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.

Q: Which of the following is used as an antibacterial agent?

Lysogenic bacteriophage. **Explanation:** The correct answer is **Lysogenic bacteriophage**. This concept revolves around **Lysogenic Conversion**, where a temperate (lysogenic) bacteriophage infects a bacterium and integrates its genome into the bacterial chromosome as a prophage. This process can introduce new genetic traits to the host bacterium, including the production of potent exotoxins. **Why Lysogenic Bacteriophage is correct:** Several medically important bacterial toxins are encoded by lysogenic phages rather than the bacterial chromosome itself. Without the presence of these specific phages, the bacteria remain non-pathogenic. A classic mnemonic for toxins acquired via lysogenic conversion is **ABCDES**: * **A:** Group A *Streptococcus* (Pyrogenic exotoxin/Scarlet fever) * **B:** *Botulinum* toxin * **C:** *Cholera* toxin * **D:** *Diphtheria* toxin * **E:** *E. coli* (Shiga-like toxin/VTEC) * **S:** *Shiga* toxin **Analysis of Incorrect Options:** * **Lytic bacteriophage:** These phages follow the lytic cycle, leading to the immediate multiplication of viruses and the death (lysis) of the host cell. While they kill bacteria, they are not the primary mechanism for toxin production (virulence). * **Plasmid:** These are extrachromosomal DNA molecules. While they often carry antibiotic resistance genes (R-plasmids), they are distinct from bacteriophages. * **Transposon:** Known as "jumping genes," these are mobile genetic elements that move within or between DNA molecules. They contribute to genetic diversity but are not independent viral agents. **NEET-PG High-Yield Pearls:** * **Diphtheria toxin** is the classic example of lysogenic conversion; only strains of *C. diphtheriae* infected by the **Beta-phage** cause disease. * **Transduction** is the process of genetic transfer mediated by bacteriophages (Generalized = Lytic; Specialized = Lysogenic).

Q: Which of the following functions are associated with plasmids?

Involved in multidrug resistance transfer and conjugation. Plasmids are extrachromosomal, double-stranded, circular DNA molecules that replicate autonomously within a bacterial cell. They are not essential for basic survival but provide significant evolutionary advantages. **Explanation of the Correct Answer:** The correct answer is **Involved in multidrug resistance transfer and conjugation**. 1. **Multidrug Resistance (R-plasmids):** Plasmids often carry "R-factors" (Resistance factors) which contain genes that encode for enzymes (like beta-lactamases) that neutralize antibiotics. 2. **Conjugation (F-plasmids):** The Fertility or F-plasmid encodes for the sex pilus, allowing the horizontal transfer of genetic material (including R-factors) between bacteria. This is the primary mechanism for the rapid spread of multidrug resistance in clinical settings. **Analysis of Options:** * **Option B (Incorrect):** While some plasmids can carry virulence factors (like the Vi antigen in *Salmonella Typhi* or toxins in *B. anthracis*), capsule formation is typically governed by chromosomal DNA in most pathogenic bacteria (e.g., *Streptococcus pneumoniae*). * **Options A, C, and D:** These options are identical in the provided text. In a standard NEET-PG format, distractors would include functions like "essential for cellular metabolism" or "part of the nucleoid," both of which are incorrect as plasmids are non-essential and extrachromosomal. **High-Yield Clinical Pearls for NEET-PG:** * **Col-plasmids:** Produce colicins (bacteriocins) which kill other bacteria. * **Ti-plasmid:** Derived from *Agrobacterium tumefaciens*, used extensively in genetic engineering. * **Episome:** A plasmid that can integrate into the bacterial chromosome (e.g., Hfr cells). * **Medical Significance:** Plasmids are the chief reason for the emergence of "Superbugs" like MRSA and Carbapenem-resistant Enterobacteriaceae (CRE).

Q: Which segment of DNA is found between chromosomal and extrachromosomal DNA molecules within cells?

Transposons. ### Explanation **Correct Option: A. Transposons** Transposons, often referred to as **"jumping genes,"** are segments of DNA that can move from one location to another within the same or different DNA molecules (e.g., from a chromosome to a plasmid, or vice versa). They do not exist as independent autonomous units but function as mobile genetic elements. Because they frequently shuttle between the bacterial chromosome and extrachromosomal elements like plasmids, they are described as the genetic link found between these two DNA molecules. **Analysis of Incorrect Options:** * **B. Cosmid:** These are artificial hybrid cloning vectors containing the *cos* sequences of lambda phage and plasmid DNA. They are used in genetic engineering to carry large DNA fragments and do not naturally occur as "jumping" segments between cellular DNA. * **C. Plasmid:** These are extrachromosomal, circular, double-stranded DNA molecules capable of autonomous replication. While they are extrachromosomal DNA themselves, they are not the "segment found between" the two; rather, they are the destination or source for transposons. * **D. Episomes:** These are a subset of plasmids that have the ability to integrate into the host chromosome (e.g., the F-plasmid in *E. coli*). While they can exist in both states, the term refers to the entire genetic unit, not the specific mobile segment moving between them. **High-Yield Clinical Pearls for NEET-PG:** * **Medical Significance:** Transposons often carry **antibiotic resistance genes** (e.g., *Tn5* carries kanamycin resistance). Their movement is a primary driver for the rapid spread of multi-drug resistance (MDR) among bacterial populations. * **Mechanism:** Transposition is mediated by the enzyme **transposase**. * **Simple vs. Complex:** Simple transposons (Insertion Sequences) carry only the information for movement, while complex transposons carry additional genes (like toxins or resistance).

Q: Non-toxigenic C. diphtheriae is converted to toxigenic C. diphtheriae with the help of a bacteriophage. By which method does this conversion occur?

Transduction. **Explanation:** The conversion of a non-toxigenic strain of *Corynebacterium diphtheriae* into a toxigenic one is a classic example of **Lysogenic Conversion**, which is a specialized form of **Transduction**. **1. Why Transduction is Correct:** Transduction is the transfer of bacterial DNA from one cell to another via a **bacteriophage** (a virus that infects bacteria). In the case of *C. diphtheriae*, the gene responsible for toxin production (the *tox* gene) is not part of the bacterial chromosome itself; it is carried by a specific temperate bacteriophage called the **Beta-phage**. When this phage infects a non-toxigenic bacterium and integrates its genome into the bacterial chromosome (lysogeny), the bacterium begins to produce the diphtheria toxin. **2. Why Other Options are Incorrect:** * **Transformation:** This involves the uptake of "naked" DNA directly from the surrounding environment. It does not require a viral vector. * **Conjugation:** This is "bacterial sex" involving direct cell-to-cell contact through a sex pilus to transfer genetic material (usually plasmids). * **Recombinant Technology:** This is an artificial laboratory technique used to manipulate DNA; it is not the natural biological process by which *C. diphtheriae* acquires virulence. **Clinical Pearls for NEET-PG:** * **Key Concept:** "No Phage = No Toxin." Only strains of *C. diphtheriae* lysogenized by the Beta-phage cause the clinical disease Diphtheria. * **Other examples of Lysogenic Conversion:** * *Vibrio cholerae* (CTX phage) * *Streptococcus pyogenes* (Erythrogenic toxin/Scarlet fever) * *Clostridium botulinum* (Botulinum toxin) * *Shigella dysenteriae* (Shiga toxin) * **High-Yield Test:** The **Elek’s Gel Precipitation Test** is used to detect the production of this toxin in vitro.

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

Question 6

Which of the following is used as an antibacterial agent?

Accepted Answer

Lysogenic bacteriophage

Answer

Lytic bacteriophage

Answer

Plasmid

Answer

Transposon

Question 7

Which of the following functions are associated with plasmids?

Accepted Answer

Involved in multidrug resistance transfer and conjugation

Answer

Involved in conjugation and imparts capsule formation

Question 8

Which segment of DNA is found between chromosomal and extrachromosomal DNA molecules within cells?

Accepted Answer

Transposons

Answer

Cosmid

Answer

Plasmid

Answer

Episomes

Question 9

Non-toxigenic C. diphtheriae is converted to toxigenic C. diphtheriae with the help of a bacteriophage. By which method does this conversion occur?

Accepted Answer

Transduction

Answer

Transformation

Answer

Conjugation

Answer

Recombinant technology

Question 10

Bacteria can acquire new genetic characteristics by all of the following mechanisms except?

Accepted Answer

Transduction: Transfer of bacterial DNA via bacteriophages.

Answer

Transformation: Uptake of free DNA from the environment.

Answer

Conjugation: Direct transfer of genetic material between bacterial cells.

Answer

Recombination with host DNA.

Microbial Genetics — MCQs

Microbial Genetics — MCQs

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