Microbial Genetics Practice Questions

Q: Transmission of R factor is by which mechanism?

Conjugation. **Explanation:** **Why Conjugation is Correct:** The **R factor (Resistance factor)** is a type of plasmid that carries genes for antibiotic resistance. It consists of two components: the **Resistance Transfer Factor (RTF)**, which contains genes for autonomous replication and conjugative transfer, and the **r-determinant**, which carries the actual resistance genes. The primary mechanism for the spread of R factors between bacteria (especially Gram-negative bacilli like *E. coli* and *Salmonella*) is **Conjugation**. This process involves direct cell-to-cell contact via a sex pilus, allowing the rapid horizontal transfer of multi-drug resistance across different bacterial species. **Why Other Options are Incorrect:** * **Transduction:** This involves the transfer of DNA via a **bacteriophage** (virus). While some resistance genes (like those for Penicillinase in *Staphylococci*) can be transduced, the large R factor plasmid is typically transferred via conjugation. * **Transformation:** This is the uptake of **naked DNA** from the environment. It is a significant mechanism for species like *S. pneumoniae* and *Neisseria*, but it is not the classic route for R factor transmission. * **Lysogenic Conversion:** This occurs when a temperate phage integrates into the bacterial chromosome (prophage), imparting new phenotypic traits (e.g., **Diphtheria toxin**, Cholera toxin, or Erythrogenic toxin). It does not involve R factor transfer. **High-Yield Clinical Pearls for NEET-PG:** * **R Factor:** Responsible for "Infectious Drug Resistance." One R factor can carry resistance to multiple drugs (e.g., Sulfonamides, Streptomycin, Chloramphenicol). * **Conjugation:** The most common method for the spread of multidrug resistance in clinical settings. * **Transposons ("Jumping Genes"):** These are DNA sequences that can move from a plasmid to a chromosome (or vice versa) and are often found within R factors.

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: Which statement is true regarding phage DNA?

Restriction enzyme sites. ### Explanation **Correct Answer: B. Restriction enzyme sites** **Why it is correct:** Bacteriophages (viruses that infect bacteria) possess specific DNA sequences known as **restriction enzyme sites**. These are short, palindromic sequences recognized and cleaved by bacterial **Restriction Endonucleases (REs)**. In nature, these enzymes act as a primitive immune system for bacteria, protecting them by "restricting" or cutting up foreign phage DNA upon entry. In molecular biology, these sites are crucial because phages (like Phage Lambda) are used as **cloning vectors**; scientists use REs to cut the phage DNA at these specific sites to insert foreign genes. **Analysis of Incorrect Options:** * **A. Antibiotic susceptibility:** Antibiotics target bacterial structures (cell walls, 70S ribosomes). Phages are viruses and are inherently resistant to antibiotics. * **C. Hexagonal DNA:** This is a distractor. While the **capsid (head)** of many phages (like T4) is icosahedral or hexagonal in shape, the **DNA itself** is a standard double-stranded linear or circular molecule, not "hexagonal" in structure. * **D. Carries short segments of host DNA:** While this occurs during **transduction**, it is not a defining characteristic of "phage DNA" itself. It is a process of horizontal gene transfer where host DNA is accidentally packaged *instead* of or *with* viral DNA. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Restriction Endonucleases:** Type II REs are the most commonly used in recombinant DNA technology because they cut at specific, predictable sites. * **Transduction:** A key mechanism of transfer of virulence factors (e.g., Diphtheria toxin, Cholera toxin, and Shiga toxin are all encoded by lysogenic phages). * **Phage Typing:** Used in epidemiology to strain-type bacteria like *Staphylococcus aureus* and *Salmonella Typhi* based on their susceptibility to specific phages.

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 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: What is formed when plasmid DNA combines with chromosomal DNA?

Episome. **Explanation:** **1. Why the Correct Answer is Right:** An **episome** is a type of extrachromosomal genetic element (plasmid) that possesses the unique ability to integrate itself into the bacterial chromosome. While all plasmids are extrachromosomal, only those that can physically combine with the host’s genomic DNA are termed episomes. A classic example is the **F-plasmid** (Fertility factor) in *E. coli*. When the F-plasmid integrates into the chromosome, the bacterium is transformed into an **Hfr (High-frequency recombination) cell**. **2. Why Other Options are Incorrect:** * **Mesosome:** These are convoluted invaginations of the bacterial plasma membrane. They were historically thought to assist in cell division and respiration, though they are now largely considered artifacts of chemical fixation for electron microscopy. * **Ribosome:** These are the protein-synthesizing organelles of the cell (70S in bacteria). They consist of RNA and proteins, not integrated DNA. * **Transposon:** Also known as "jumping genes," these are small segments of DNA that can move from one location to another (e.g., from plasmid to chromosome). However, the term for the *entire* combined genetic unit is an episome. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Hfr Cells:** When an episome (F-factor) integrates, it can transfer chromosomal genes to a recipient cell during conjugation with high efficiency. * **R-Plasmids:** These carry genes for antibiotic resistance. If an R-plasmid acts as an episome, multi-drug resistance can be rapidly spread and stabilized within a bacterial population. * **Temperate Phages:** The DNA of lysogenic bacteriophages (like the Lambda phage) also acts as an episome when it integrates into the host genome as a **prophage**.

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).

Question 1

Transmission of R factor is by which mechanism?

Accepted Answer

Conjugation

Answer

Transduction

Answer

Transformation

Answer

Lysogenic conversion

Question 2

What is a gene cassette?

Accepted Answer

Circular, non-replicating DNA segments containing only open reading frames

Answer

A circular, autonomously replicating element

Answer

Integrated elements that can excise to form a non-replicating circular transfer intermediate

Answer

An integrated DNA segment that contains an integrase, a promoter, and an integration site

Question 3

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 4

Which of the following statements is true regarding kappa, lambda, and heavy chain immunoglobulins?

Accepted Answer

The chains are formed by genetic rearrangement after maturation.

Answer

Coded at the same site on a chromosome.

Answer

Coded at different sites on the same chromosome.

Answer

All of the above.

Question 5

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 6

Which statement is true regarding phage DNA?

Accepted Answer

Restriction enzyme sites

Answer

Antibiotic susceptibility

Answer

Hexagonal DNA

Answer

Carries short segments of host DNA

Question 7

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 8

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 9

What is formed when plasmid DNA combines with chromosomal DNA?

Accepted Answer

Episome

Answer

Mesosome

Answer

Ribosome

Answer

Transposon

Question 10

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

Accepted Answer

Transposons

Answer

Cosmid

Answer

Plasmid

Answer

Episomes

Microbial Genetics — MCQs

Microbial Genetics — MCQs

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