Microbial Genetics Practice Questions

Q: When phage DNA integrates with the bacterial chromosome as a prophage and confers new characteristics to the host bacterium, this phenomenon is referred to as:

Phage conversion. ### Explanation **Correct Answer: B. Phage conversion** **1. Why Phage Conversion is Correct:** Phage conversion (also known as **Lysogenic conversion**) occurs when a temperate bacteriophage infects a bacterium and integrates its genome into the host chromosome as a **prophage**. Unlike the lytic cycle, the bacterium survives, and the newly acquired viral genes are expressed, conferring new phenotypic characteristics (often virulence factors) to the host. The bacterium remains "converted" as long as it harbors the prophage. **2. Why Other Options are Incorrect:** * **A. Transformation:** This involves the uptake of "naked" DNA from the surrounding environment by a competent bacterium. It does not require a viral vector. * **C. Conjugation:** This is the transfer of genetic material (usually plasmids) through direct cell-to-cell contact via a sex pilus. It is often referred to as bacterial "mating." * **D. Transduction:** This is the process where a bacteriophage acts as a vector to transfer bacterial DNA from one cell to another. While it involves phages, the goal is the transfer of *host* genes, whereas phage conversion involves the expression of *viral* genes. **3. High-Yield Clinical Pearls (NEET-PG):** Several medically significant bacterial toxins are encoded by phages via lysogenic conversion. Remember the mnemonic **"COBEDS"**: * **C**holera toxin (*Vibrio cholerae*) * **O** antigen of *Salmonella* * **B**otulinum toxin (*Clostridium botulinum*) * **E**rythrogenic toxin (*Streptococcus pyogenes* - causes Scarlet Fever) * **D**iphtheria toxin (*Corynebacterium diphtheriae*) * **S**higa toxin (*Shigella* and EHEC/STEC) *Note: If a bacterium loses its prophage, it loses its ability to produce these toxins and becomes non-pathogenic.*

Q: Transfer of genetic material in between bacteria through pili is termed as:

Conjugation. **Explanation:** **Conjugation** is the correct answer because it is the process of horizontal gene transfer that requires **direct cell-to-cell contact**. This process is mediated by a specialized proteinaceous tube called the **sex pilus** (encoded by the **F plasmid**). The donor cell ($F^+$) attaches to the recipient cell ($F^-$) via the pilus, which then retracts to bring the cells together, allowing for the transfer of a single strand of DNA. **Analysis of Incorrect Options:** * **Transduction (A):** This involves the transfer of bacterial DNA from one cell to another via a **bacteriophage** (virus). It does not involve pili. * **Transformation (C):** This is the uptake of **"naked" DNA** directly from the surrounding environment by a competent bacterium. No cell-to-cell contact or pili are required. * **Transfection (D):** This term typically refers to the process of deliberately introducing naked or purified nucleic acids into **eukaryotic cells** (often in a laboratory setting). **High-Yield Clinical Pearls for NEET-PG:** * **Medical Significance:** Conjugation is the most common mechanism for the spread of **multidrug resistance (R-plasmids)** among Gram-negative bacteria. * **Hfr Cells:** When an F plasmid integrates into the bacterial chromosome, the cell becomes a **High Frequency of Recombination (Hfr)** cell. * **Gram-Positive Conjugation:** Unlike Gram-negatives, Gram-positive bacteria (like *Enterococcus faecalis*) do not use pili; they use **sticky surface molecules** (adhesins) to facilitate contact. * **Transformation** is the primary mechanism for *Streptococcus pneumoniae*, *Haemophilus influenzae*, and *Neisseria* (the "SHiN" organisms).

Q: Which of the following statements regarding plasmids is false?

Plasmids are synchronized with chromosomal multiplication.. ### Explanation **Why Option D is the correct (False) statement:** Plasmids are characterized by **autonomous replication**. Unlike chromosomal DNA, which replicates only once during the cell cycle, plasmids possess their own **origin of replication (oriP)**. This allows them to replicate independently of the host cell's chromosomal division. Consequently, a single bacterial cell can contain multiple copies of a plasmid (high copy number), and their multiplication is **not synchronized** with the chromosomal multiplication. **Analysis of Incorrect Options:** * **Option A (Circular):** Most bacterial plasmids exist as covalently closed circular DNA molecules. While some linear plasmids exist in specific genera (like *Borrelia*), the standard definition for medical microbiology is circular. * **Option B (Double-stranded DNA):** Plasmids are composed of double-stranded DNA (dsDNA). They utilize the host cell's machinery (DNA polymerase) for their replication. * **Option C (Extra-chromosomal):** By definition, plasmids are genetic elements that exist physically separate from the main bacterial chromosome. They are not essential for the basic survival of the bacteria but provide selective advantages. **High-Yield Clinical Pearls for NEET-PG:** * **R-Plasmids:** Carry genes for antibiotic resistance (e.g., Beta-lactamases). This is the most clinically significant function. * **F-Plasmids (Fertility):** Code for the **sex pilus**, essential for the process of **Conjugation** (horizontal gene transfer). * **Col-Plasmids:** Produce **bacteriocins** (e.g., Colicin), which are proteins that kill other closely related bacteria. * **Virulence Plasmids:** Carry genes for toxins (e.g., Anthrax toxin, *E. coli* enterotoxins). * **Episomes:** Plasmids that have the unique ability to integrate into the bacterial chromosome.

Q: What is the mechanism of direct transfer of free DNA from one bacterium to another?

Transformation. **Explanation:** The correct answer is **Transformation (Option A)**. **Why Transformation is correct:** Transformation is the process by which a recipient bacterium takes up **"naked" or free DNA** directly from the surrounding medium. This DNA is typically released into the environment following the lysis of a donor bacterium. For transformation to occur, the recipient cell must be in a state of **competence**. This was famously demonstrated by Griffith’s experiment using *Streptococcus pneumoniae*. **Why other options are incorrect:** * **Conjugation (Option B):** This involves the transfer of genetic material (usually plasmids) through **direct cell-to-cell contact** via a sex pilus. It is often referred to as "bacterial mating." * **Transduction (Option C):** This is the transfer of bacterial DNA from one cell to another mediated by a **bacteriophage** (virus). There is no "free" DNA involved as the genetic material is packaged within a viral capsid. **High-Yield Clinical Pearls for NEET-PG:** * **Natural Competence:** Bacteria that naturally undergo transformation include *Haemophilus influenzae*, *Streptococcus pneumoniae*, and *Neisseria* species (Mnemonic: **H**is **S**exy **N**eice). * **Griffith Effect:** The "Transforming Principle" was the first evidence that DNA is the genetic material. * **DNAse Sensitivity:** Transformation is the only horizontal gene transfer mechanism that is **inhibited by DNAse** in the medium, as the DNA is exposed and "free." * **Clinical Significance:** Transformation plays a role in the spread of antibiotic resistance and antigenic variation in *Neisseria gonorrhoeae*.

Q: The image given below shows:

Conjugation. ***Conjugation*** - The image clearly depicts a **sex pilus** connecting a donor cell (F+ cell) to a recipient cell (F- cell), facilitating the transfer of an **F factor (plasmid)**. - This process involves direct cell-to-cell contact and genetic material transfer, which is the hallmark of bacterial conjugation. *Transformation* - This process involves the uptake of **naked DNA** from the environment by a bacterial cell, without direct cell-to-cell contact. - The image shows a physical connection (sex pilus) and directed DNA transfer, not uptake of free DNA. *Transduction* - This is the process where a **bacteriophage** (virus) transfers genetic material from one bacterium to another. - The image does not show any viral particles or their involvement in DNA transfer. *Lysogenic conversion* - This occurs when a bacterium acquires new genes from a **lysogenic bacteriophage** (prophage) integrated into its genome, often altering the host's phenotype. - The diagram illustrates plasmid transfer between bacteria via a pilus, not integration of viral DNA into the bacterial chromosome.

Question 1

When phage DNA integrates with the bacterial chromosome as a prophage and confers new characteristics to the host bacterium, this phenomenon is referred to as:

Accepted Answer

Phage conversion

Answer

Transformation

Answer

Conjugation

Answer

Transduction

Question 2

In transduction, what is the origin of the DNA transmitted by the vector to the recipient bacterium?

Accepted Answer

Another bacterium

Answer

Human cell

Answer

Bacteriophage

Answer

Virus

Question 3

Drug resistance in Staphylococcus aureus is most commonly acquired by which mechanism?

Accepted Answer

Transduction

Answer

Mutation

Answer

Transformation

Answer

Conjugation

Question 4

Transfer of genetic material in between bacteria through pili is termed as:

Accepted Answer

Conjugation

Answer

Transduction

Answer

Transformation

Answer

Transfection

Question 5

Bacteria acquire restriction endonuclease by horizontal gene transfer. What is the likely outcome of this acquisition?

Accepted Answer

The bacteria cannot survive in the absence of DNA methylase to protect the host DNA.

Answer

The bacteria can undergo mutation.

Answer

The bacteria causes degradation of the host DNA.

Answer

It aids in DNA proofreading within the bacteria.

Question 6

What is the most common mechanism of transfer resistance in Staphylococcus aureus?

Accepted Answer

Transduction

Answer

Conjugation

Answer

Transformation

Answer

Mutation

Question 7

Which of the following statements regarding plasmids is false?

Accepted Answer

Plasmids are synchronized with chromosomal multiplication.

Answer

Plasmids are circular genetic material.

Answer

Plasmids are double-stranded DNA.

Answer

Plasmids are extra-chromosomal.

Question 8

What is the mechanism of direct transfer of free DNA from one bacterium to another?

Accepted Answer

Transformation

Answer

Conjugation

Answer

Transduction

Answer

None of the above

Question 9

Genetic material is transferred from one bacterium to another by which of the following mechanisms?

Accepted Answer

Transfection

Answer

Transduction

Answer

Transformation

Answer

Conjugation

Question 10

The image given below shows:

Accepted Answer

Conjugation

Answer

Transformation

Answer

Transduction

Answer

Lysogenic conversion

Microbial Genetics — MCQs

Microbial Genetics — MCQs

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