Microbial Genetics Practice Questions

Q: Which of the following mechanisms of genetic transfer is illustrated here?

Transduction. ***Transduction*** - Involves the transfer of bacterial DNA from one bacterium to another by a **bacteriophage** (bacterial virus) acting as a vector. - This can occur via **generalized transduction** (random segments of DNA packaged) or **specialized transduction** (specific genes near the phage integration site). *Transformation* - Involves the uptake of **naked or environmental DNA** by competent bacteria from the surrounding medium. - Does not require a vector or cell-to-cell contact and is historically demonstrated in **Streptococcus pneumoniae**. *Conjugation* - Requires **direct cell-to-cell contact** between bacteria, typically mediated by a **sex pilus** encoded by an **F-plasmid**. - Results in the unidirectional transfer of plasmid DNA or sometimes chromosomal DNA, often leading to rapid spread of **antibiotic resistance**. *Transposition* - Refers to the movement of **transposons** ('jumping genes') from one location to another within or between DNA molecules. - This is an **intracellular process** and does not act as a primary mechanism for horizontal gene transfer between distinct bacterial cells.

Q: The transfer of genetic information from one cell to another can occur by three methods. Phage mediated transfer of cell DNA into host is known as?

Transduction. **Explanation:** The transfer of genetic material between bacteria occurs via horizontal gene transfer. The correct answer is **Transduction**, which is defined as the process by which DNA is transferred from one bacterium to another by a **bacteriophage** (a virus that infects bacteria). During the phage replication cycle, segments of bacterial DNA are accidentally packaged into the viral capsid and subsequently injected into a new host cell. **Analysis of Options:** * **Transformation:** This involves the uptake of **"naked" DNA** directly from the surrounding environment by a competent recipient cell. It does not require a vector or cell-to-cell contact. * **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." * **Transmission:** This is a general term referring to the spread of an infectious agent from one host to another (e.g., respiratory droplets, fecal-oral route) and is not a specific mechanism of microbial genetic transfer. **High-Yield Clinical Pearls for NEET-PG:** * **Generalized Transduction:** Occurs during the lytic cycle; any part of the bacterial genome can be transferred. * **Specialized Transduction:** Occurs during the lysogenic cycle; only specific genes adjacent to the viral integration site are transferred. * **Clinical Significance:** Transduction is a key mechanism for the transfer of **virulence factors** (e.g., Shiga toxin, Diphtheria toxin, and Cholera toxin) and antibiotic resistance genes. * **Competence:** Only certain bacteria (e.g., *S. pneumoniae*, *H. influenzae*, *Neisseria*) are naturally "competent" for transformation.

Q: A bacterium acquires an antibiotic-resistance gene that is carried by a phage vector. The phage vector carries primarily the antibiotic-resistance gene, rather than randomly chosen pieces of bacterial DNA. This is an example of which of the following?

Site-specific transduction. **Explanation:** The process described is **Specialized (Site-specific) Transduction**. This occurs when a temperate bacteriophage integrates its genome into a specific site on the bacterial chromosome (prophage). Upon induction, the phage DNA sometimes excises imprecisely, carrying with it specific adjacent bacterial genes (like antibiotic resistance or toxins). Because the phage integrates at a fixed site, it consistently carries the same set of neighboring genes rather than random fragments. **Analysis of Options:** * **Generalized Transduction (B):** Occurs during the lytic cycle when random fragments of bacterial DNA are accidentally packaged into the phage head. It can carry *any* part of the bacterial genome, unlike the specific genes mentioned in the question. * **Conjugation (A):** Involves the transfer of DNA (usually plasmids) through direct cell-to-cell contact via a sex pilus. No phage vector is involved. * **Transformation (D):** The uptake of "naked" DNA from the surrounding environment by a competent bacterium. It does not require a viral vector. **High-Yield Clinical Pearls for NEET-PG:** * **Specialized Transduction** is responsible for the transfer of potent bacterial toxins. Remember the mnemonic **ABCDE**: * **A**ntigen (Streptococcal pyrogenic exotoxin) * **B**otulinum toxin * **C**holera toxin * **D**iphtheria toxin * **E**rythrogenic toxin (Shiga toxin) * **Lysogenic conversion:** When a non-pathogenic bacterium becomes pathogenic after acquiring a phage containing a toxin gene (e.g., *Corynebacterium diphtheriae*). * **Generalized transduction** occurs during the **lytic** cycle; **Specialized transduction** occurs during the **lysogenic** cycle.

Question 1

Which of the following mechanisms of genetic transfer is illustrated here?

Accepted Answer

Transduction

Answer

Transformation

Answer

Conjugation

Answer

Transposition

Question 2

The transfer of genetic information from one cell to another can occur by three methods. Phage mediated transfer of cell DNA into host is known as?

Accepted Answer

Transduction

Answer

Transformation

Answer

Transmission

Answer

Conjugation

Question 3

Bacteria are simple genetic units with all of the following properties, EXCEPT:

Accepted Answer

Their DNA has intervening sequences (introns) in almost all genes

Answer

They are diploid

Answer

Their genetic material is organized into a single chromosome

Answer

They use the same genetic code as all eukaryotes

Question 4

A bacterium acquires an antibiotic-resistance gene that is carried by a phage vector. The phage vector carries primarily the antibiotic-resistance gene, rather than randomly chosen pieces of bacterial DNA. This is an example of which of the following?

Accepted Answer

Site-specific transduction

Answer

Conjugation

Answer

Generalized transduction

Answer

Transformation

Question 5

What method of bacterial gene transfer involves the role of viruses?

Accepted Answer

Transduction

Answer

Conjugation

Answer

Translation

Answer

Transformation

Microbial Genetics — MCQs

Microbial Genetics — MCQs

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