Plasmids Practice Questions
20 free Plasmids practice questions for the Microbiology. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which of the following best defines a plasmid in a bacterial cell?
- A A permanently attached segment of the bacterial chromosome
- B A specialized protein involved in DNA repair
- C The RNA template used for protein synthesis
- D Extrachromosomal DNA that replicates independently
Correct answer: Extrachromosomal DNA that replicates independently
Plasmids are small, usually circular, double-stranded DNA molecules that are distinct from the cell's chromosomal DNA. They possess their own origin of replication (ori), allowing them to replicate autonomously within the host.
Which specific region of a plasmid is essential for initiating its autonomous replication?
- A The tra locus
- B The antibiotic resistance gene
- C The origin of replication (ori)
- D The multiple cloning site (MCS)
Correct answer: The origin of replication (ori)
The origin of replication (ori) is the DNA sequence where replication begins. It determines the host range of the plasmid and helps control the number of plasmid copies maintained in the cell.
Plasmids that can be transferred from one bacterium to another through cell-to-cell contact are known as:
- A Cryptic plasmids
- B Non-mobilizable plasmids
- C Episomes
- D Conjugative plasmids
Correct answer: Conjugative plasmids
Conjugative plasmids carry a set of 'tra' (transfer) genes which encode the structures and enzymes necessary for bacterial conjugation. The F-plasmid is the most well-studied example of a conjugative plasmid.
What is the function of the 'tra' genes found on some bacterial plasmids?
- A They provide resistance to heavy metal compounds
- B They allow the host bacterium to ferment lactose
- C They trigger programmed lysis of the host cell
- D They direct conjugative transfer of the plasmid
Correct answer: They direct conjugative transfer of the plasmid
The 'tra' operon includes genes for pilus formation and DNA transfer. These genes enable the donor cell to physically connect with a recipient cell and move a copy of the plasmid across.
Which type of plasmid carries genes that provide the host bacterium with resistance to antibiotics?
- A F-plasmids
- B Col-plasmids
- C Ti-plasmids
- D R-plasmids
Correct answer: R-plasmids
R-plasmids (Resistance plasmids) contain genes that encode enzymes capable of inactivating antibiotics or pumping them out of the cell. They are of significant clinical concern because they can spread multi-drug resistance rapidly.
The 'copy number' of a plasmid refers to which of the following?
- A The number of genes present on the plasmid
- B The total number of base pairs in the plasmid DNA
- C The number of plasmid copies maintained per cell
- D The number of times a plasmid can be transferred
Correct answer: The number of plasmid copies maintained per cell
Copy number is a regulated characteristic of a plasmid, ranging from one or two (low copy) to hundreds (high copy). It is primarily determined by the initiation frequency at the origin of replication.
Which term describes a plasmid that has the ability to integrate into the host's bacterial chromosome?
- A Bacteriocin
- B Cosmid
- C Phagemid
- D Episome
Correct answer: Episome
An episome is a genetic element that can replicate either independently in the cytoplasm or as an integrated part of the host chromosome. The F-factor in Hfr (High Frequency of Recombination) strains is a classic episome.
Plasmids that encode bacteriocins, which are proteins that inhibit or kill closely related bacterial species, are called:
- A Resistance plasmids
- B Degradative plasmids
- C Col-plasmids
- D Ti-plasmids
Correct answer: Col-plasmids
Col-plasmids (Colicinogenic plasmids) carry genes for bacteriocins like colicins. These proteins provide a competitive advantage by eliminating sensitive neighboring bacteria of the same or similar species.
What occurs during 'plasmid incompatibility' (Inc groups)?
- A Two plasmids cannot stably coexist in one cell
- B The plasmid prevents the host cell from dividing
- C The plasmid kills the host cell upon entry
- D The plasmid DNA is converted into RNA
Correct answer: Two plasmids cannot stably coexist in one cell
Incompatibility occurs when two plasmids share the same replication or partitioning mechanisms. One plasmid will eventually be lost during cell division because the cell cannot distinguish between them to ensure both are inherited.
The Ti-plasmid, used extensively in plant genetic engineering, is naturally found in which bacterium?
- A Escherichia coli
- B Corynebacterium diphtheriae
- C Streptococcus pyogenes
- D Agrobacterium tumefaciens
Correct answer: Agrobacterium tumefaciens
The Ti (Tumor-inducing) plasmid is found in Agrobacterium tumefaciens. It is capable of transferring a segment of its DNA (T-DNA) into the genome of plant cells, causing crown gall disease.
Which of the following is a characteristic of 'stringent' plasmid replication control?
- A Replication is independent of the host chromosome replication
- B The plasmid maintains a very high copy number
- C The plasmid replicates in sync with the host chromosome
- D The plasmid lacks an origin of replication
Correct answer: The plasmid replicates in sync with the host chromosome
Stringent control results in a low copy number (1–2 copies per cell). This ensures that the plasmid replicates only once per cell cycle, closely following the timing of the host's chromosomal replication.
What is 'plasmid curing'?
- A The process of repairing damaged plasmid DNA
- B The insertion of a new gene into a plasmid
- C The loss of a plasmid from a bacterial population
- D The process of a plasmid integrating into a chromosome
Correct answer: The loss of a plasmid from a bacterial population
Curing is the loss of a plasmid, which can occur spontaneously or be induced by chemical agents or environmental stress (like heat). Without selective pressure, cells that lose the plasmid may outcompete those that still carry the metabolic burden of the plasmid.
Which type of plasmid allows bacteria to break down unusual substances like toluene, camphor, or petroleum hydrocarbons?
- A Virulence plasmids
- B Fertility plasmids
- C Degradative plasmids
- D Drug resistance plasmids
Correct answer: Degradative plasmids
Degradative plasmids carry genes for specialized enzymes that break down complex or toxic organic compounds. This allows the host bacteria to survive in unique ecological niches or contaminated environments.
In the context of recombinant DNA technology, a 'selectable marker' on a plasmid is used to:
- A Identify cells that have taken up the plasmid
- B Increase the copy number of the plasmid
- C Allow the plasmid to integrate into the chromosome
- D Terminate the replication process
Correct answer: Identify cells that have taken up the plasmid
A selectable marker, usually an antibiotic resistance gene, allows researchers to kill off any bacteria that did not take up the plasmid. Only the 'transformed' bacteria will survive on media containing the antibiotic.
Which structural feature allows 'relaxed' plasmids to maintain a high copy number?
- A A lack of a protein-based replication inhibitor
- B Multiple origins of replication on a single ring
- C Integration into the host chromosome
- D The presence of a thick protein coat
Correct answer: A lack of a protein-based replication inhibitor
Relaxed plasmids have replication control mechanisms that are not tightly coupled to the host cell cycle. This allows them to initiate replication frequently, leading to high copy numbers (10–100 or more).
A plasmid that carries all the information required for its own transfer but lacks the genes to initiate the process is called:
- A Mobilizable
- B Conjugative
- C Transposable
- D Incompatible
Correct answer: Mobilizable
Mobilizable plasmids contain an 'oriT' (origin of transfer) but do not encode the 'tra' genes for a pilus. They can only be transferred if a conjugative plasmid is also present in the cell to provide the necessary machinery.
Which of the following statements about plasmid size is generally true?
- A Plasmids typically range from 1 kb to over 200 kb
- B Plasmids are usually larger than the chromosome
- C Plasmid sizes are confined to a single fixed range
- D Plasmids rarely carry more than a single gene
Correct answer: Plasmids typically range from 1 kb to over 200 kb
Plasmids vary significantly in size, but they are much smaller than the bacterial chromosome. Small plasmids are often used in cloning, while larger plasmids often carry complex operons like those for multi-drug resistance.
Which term describes a plasmid that does not appear to have any detectable effect on the phenotype of the host cell?
- A Cryptic plasmid
- B Killer plasmid
- C Active plasmid
- D Reporter plasmid
Correct answer: Cryptic plasmid
Cryptic plasmids are those that do not carry any known genes other than those required for their own replication and maintenance. Their presence is often only discovered through DNA sequencing or physical isolation.
The 'Multiple Cloning Site' (MCS) in a laboratory plasmid is characterized by:
- A Having many origins of replication
- B Containing several unique restriction enzyme sites
- C Encoding several different antibiotic resistance genes
- D Being the site where the 'tra' genes are located
Correct answer: Containing several unique restriction enzyme sites
The MCS, or polylinker, is a short DNA region containing several different restriction sites. This allows researchers to use various enzymes to cut the plasmid and insert foreign DNA at that specific location.
Which mechanism ensures that at least one copy of a low-copy-number plasmid is passed to each daughter cell during division?
- A Random cytoplasmic diffusion
- B Partitioning systems
- C Programmed host cell lysis
- D Gene transposition
Correct answer: Partitioning systems
Partitioning systems are active mechanisms that physically push or pull plasmid copies to opposite poles of the dividing cell. This prevents 'segregational loss,' ensuring both daughter cells remain plasmid-positive.