 120 colonies. How many viable cells per mL were in the undiluted broth?
- A. 1.2 × 10⁶
- B. 1.2 × 10⁵
- C. 1.2 × 10⁷
- D. 1.2 × 10⁸ ✓
Answer: D. 1.2 × 10⁸
Explanation: Colonies ÷ volume plated × dilution factor = 120 ÷ 0.1 × 10⁵ = 1.2 × 10⁸ cells/mL.
Q4. A Gram-positive species is treated with lysozyme in an isotonic sucrose solution before Gram staining. After the full stain procedure, what is the most likely colour of its cells?
- A. Pink-red, because the peptidoglycan was removed so crystal violet washes out ✓
- B. Pink-red, because the lipopolysaccharide layer prevents the stain entering
- C. Purple, because the cell membrane is still thick enough to hold the stain
- D. Purple, because the sucrose solution fixes the stain permanently
Answer: A. Pink-red, because the peptidoglycan was removed so crystal violet washes out
Explanation: Lysozyme cuts peptidoglycan, which normally traps crystal violet–iodine; the protoplasts decolourise with alcohol and take up the safranin counterstain.
Q5. A growing culture is exposed to penicillin and its cells lyse in an ordinary medium but survive in a medium of high osmotic strength. What does this show?
- A. Penicillin blocks protein synthesis, so the cells burst without ribosomes in a hypertonic medium
- B. Penicillin blocks wall synthesis, so the cells burst without a wall in a hypotonic medium ✓
- C. Penicillin dissolves the membrane, so the cells leak in a hypertonic medium
- D. Penicillin cuts DNA, so the cells burst without chromosomes in a hypotonic medium
Answer: B. Penicillin blocks wall synthesis, so the cells burst without a wall in a hypotonic medium
Explanation: Penicillin inhibits cross-linking of peptidoglycan, so the weakened wall cannot resist osmotic uptake; a protective high-osmolarity medium prevents lysis.

The other 25 questions

Take the timed test (free, no sign-up) to see your score and the answer and explanation for every question.

Q6. Two auxotrophic strains of bacteria are grown in the two arms of a U-tube separated by a filter whose pores are too small for cells but large enough for viruses. Prototrophic recombinants still appear. Which gene-transfer process is the most probable?
- A. Transduction by phage particles
- B. Transformation by free-floating plasmids
- C. Binary fission with crossing over
- D. Conjugation through a sex pilus
Q7. A bacterial culture clears rapidly after brief exposure to UV light, and large numbers of virus particles are found in the broth. Which statement fits?
- A. The cells carried a transposon that moved into the bacterial membrane
- B. The cells carried a prophage that entered the lytic cycle after induction
- C. The cells carried a plasmid that began conjugation after the UV dose
- D. The cells carried a virus that was in the lytic cycle before induction
Q8. A man is tested for HIV antibodies two weeks after exposure and is negative, but he tests positive three months later. Which explanation is correct?
- A. Antibodies can only be made when CD4 cells reach zero
- B. The virus had been eliminated and then returned
- C. Antibodies were not yet at detectable levels (window period)
- D. The test detects only the integrated provirus in T cells
Q9. In a retrovirus infecting a human T cell, which sequence of events is correct?
- A. RNA → protein by integrase, then DNA copy by reverse transcriptase
- B. RNA → DNA by reverse transcriptase, then integration by integrase
- C. DNA → RNA by reverse transcriptase, then integration by ligase
- D. RNA → DNA by integrase, then integration by reverse transcriptase
Q10. An infectious extract from diseased sheep brain keeps its infectivity after treatment with DNase, RNase and UV light, but loses it after treatment with proteases. What is the agent most probably?
- A. A prion
- B. A bacteriophage
- C. A viroid
- D. A retrovirus
Q11. Oral polio vaccine (OPV) can induce herd protection even among unvaccinated contacts, whereas injected inactivated vaccine (IPV) cannot. What explains this?
- A. OPV contains viral RNA that is passed in urine
- B. OPV contains live attenuated virus that is shed in faeces
- C. OPV contains killed virus that is shed in saliva
- D. OPV contains antibodies that are shed in faeces
Q12. In a hospital ward, the proportion of antibiotic-resistant bacteria rises after widespread use of the antibiotic. Which explanation is correct?
- A. The antibiotic causes the resistance mutations in bacteria
- B. Bacteria choose to develop resistance after exposure
- C. Resistant bacteria arise only after conjugation with the antibiotic
- D. Resistant mutants already present survive and multiply under selection
Q13. A farm soil sample accumulates nitrite and little nitrate. Which group of bacteria is most probably inhibited?
- A. Ammonia-oxidising bacteria such as Nitrosomonas
- B. Denitrifying bacteria such as Pseudomonas
- C. Nitrite-oxidising bacteria such as Nitrobacter
- D. Nitrogen-fixing bacteria such as Rhizobium
Q14. A microbe lives in a hot spring at 90 °C without oxygen, produces methane from CO₂ and H₂, and has no peptidoglycan in its wall. It most likely belongs to
- A. Cyanobacteria
- B. Fungi
- C. Archaea
- D. Eubacteria
Q15. Why can the filamentous cyanobacterium Anabaena fix nitrogen in light even though it also makes oxygen in photosynthesis?
- A. Heterocysts lack photosystem I, so they make no ATP in the light
- B. Heterocysts lack photosystem II, so nitrogenase is not exposed to oxygen
- C. Vegetative cells lack nitrogenase, so oxygen cannot reach heterocysts
- D. Akinetes make oxygen, so nitrogenase stays active in the vegetative cells
Q16. Lichens grow well on bark in a rural area but disappear around a factory emitting sulphur dioxide. Which feature makes them useful as pollution indicators?
- A. They absorb water and gases over their whole surface with no cuticle
- B. They absorb minerals from deep soil through long roots
- C. They take in gases through stomata that close in polluted air
- D. They make their own sulphur compounds and tolerate no extras
Q17. A technician observes that a bread mould grows hyphae into the bread and the bread softens around them. Which description of fungal nutrition is correct?
- A. Hyphae engulf food particles by phagocytosis, then digest them inside
- B. Hyphae make food by photosynthesis, then store it as starch
- C. Hyphae secrete enzymes that digest food outside, then absorb the products
- D. Hyphae secrete acids to kill food, then absorb it without digestion
Q18. In the life cycle of Plasmodium, in which host or organ do the gametes fuse to form the zygote?
- A. Red blood cells of the human host
- B. Stomach of the female Anopheles mosquito
- C. Liver of the human host
- D. Salivary glands of the mosquito
Q19. Euglena kept in total darkness for weeks loses its chlorophyll but survives and grows in a medium rich in organic nutrients. This shows that it
- A. can switch between autotrophic and heterotrophic nutrition
- B. is strictly heterotrophic and never makes chlorophyll
- C. is strictly photosynthetic and needs no organic matter
- D. can switch between fermentation and photosynthesis only
Q20. Which sterilisation condition reliably kills bacterial endospores in a culture medium?
- A. Autoclaving at 121 °C for 15 minutes at 15 psi
- B. Pasteurising at 72 °C for 15 seconds
- C. Boiling at 100 °C for 15 minutes at normal pressure
- D. Irradiating with UV light for 15 minutes
Q21. In an experiment with phage T2, one batch is labelled with ³⁵S and another with ³²P. After infection and violent blending, most radioactivity of the ³²P batch is in the infected cells. What does this show?
- A. Phage DNA enters the bacterium and carries the genetic information
- B. Phage protein stays outside the cell and builds new virus coats
- C. Phage DNA stays outside the cell and acts as a signal
- D. Phage protein enters the bacterium and carries the genetic information
Q22. Pandemic influenza strains sometimes arise when a pig is infected with both a human and an avian influenza virus. Which feature of the virus makes this possible?
- A. Its genome is a circular RNA that integrates into pig DNA
- B. Its genome is in separate RNA segments that can reassort
- C. Its genome is a single DNA molecule that recombines
- D. Its genome is a single RNA strand that lacks any protein
Q23. A smear shows rod-shaped cells arranged in a chain, comma-shaped cells, and spherical cells in grape-like clusters. Which pairing of shape and example is correct?
- A. Comma-shaped — Spirillum; clusters of cocci — Streptococcus
- B. Comma-shaped — Vibrio; clusters of cocci — Streptococcus
- C. Comma-shaped — Vibrio; clusters of cocci — Staphylococcus
- D. Comma-shaped — Bacillus; clusters of cocci — Diplococcus
Q24. Cells from the middle of the log phase of growth in a rich medium are transferred into fresh medium of exactly the same composition at the same temperature. Which phase is expected to be much shorter or absent?
- A. Death phase
- B. Lag phase
- C. Stationary phase
- D. Log phase
Q25. Paper discs soaked in antibiotics A, B and C are placed on a lawn of bacteria. After incubation the clear zones are: A 22 mm, B 0 mm, C 10 mm. Which interpretation is correct?
- A. The bacteria are most sensitive to C and resistant to A
- B. The bacteria are most sensitive to B and resistant to A
- C. The bacteria are most sensitive to A and resistant to B
- D. The bacteria are equally sensitive to A and C
Q26. The sap of a diseased tobacco plant remains infectious after being forced through a porcelain filter that holds back bacteria. What does this show?
- A. The infectious agent is larger than a bacterium
- B. The infectious agent is a toxin and not alive
- C. The infectious agent is a fungal spore
- D. The infectious agent is smaller than a bacterium
Q27. Cells infected with a virus release a protein that makes neighbouring uninfected cells resistant to several different viruses. This protein is
- A. complement
- B. antitoxin
- C. lysozyme
- D. interferon
Q28. RNA from tobacco mosaic virus strain A is mixed with coat protein from strain B and used to infect tobacco leaves. The progeny virus particles would be expected to have
- A. strain A RNA and strain A coat protein
- B. strain B RNA and strain B coat protein
- C. strain B RNA and strain A coat protein
- D. strain A RNA and strain B coat protein
Q29. People who drink water contaminated with human faeces develop amoebic dysentery. Which stage of Entamoeba histolytica is the infective form?
- A. The gametocyte
- B. The sporozoite
- C. The trophozoite
- D. The cyst
Q30. A field that grew a legume crop has more soil nitrogen than a neighbouring cereal field. Which organism accounts for the difference?
- A. Rhizobium in root nodules, converting N₂ to ammonia
- B. Rhizobium in leaves, converting nitrate to N₂
- C. Nitrosomonas in root nodules, converting nitrate to ammonia
- D. Nitrobacter in root nodules, converting N₂ to nitrate