Bacteria and Viruses

Biology, HSSC-Punjab First Year. This chapter is split into 20 topics and has 473 practice questions. The topics covered are listed below, followed by 10 free sample questions from the chapter.

Topics in this chapter

1. Nature and structure of viruses 25 questions
2. Importance and applications of bacteria 25 questions
3. Endospore formation in bacteria 25 questions
4. Motility in bacteria and flagella structure 25 questions
5. Bacteriophage, HIV and viral disease 25 questions
6. Normal flora 25 questions
7. Bacterial ecology and diversity 25 questions
8. Structure of the bacterial cell 25 questions
9. Bacterial cell envelope, cytoplasm and Gram reaction (Figures 2.1-2.3) 26 questions
10. Sporulation steps in Figure 2.4 and motility patterns in Figures 2.5-2.6 21 questions
11. Bacterial flagella and pili: arrangements, structure and Figures 2.7-2.8 21 questions
12. Bacterial diversity: Margulis and Schwartz groups, Omnibacteria to Spirochaetes 21 questions
13. Pseudomonads, actinomycetes, nitrogen-fixing and chemosynthetic bacteria 22 questions
14. Table of characteristics of bacterial groups: form, motility, nutrition and role 28 questions
15. Importance of bacteria and normal flora: recyclers, products, cleaners and research tools 21 questions
16. Virus structure, size and shape: capsid, envelope and size calculations 21 questions
17. Structure of the T4 bacteriophage and phages as gene carriers (Figure 2.11) 20 questions
18. HIV and AIDS: structure, reverse transcription, history and origin (Figure 2.12) 22 questions
19. Exercise-based questions and assertion-reason items for bacteria and viruses 20 questions
20. Comparing bacteria and viruses, and integrated questions on bacterial structure and function 30 questions

Sample questions: Bacteria and Viruses

10 sample questions with the correct answer and an explanation. Try to answer each one before reading the explanation.

Q1. Which sequence correctly describes the structures from outside to inside in a capsulated Gram-negative bacterium?

• A. Outer membrane -> capsule -> cell membrane -> peptidoglycan
• B. Peptidoglycan -> capsule -> cell membrane -> outer membrane
• C. Cell membrane -> peptidoglycan -> outer membrane -> capsule
• D. Capsule -> outer membrane -> peptidoglycan layer -> periplasmic space -> cell membrane(correct answer)

Answer: D.In a capsulated Gram-negative bacterium the capsule lies outside the cell wall; the cell wall itself consists of an outer membrane (lipopolysaccharides and lipoproteins containing porin), then the thin peptidoglycan layer, then the periplasmic space, and finally the cell membrane.

Q2. Arrange the steps of sporulation in the correct order: (1) spore coat forms (2) DNA is isolated by a septum (3) vegetative cell breaks and the endospore is released (4) the new DNA becomes surrounded by two membranes

• A. 1, 2, 3, 4
• B. 4, 2, 3, 1
• C. 2, 4, 1, 3(correct answer)
• D. 2, 1, 4, 3

Answer: C.The DNA is first isolated by a septum (2), the cell membrane then grows around it so that the DNA is enclosed by two membranes (4), a peptidoglycan layer and spore coat form (1), and finally the vegetative cell breaks and the endospore is released (3).

Q3. Which of the following is NOT part of the structure of a mature endospore as described in the text?

• A. A core containing DNA and cytoplasm
• B. Two membranes around the core
• C. A capsule of lipopolysaccharides(correct answer)
• D. A peptidoglycan layer and a spore coat

Answer: C.The maturing endospore has its DNA and cytoplasm enclosed by two membranes, with a new peptidoglycan layer between the membranes and a spore coat around it. A lipopolysaccharide capsule is a structure of some Gram-negative bacterial cell walls, not of the endospore.

Q4. Why is endospore formation important in food preservation and sterilisation?

• A. Because endospores make food taste better
• B. Because endospores prevent all bacterial growth permanently
• C. Because endospores are extremely resistant dormant structures that can survive conditions which kill vegetative cells(correct answer)
• D. Because endospores increase the nutritional value of food

Answer: C.Because the endospore is a thick-walled, dehydrated and metabolically inactive resting structure, it survives conditions that destroy the active vegetative cell. This resistance is why sterilisation must destroy endospores as well as vegetative cells.

Q5. The classification of viruses follows guidelines established by the:

• A. Linnaean Society
• B. International Committee on Taxonomy of Viruses (ICTV)(correct answer)
• C. Punjab Textbook Board
• D. World Health Organization only

Answer: B.Viruses are classified based on several characteristics, including their genetic material, replication strategy, morphology and the hosts they infect. The classification of viruses follows guidelines established by the International Committee on Taxonomy of Viruses (ICTV).

Q6. A negative-sense RNA virus is one in which:

• A. The genome is DNA and is transcribed into RNA
• B. The RNA genome is transcribed into mRNA by a viral RNA polymerase before translation(correct answer)
• C. The RNA genome is directly translated into proteins by the host ribosomes
• D. The genome is never translated

Answer: B.In negative-sense RNA viruses the RNA genome is transcribed into mRNA by a viral RNA polymerase before translation; rabies virus and Ebola virus are examples. In positive-sense RNA viruses the RNA genome is directly translated by the host cell's ribosomes.

Q7. Which enzyme is carried by retroviruses such as HIV?

• A. DNA ligase
• B. RNA polymerase only
• C. Lysozyme
• D. Reverse transcriptase(correct answer)

Answer: D.The most distinguishing character of retroviruses is the presence of a specific enzyme, reverse transcriptase, which catalyses reverse transcription — the conversion of single-stranded RNA into a DNA strand, which is then completed into a DNA double helix.

Q8. Bacteriophages are used in genetic engineering because they:

• A. Destroy all bacterial DNA
• B. Produce antibiotics inside bacteria
• C. Convert bacteria into viruses
• D. Act as carriers that deliver a gene of interest into the target bacterial cell(correct answer)

Answer: D.Bacteriophages are used as carriers in genetic engineering. The gene of interest is inserted into the DNA of the bacteriophage, which carries it to the target bacterial cell. When the virus incorporates its DNA into the bacterial chromosome, the gene of interest also becomes part of the bacterial DNA.

Q9. Which statement about the use of bacteriophages is correct?

• A. The gene of interest is inserted into the phage DNA, and once the phage DNA joins the bacterial chromosome the gene becomes part of the bacterial DNA(correct answer)
• B. The phage cannot carry any foreign DNA
• C. The phage converts bacteria into plants
• D. The phage destroys the gene of interest

Answer: A.In genetic engineering the gene of interest is inserted into the DNA of the bacteriophage, which carries it to the target bacterial cell. When the virus incorporates its DNA into the bacterial chromosome, the gene of interest also becomes part of the bacterial DNA.

Q10. A patient's immune system is severely weakened, and a retrovirus with reverse transcriptase is isolated from the blood. Which virus is most likely responsible, and which genome would it carry?

• A. HIV, carrying an RNA genome(correct answer)
• B. A bacteriophage, carrying a DNA genome
• C. A prion, carrying protein only
• D. A viroid, carrying circular RNA only

Answer: A.HIV is a retrovirus, and retroviruses contain RNA in their capsids together with the enzyme reverse transcriptase. HIV weakens the immune system and causes AIDS. Prions contain only protein and viroids only circular RNA, and neither carries reverse transcriptase.