Take the NUMS Biology — Biodiversity: Viruses, Bacteria and Kingdoms (Set 7): 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. In a classic experiment, RNA from tobacco mosaic virus strain A was mixed with coat protein of strain B and the reconstituted particles infected tobacco leaves. The progeny viruses had the coat protein of:
- A. neither strain, because plant cells make the coat themselves
- B. a mixture of A and B, because both components are inherited
- C. strain A, because the viral RNA carries the genetic information ✓
- D. strain B, because the protein coat carries the genetic information
Answer: C. strain A, because the viral RNA carries the genetic information
Explanation: The infecting RNA directed synthesis of new coat protein of its own strain, showing that RNA is the genetic material of TMV.
Q2. One phage particle infects a bacterium and the bacterium releases 50 new phages. In each following cycle every newly released phage infects its own fresh bacterium and also releases 50 phages. Assuming every particle succeeds, how many phages are released during the third lytic cycle alone?
- A. 127 550
- B. 125 000 ✓
- C. 25 000
- D. 625 000
Answer: B. 125 000
Explanation: Each cycle multiplies the number of released phages by 50: 50 → 2 500 → 125 000. The third cycle alone releases 50 × 50 × 50 = 125 000 (the running total of all progeny would be 127 550).
Q3. A bacterial culture is growing normally with no phage added. After brief UV irradiation the culture clears as the cells burst and phage particles appear. This indicates that the bacteria were:
- A. lysogenic, carrying a plasmid that UV converted into a virus
- B. contaminated by fungi whose spores were killed by UV
- C. lysogenic, with a prophage induced into the lytic cycle ✓
- D. infected by viroids that UV turned into RNA viruses
Answer: C. lysogenic, with a prophage induced into the lytic cycle
Explanation: A prophage integrated into the host chromosome can be induced by stress such as UV to excise, replicate and lyse the cell.
Q4. A patient has pneumonia caused by Mycoplasma pneumoniae, a bacterium with no cell wall, and penicillin fails to work. What is the best explanation?
- A. It is an archaeon, whose wall penicillin cannot cross
- B. It lacks peptidoglycan, the cell-wall component penicillin interferes with ✓
- C. It builds its wall from chitin, which penicillin cannot attack
- D. It lacks ribosomes, which are the target of penicillin
Answer: B. It lacks peptidoglycan, the cell-wall component penicillin interferes with
Explanation: Penicillin blocks cross-linking of peptidoglycan during wall synthesis. Mycoplasma has no wall and no peptidoglycan, so the drug has no target.
Q5. A person is infected with HIV two weeks ago and an antibody-based screening test is negative today. The most likely explanation is:
- A. the test fell in the window period before antibodies appear ✓
- B. antibody tests detect the viral RNA, which is not yet made
- C. the virus is destroyed by the immune system within two weeks
- D. the person cannot be infected because HIV does not cause antibodies
Answer: A. the test fell in the window period before antibodies appear
Explanation: Antibody tests need time (weeks) for seroconversion. In this window the person is infected and infectious, but antibodies are low.
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Q6. A patient with HIV has a CD4 T-cell count that falls from 1200 to 180 cells per µL over some years and now develops pneumocystis pneumonia. Taking AIDS to be defined as fewer than 200 CD4 cells per µL, the best interpretation is:
- A. progression to AIDS, because helper T cells are depleted
- B. early infection, because the virus attacks only B cells
- C. vaccination response, because opportunistic infection is mild
- D. recovery, because CD4 cells fall as the virus is cleared
Q7. Drugs such as AZT are used for HIV infection. They resemble a DNA nucleotide and stop the synthesis of the DNA copy of viral RNA. Which enzyme is the target?
- A. RNA polymerase, which makes mRNA from a DNA template
- B. DNA polymerase, which copies the host cell DNA
- C. Protease of the host cell, which digests peptides
- D. Reverse transcriptase, which makes DNA from viral RNA
Q8. Rabies virus enters through a bite and travels along peripheral nerves to the brain. Which prediction follows?
- A. A bite on the foot leads to a shorter incubation period than one on the face
- B. Bites cause illness only when the virus is swallowed with saliva
- C. The incubation period is the same, because the virus travels in blood
- D. A bite on the face leads to a shorter incubation period than one on the foot
Q9. Cells infected by a virus release interferon. Which statement describes its role?
- A. It converts infected cells into antibody-making plasma cells
- B. It carries the viral genome from one cell to the next
- C. It binds the virus particle directly and destroys its protein coat
- D. It signals neighbours to make proteins blocking viral replication
Q10. An infectious extract keeps its infectivity after treatment with RNase, DNase and ultraviolet light, but loses it after protease treatment. The agent is most likely:
- A. a bacteriophage, which is a DNA virus of bacteria
- B. a viroid, which is a naked circular RNA molecule
- C. a prion, an infectious protein with no nucleic acid
- D. a retrovirus, which has an RNA genome and an envelope
Q11. A hepatitis B vaccine contains a recombinant viral surface protein only. Why can it not cause the disease?
- A. It contains the virus coat plus RNA that cannot be read at all
- B. It contains live virus that has lost its polymerase, so it is weak
- C. It contains no viral genome and so cannot make new virus
- D. It contains antibodies that destroy the virus when it enters
Q12. The tail fibres of a T4 phage bind to the surface of E. coli. What happens next?
- A. The tail fibres cut the bacterial chromosome into small fragments
- B. The sheath contracts and DNA is injected, capsid left outside
- C. The whole phage particle enters the cell by endocytosis
- D. The capsid injects protein while the DNA stays outside the cell
Q13. A bacterium divides every 40 minutes. A culture starts with 1000 cells in fresh medium, and growth is exponential from the start. How many cells will there be after 4 hours?
- A. 6000
- B. 64 000
- C. 24 000
- D. 96 000
Q14. Penicillin is most effective on a bacterial culture in which phase of growth?
- A. The lag phase, because cells make no proteins at all then
- B. The stationary phase, because cells divide faster than they die
- C. The log phase, as cells are building new walls while dividing
- D. The death phase, because the walls are thickest at that time
Q15. In the Gram stain procedure, a student leaves out the final counterstain (safranin). Which result is expected?
- A. Gram-positive cells look purple and Gram-negative look colourless
- B. Both kinds of cells look colourless after the alcohol wash
- C. Both kinds of cells look purple after the alcohol wash
- D. Gram-positive cells look colourless and Gram-negative look pink
Q16. Bacteria treated with penicillin burst in dilute medium but survive as round bodies in a medium containing 20% sucrose. Why?
- A. Sucrose blocks penicillin from entering through the membrane
- B. Sucrose repairs the peptidoglycan wall that penicillin damaged
- C. Sucrose is used as food that lets the cells divide faster
- D. The medium is hypertonic, which stops water bursting the cells
Q17. An F⁺ bacterium is mixed with an F⁻ bacterium. After conjugation, which is the usual outcome?
- A. Both cells become F⁻, because the plasmid is lost in mating
- B. The F⁻ cell becomes Hfr, because its chromosome is transferred
- C. The F⁺ cell becomes F⁻, because it gives up its plasmid
- D. Both cells become F⁺, as a copy of the F plasmid passes over
Q18. Extracts from heat-killed S-type pneumococci are treated separately with protease, RNase or DNase, and each is mixed with live R-type bacteria. Only the DNase-treated mixture fails to produce S-type colonies. What is the conclusion?
- A. Polysaccharide capsule is the transforming principle
- B. DNA is the transforming principle
- C. RNA is the transforming principle
- D. Protein is the transforming principle
Q19. In generalised transduction, a phage particle that infects a second bacterium carries some DNA from the first bacterium. How did this happen?
- A. Host DNA fragments were wrongly packaged into the phage head
- B. The phage took up naked DNA from the medium by transformation
- C. A pilus connected the two cells and carried the DNA across
- D. The phage used bacterial DNA as its own genome on purpose
Q20. A hospital finds that unrelated bacterial species have become resistant to the same antibiotic at the same time. The most likely mechanism is:
- A. resistance arising from the immune cells of the patients
- B. simultaneous mutation of the same gene in all the species
- C. the antibiotic itself changing the DNA of each species
- D. transfer of a resistance plasmid by conjugation between species
Q21. Root nodule cells of legumes contain leghaemoglobin. Its role is to:
- A. transport nitrogen gas, so that it reaches the ribosomes
- B. carry carbon dioxide, so that photosynthesis is faster
- C. store nitrate, so that roots can absorb ammonia
- D. bind oxygen, so nitrogenase of Rhizobium is protected
Q22. After heavy flooding, a farm soil loses nitrate and releases nitrogen gas. Which bacteria and conditions are responsible?
- A. Denitrifying bacteria acting in waterlogged, oxygen-poor soil
- B. Decomposing bacteria acting in acidic, aerobic soil
- C. Nitrifying bacteria acting in well-aerated, warm, dry soil
- D. Nitrogen-fixing bacteria acting in dry, loose, sandy soil
Q23. In the filamentous cyanobacterium Anabaena, special thick-walled heterocysts appear when combined nitrogen is scarce. They are the site of:
- A. spore formation, protected from nitrogen in the neighbouring cells
- B. cell division, protected from light by neighbouring cells
- C. photosynthesis, protected from nitrogen made by the neighbouring cells
- D. nitrogen fixation, protected from oxygen made by neighbouring cells
Q24. A microbe lives in a hot spring at 90 °C. It has no peptidoglycan in its wall and its membrane lipids contain ether linkages. To which group does it belong?
- A. Protists
- B. Archaea
- C. Eubacteria
- D. Cyanobacteria
Q25. A fungus has aseptate (coenocytic) hyphae, forms sporangiospores in sporangia, and reproduces sexually by a thick-walled zygospore. It is most likely:
- A. Agaricus, a mushroom
- B. Rhizopus, a bread mould
- C. Penicillium, a blue-green mould
- D. Saccharomyces, a budding yeast
Q26. In an ectomycorrhizal association, a pine root and a fungus exchange materials. What does each partner mainly receive?
- A. The fungus receives minerals and the plant receives sugars
- B. The fungus receives nitrogen gas and the plant receives light
- C. The fungus gets sugars and the plant gets minerals and water
- D. The fungus receives water and the plant receives chitin
Q27. In the life cycle of Plasmodium, in which host does sexual reproduction (fusion of gametes) take place?
- A. A female Anopheles mosquito
- B. The human red blood cell
- C. The human liver cell
- D. The male Anopheles mosquito
Q28. Which stage of Entamoeba histolytica is passed in faeces and infects a new host through contaminated water?
- A. The merozoite
- B. The sporozoite
- C. The cyst
- D. The trophozoite
Q29. Four organisms are named: Canis lupus, Canis latrans, Vulpes vulpes and Felis catus. Which pair is most closely related on the basis of their names?
- A. Canis lupus and Canis latrans, because they share a genus
- B. Vulpes vulpes and Felis catus, because both end in -s
- C. Canis latrans and Felis catus, because they share a species
- D. Canis lupus and Vulpes vulpes, because they share a family
Q30. A culture of Bacillus survives boiling for 10 minutes, but no growth occurs after 15 minutes at 121 °C under pressure. What explains the first result?
- A. Endospores resist boiling but are destroyed by autoclaving
- B. Capsules protect the cells from all temperatures below 121 °C
- C. Vegetative cells resist boiling but are destroyed by autoclaving
- D. Plasmids make the cells resistant to heat of any kind