Phage genetics, plasmid mapping and applications MCQs With Answer

Introduction: Phage genetics and plasmid mapping are foundational topics in microbial and cellular biology with direct relevance to M.Pharm training. This set of MCQs focuses on molecular mechanisms of bacteriophage replication, lysis–lysogeny regulation, genetic recombination in phages, and practical strategies for plasmid characterization, mapping and application in biotechnology. Questions explore experimental approaches (transduction, complementation, restriction and linkage mapping), plasmid biology (replication origins, copy-number control, incompatibility, mobilization), and applied uses such as cloning vectors, phage display and gene delivery. These items are designed to test conceptual depth, experimental reasoning and problem-solving skills required in pharmaceutical research and development.

Q1. Which genetic event primarily determines whether a temperate bacteriophage enters the lytic or lysogenic cycle?

• Mutation rate in the host chromosome
• Multiplicity of infection only
• Relative expression levels of phage repressor and cro-like regulatory proteins
• Presence of plasmid-borne resistance genes in the host

Correct Answer: Relative expression levels of phage repressor and cro-like regulatory proteins

Q2. In generalized transduction by a bacteriophage, which statement is correct?

• Only phage DNA is packaged into transducing particles
• Any fragment of host DNA can be mistakenly packaged and transferred
• Only genes adjacent to the phage integration site are transferred
• Transduction requires the host to be lysogenized

Correct Answer: Any fragment of host DNA can be mistakenly packaged and transferred

Q3. Specialized transduction differs from generalized transduction because:

• It transfers only plasmid DNA
• It packages random fragments of bacterial DNA
• It transfers specific host genes adjacent to prophage integration sites
• It occurs only in lytic phages

Correct Answer: It transfers specific host genes adjacent to prophage integration sites

Q4. In phage complementation tests, failure to complement between two phage mutants indicates:

• Mutations are in different genes
• Mutations are in the same gene (allelic)
• The mutants are from different phage species
• One mutant is revertant

Correct Answer: Mutations are in the same gene (allelic)

Q5. A standard method to create a genetic map of a bacteriophage based on recombination frequency uses which experimental approach?

• Measuring plaque size only
• Co-infection of a sensitive host with two marked phage strains and scoring recombinant phenotypes
• Sequencing host RNA transcripts during infection
• Measuring adsorption rate differences

Correct Answer: Co-infection of a sensitive host with two marked phage strains and scoring recombinant phenotypes

Q6. Which plasmid feature is essential for conjugative transfer between bacteria?

• High copy number
• oriC replication origin
• oriT (origin of transfer) plus tra genes or their mobilization functions
• Antibiotic resistance marker

Correct Answer: oriT (origin of transfer) plus tra genes or their mobilization functions

Q7. In restriction mapping of a plasmid, which combination of data is most useful to deduce fragment order?

• Sizes of single digests only
• Sizes of double digests and single digests
• Total plasmid size only
• GC content measurements

Correct Answer: Sizes of double digests and single digests

Q8. A plasmid with an iteron-containing origin and tight copy-number control most likely replicates by:

• Rolling-circle replication
• Theta replication with replication initiation regulated by Rep proteins binding iterons
• RNA-primed plasmid replication identical to phage lambda
• Host chromosome-like bidirectional replication from oriC

Correct Answer: Theta replication with replication initiation regulated by Rep proteins binding iterons

Q9. Which technique allows mapping of gene order on a plasmid by identifying co-transference frequencies during conjugation?

• Pulse-field gel electrophoresis
• Interrupted mating (time-of-entry mapping) adapted for large plasmids
• Blue–white screening
• Phage plaque hybridization

Correct Answer: Interrupted mating (time-of-entry mapping) adapted for large plasmids

Q10. Phage display technology primarily exploits which principle?

• Integration of phage DNA into the host chromosome
• Fusion of peptide or protein coding sequences to a phage coat protein gene so displayed peptides are linked to their encoding DNA
• Use of generalized transduction to transfer plasmids
• Induction of lysogeny to express host genes

Correct Answer: Fusion of peptide or protein coding sequences to a phage coat protein gene so displayed peptides are linked to their encoding DNA

Q11. A plasmid belonging to the same incompatibility (Inc) group as another plasmid in the same cell will typically:

• Be stably inherited together at equal copy numbers
• Compete for the same replication/partitioning system leading to loss of one plasmid
• Undergo immediate recombination to form a hybrid plasmid
• Become integrated into the chromosome

Correct Answer: Compete for the same replication/partitioning system leading to loss of one plasmid

Q12. When performing phage genetic mapping by recombination, which factor increases the frequency of recombinants between two markers?

• Markers being very close genetically
• Using a high multiplicity of infection to allow multiple genomes per cell
• Using a completely different host species
• Eliminating host recombination functions

Correct Answer: Using a high multiplicity of infection to allow multiple genomes per cell

Q13. Which plasmid feature increases its stability in the absence of selection pressure by antibiotic?

• Lack of partitioning (par) systems
• Presence of toxin–antitoxin addiction systems and active partitioning loci
• Extremely high copy number without partitioning
• Large noncoding regions

Correct Answer: Presence of toxin–antitoxin addiction systems and active partitioning loci

Q14. In mapping phage genes by complementation, two mutants that complement each other indicate:

• They carry identical mutations in the same codon
• They have lesions in different genes required for the same function
• One mutant enhances the other’s defect
• They are derived from different phage strains and cannot recombine

Correct Answer: They have lesions in different genes required for the same function

Q15. Phagemids are useful in cloning because they combine features of plasmids and phage. Which property describes a phagemid?

• It cannot be packaged into phage particles
• It contains an f1 origin allowing single-stranded DNA production when helper phage is provided
• It always integrates into the host chromosome
• It is exclusively a conjugative element

Correct Answer: It contains an f1 origin allowing single-stranded DNA production when helper phage is provided

Q16. When constructing a restriction map of a plasmid, two enzymes produce fragments that, when combined in a double digest, yield fragments equal to the single digest sums. This observation implies:

• The enzymes cut at identical sites
• Their cut sites are in the same linear order and the fragments do not overlap unexpectedly
• The plasmid is circular and cannot be mapped
• One enzyme did not cut

Correct Answer: Their cut sites are in the same linear order and the fragments do not overlap unexpectedly

Q17. In a generalized transduction experiment, the frequency of transductants for two markers is much higher when the markers are close together; this is because:

• Phage preferentially packages contiguous host DNA fragments, so closely linked genes co-package more often
• Closely linked genes have identical promoters
• Close genes increase host cell lysis rate
• Recombination between close genes is suppressed

Correct Answer: Phage preferentially packages contiguous host DNA fragments, so closely linked genes co-package more often

Q18. A cloning vector intended for high-yield protein expression in E. coli typically requires which plasmid features?

• Low copy-number origin and lack of promoter
• Strong, inducible promoter, high-expression ribosome-binding site, selectable marker and a suitable origin of replication
• Only an origin of transfer (oriT)
• Phage integration site for lysogeny

Correct Answer: Strong, inducible promoter, high-expression ribosome-binding site, selectable marker and a suitable origin of replication

Q19. Rolling-circle replication (RCR) plasmids are characterized by which features?

• Bidirectional replication from a fixed origin and high dependence on iterons
• Single-stranded DNA intermediates, a nicking initiator protein (Rep), and often high copy number
• Integration into the host chromosome prior to replication
• Strict incompatibility with all theta-replicating plasmids

Correct Answer: Single-stranded DNA intermediates, a nicking initiator protein (Rep), and often high copy number

Q20. CRISPR–Cas systems in bacteria can limit phage infection. In phage genetics experiments, why is it important to consider host CRISPR activity?

• CRISPR systems only affect plasmids, not phages
• Active CRISPR can bias outcomes by selectively destroying specific phage genotypes, reducing recovery of recombinants or transductants
• CRISPR enhances phage recombination uniformly
• CRISPR systems cause plasmid integration into phage genomes

Correct Answer: Active CRISPR can bias outcomes by selectively destroying specific phage genotypes, reducing recovery of recombinants or transductants

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