Microbial Genetics — MCQs

Microbial Genetics Indian Medical PG Practice Questions and MCQs

Question 61: All the following terms are used to describe bacterial chromosome Except

A. Diploid (Correct Answer)
B. Circular
C. Nucleoid
D. Haploid

Explanation: ***Diploid*** - Bacterial chromosomes are typically **haploid**, meaning they contain only one copy of each gene, not two sets as in diploid organisms. - While bacteria can sometimes have multiple copies of their chromosome, these copies are usually identical replicas and do not represent distinct sets of genetic information. *Circular* - The bacterial chromosome is characteristically a **single, closed circular DNA molecule**. - This circular structure distinguishes bacterial chromosomes from the linear chromosomes found in eukaryotes. *Nucleoid* - The bacterial chromosome is located in a region of the cytoplasm called the **nucleoid**, as bacteria lack a membrane-bound nucleus. - The nucleoid is where the genetic material is condensed and organized within the bacterial cell. *Haploid* - Bacteria are essentially **haploid organisms**, possessing a single set of chromosomes. - This means they have only one allele for each gene, which can lead to rapid expression of mutations.

Question 62: Transfer of a portion of DNA from one bacterium to another by bacteriophage is known as

A. Conjugation
B. Transformation
C. Mutation
D. Transduction (Correct Answer)

Explanation: ***Transduction*** - **Transduction** is a process where bacterial DNA is transferred from one bacterium to another via a **bacteriophage** (a virus that infects bacteria). - The bacteriophage accidentally packages bacterial DNA instead of its own genetic material and then injects it into a new host bacterium. - This is the defining mechanism that distinguishes transduction from other forms of horizontal gene transfer. *Conjugation* - **Conjugation** involves the direct transfer of genetic material (usually plasmids) between bacterial cells through physical contact, often via a **pilus**. - It does not involve a bacteriophage as an intermediary for DNA transfer. *Transformation* - **Transformation** is the process by which bacteria take up **free DNA** from their environment. - This DNA can come from lysed bacterial cells and does not involve bacteriophages. *Mutation* - A **mutation** is a change in the DNA sequence of an organism. - While it alters genetic material, it is not a mechanism for the transfer of DNA between bacteria.

Question 63: Which of the following is the most widespread method of genetic transfer among bacteria?

A. Transformation
B. Transduction
C. Lysogenic conversion
D. Conjugation (Correct Answer)

Explanation: ***Conjugation*** - This method involves **direct cell-to-cell contact** between bacteria, typically through a **pilus**, allowing for the transfer of **plasmids** or chromosomal DNA. - Its widespread nature is due to the efficient transfer of large amounts of genetic material, including **antibiotic resistance genes**, among diverse bacterial species. *Transformation* - This process involves the uptake of **naked DNA** from the environment by a competent bacterial cell. - While important, it is less common because bacteria must be in a specific **physiological state** (competent) to take up DNA, and the DNA itself can be fragile in the environment. *Transduction* - This mechanism involves the transfer of bacterial DNA from one bacterium to another via a **bacteriophage** (virus). - Its prevalence is limited by the specific range of bacteria that a phage can infect, making it less widespread compared to conjugation. *Lysogenic conversion* - This refers to the acquisition of new traits by a bacterium due to the integration of a **prophage** (temperate phage DNA) into its genome. - While it can alter bacterial phenotypes, it is a specific outcome of phage infection and not a primary mechanism for broad genetic transfer among bacteria.

Question 64: What is the transfer of genetic material by direct physical contact in bacteria called?

A. Transformation
B. Transduction
C. Conjugation (Correct Answer)
D. Lysogeny

Explanation: ***Conjugation*** - This process involves the direct transfer of genetic material (usually a **plasmid**) from a donor bacterium to a recipient bacterium through a **pilus**. - It requires direct cell-to-cell contact, which is the defining characteristic mentioned in the question. *Transformation* - This is the uptake of **naked DNA** from the environment by a bacterial cell. - It does not involve direct physical contact between two bacterial cells. *Transduction* - This is the transfer of bacterial DNA from one bacterium to another via a **bacteriophage** (virus that infects bacteria). - While it involves genetic transfer, it is mediated by a virus, not direct cell-to-cell contact. *Lysogeny* - This refers to a state where a **bacteriophage genome** integrates into the host bacterial chromosome and replicates along with it without causing lysis. - It describes a phage life cycle and not a method of bacterial genetic material transfer between two bacteria.

Question 65: Genetic information transferred by phage is called?

A. Transduction (Correct Answer)
B. Transcription
C. Lysogenic conversion
D. Transformation

Explanation: ***Transduction*** - **Transduction** is the process by which foreign DNA is introduced into a cell by a virus or viral vector (phage). - This process is a common tool used by molecular biologists to introduce a foreign gene into a host cell's genome. *Transcription* - **Transcription** is the process where a gene's information is copied from DNA into RNA, not directly transferred by phages. - This is the first step in **gene expression**, where the mRNA molecule then carries the code for protein synthesis. *Lysogenic conversion* - **Lysogenic conversion** occurs when a temperate phage integrates its DNA into the host bacterium's genome, leading to the expression of new genes. - While it involves phage DNA and can alter host characteristics (like virulence), it's a specific outcome of lysogeny, whereas transduction is the general term for phage-mediated genetic transfer. *Transformation* - **Transformation** is the genetic alteration of a cell resulting from the direct uptake and incorporation of exogenous **naked DNA** from its surroundings. - This process does not involve a viral vector (phage) for DNA transfer.

Question 66: A phenomenon in which transfer of genetic information occurs through the agency of free DNA is known as

A. Transformation (Correct Answer)
B. Transcription
C. Translation
D. Transduction

Explanation: ***Transformation*** - **Transformation** is the process where a bacterial cell takes up **naked DNA** from its environment. - This acquired genetic material can then be incorporated into the host cell's genome, leading to a change in its characteristics. *Transcription* - **Transcription** is the process of synthesizing **RNA from a DNA template**. - It involves the enzyme **RNA polymerase** and is the first step in gene expression, not the transfer of genetic information between organisms via free DNA. *Translation* - **Translation** is the process where **ribosomes synthesize proteins** by decoding mRNA sequences. - This process converts the genetic information encoded in mRNA into a functional protein, rather than the uptake of free DNA. *Transduction* - **Transduction** involves the transfer of bacterial DNA from one bacterium to another via a **bacteriophage (virus)**. - Unlike transformation, which uses naked DNA, transduction requires a viral vector to mediate the genetic transfer.

Question 67: The genetic ability of a bacterium to grow in the presence of several antibiotics is passed in vivo from one bacterium to another by:

A. Transposition
B. Conjugation (Correct Answer)
C. Transduction
D. Transformation

Explanation: ***Conjugation*** - This process involves the direct transfer of **genetic material** (often plasmids carrying **resistance genes**) from one bacterium to another through direct cell-to-cell contact, typically via a **pilus**. - It is a primary mechanism for the spread of **antibiotic resistance** among bacteria, enabling the rapid acquisition of resistance traits. *Transposition* - This involves the movement of **transposable elements** (jumping genes) within a bacterium's own genome or between a plasmid and the chromosome. - While transposons can carry antibiotic resistance genes, their movement is typically within a single cell's DNA, not directly between different bacteria (though they can be part of transferred plasmids). *Transduction* - This is the process where **bacteriophages** (viruses that infect bacteria) carry bacterial DNA from one bacterium to another. - While it can transfer antibiotic resistance genes, it requires a viral vector rather than direct cell-to-cell contact, making it a distinct mechanism from conjugation. *Transformation* - This is the uptake of **naked DNA** from the environment by a bacterial cell. - The DNA can originate from dead bacterial cells, and while it *can* include resistance genes, it's not a direct, active transfer between living bacteria in the way conjugation is.

Question 68: The mechanism of genetic transfer where a phage serves as a vehicle is:

A. Transformation
B. Lysogeny
C. Transduction (Correct Answer)
D. Conjugation

Explanation: ***Transduction*** - This is the correct answer as **transduction** is the process of genetic transfer where a **bacteriophage** (a virus that infects bacteria) serves as a vehicle to transfer **bacterial DNA** from one bacterium to another. - This process can be **generalized transduction**, where any bacterial gene can be transferred, or **specialized transduction**, where only specific genes near the prophage integration site are transferred. *Transformation* - **Transformation** is a type of horizontal gene transfer where bacteria take up **naked DNA** from their environment. - This process does **not involve a phage** and is especially common in naturally competent bacteria like Streptococcus pneumoniae and Haemophilus influenzae. *Conjugation* - **Conjugation** is the process where genetic material is directly transferred between two bacterial cells through a **pilus** (sex pilus). - This typically involves the transfer of **plasmids** and does not involve a viral vehicle. *Lysogeny* - **Lysogeny** refers to a cycle in which a **bacteriophage** integrates its **genome** into the host bacterium's chromosome as a **prophage** without immediately causing lysis. - While it involves a phage, it describes the *state* of the phage-host relationship rather than a method of gene transfer *between different bacteria* via a phage vehicle.

Question 69: Lysogenic conversion is seen in -

A. Staphylococcus
B. Corynebacterium diphtheriae (Correct Answer)
C. Salmonella
D. E. coli

Explanation: ***Corynebacterium diphtheriae*** - **Lysogenic conversion** in *Corynebacterium diphtheriae* is the **classic textbook example**, where a bacteriophage (phage β) carries the **tox gene** for the **diphtheria toxin**, converting a non-pathogenic strain into a toxigenic pathogen. - The integration of this **prophage** into the bacterial chromosome allows for stable toxin production, leading to the clinical manifestations of diphtheria. - This is the **most commonly cited example** of lysogenic conversion in medical microbiology education. *Staphylococcus* - While *Staphylococcus* can be infected by bacteriophages, **lysogenic conversion** is not the primary mechanism for acquiring major virulence factors. - Toxins like **TSST-1** and **Panton-Valentine leukocidin (PVL)** are more commonly encoded on **plasmids, pathogenicity islands, or other mobile genetic elements** rather than temperate bacteriophages. *Salmonella* - Some *Salmonella* serotypes can undergo **lysogenic conversion** where prophages confer **O-antigen modifications** or other factors. - However, this is less emphasized in standard medical curricula compared to the diphtheria toxin example. *E. coli* - **Shiga toxin-producing *E. coli* (STEC)** strains acquire **Shiga toxins (Stx1 and Stx2)** through lysogenic conversion via **Stx-encoding bacteriophages**. - While this is indeed a valid and important example of lysogenic conversion, the **diphtheria toxin** case is traditionally presented as the **primary teaching example** in Indian medical PG curricula (Ananthanarayan & Paniker, Jawetz). - For examination purposes, ***Corynebacterium diphtheriae*** is the expected answer when asked about lysogenic conversion.

Question 70: Bacteria acquire characteristics by all of the following except:

A. Through plasmids
B. Through conjugation
C. Through mitosis (Correct Answer)
D. Through bacteriophages

Explanation: ***Through mitosis - Bacteria are prokaryotes and reproduce by binary fission, not mitosis. Mitosis is a process of nuclear division in eukaryotes involving organized chromosomes and a mitotic spindle. Therefore, bacteria do not acquire new characteristics or genetic material through mitosis.*** *Through plasmids - Plasmids are small, circular, extrachromosomal DNA molecules found in bacteria that can carry genes for antibiotic resistance, virulence factors, or metabolic capabilities. Bacteria can acquire new characteristics by taking up plasmids from the environment (transformation) or through conjugation.* *Through conjugation - Conjugation is a process of horizontal gene transfer where genetic material (typically plasmids) is transferred directly from a donor bacterium to a recipient through a pilus (sex pilus) during direct cell-to-cell contact. This allows bacteria to rapidly acquire new characteristics like antibiotic resistance genes.* *Through bacteriophages - Bacteriophages are viruses that infect bacteria. During infection, phages can pick up bacterial DNA and transfer it to other bacteria through a process called transduction (specialized or generalized). This introduces new genetic information and characteristics to the recipient bacterium.*

Practice by Chapter

Bacterial Genome Organization

Practice Questions

Plasmids and Mobile Genetic Elements

Practice Questions

Bacterial Gene Expression

Practice Questions

Mutation and Mutagenesis

Practice Questions

Gene Transfer in Bacteria

Practice Questions

Transposons and Insertion Sequences

Practice Questions

Bacterial Genetic Recombination

Practice Questions

Regulation of Gene Expression

Practice Questions

CRISPR-Cas Systems

Practice Questions

Bacterial Stress Responses

Practice Questions

Genetics of Antimicrobial Resistance

Practice Questions

Genetic Basis of Bacterial Virulence

Practice Questions

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