Microbial Genetics — MCQs

Microbial Genetics Indian Medical PG Practice Questions and MCQs

Question 51: Drug resistance in tuberculosis is primarily due to which genetic mechanism?

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

Explanation: **Explanation:** In *Mycobacterium tuberculosis* (MTB), drug resistance is almost exclusively mediated by **spontaneous chromosomal mutations**. Unlike many other bacteria, MTB does not possess plasmids or transposons, which are the typical vehicles for horizontal gene transfer. Resistance arises due to random mutations in specific genes that encode drug targets or activating enzymes (e.g., *katG* mutations for Isoniazid resistance and *rpoB* mutations for Rifampicin resistance). These mutants are then "selected" when a patient receives inadequate or monotherapy. **Analysis of Incorrect Options:** * **Transformation:** This involves the uptake of free "naked" DNA from the environment. While common in *S. pneumoniae* or *Neisseria*, it is not a mechanism for resistance in MTB. * **Transduction:** This is the transfer of genetic material via a bacteriophage. While mycobacteriophages exist, they do not play a role in clinical drug resistance. * **Conjugation:** This is the transfer of DNA (usually plasmids) through direct cell-to-cell contact (sex pili). Since MTB lacks plasmids, this mechanism is absent. **High-Yield Clinical Pearls for NEET-PG:** * **Rifampicin Resistance:** Acts as a surrogate marker for Multidrug-Resistant TB (MDR-TB). It is primarily due to mutations in the **rpoB gene** (beta subunit of RNA polymerase). * **Isoniazid (INH) Resistance:** Most commonly due to mutations in the **katG gene** (loss of catalase-peroxidase activity) or **inhA gene**. * **Multi-Drug Resistance (MDR):** Defined as resistance to at least Isoniazid and Rifampicin. * **Extensively Drug-Resistant (XDR):** MDR plus resistance to any fluoroquinolone and at least one second-line injectable drug (Amikacin, Kanamycin, or Capreomycin).

Question 52: Which of the following ultrasound findings has the highest association with aneuploidy?

A. Choroid plexus cyst
B. Nuchal translucency
C. Cystic hygroma (Correct Answer)
D. Single umbilical artery

Explanation: **Explanation:** **Cystic Hygroma** is the correct answer because it has the strongest association with fetal aneuploidy among the listed options. It is a congenital malformation of the lymphatic system, appearing as a fluid-filled sac (usually in the neck). Approximately **50-60%** of fetuses diagnosed with cystic hygroma in the first trimester have an underlying chromosomal abnormality, most commonly **Turner Syndrome (45,X)**, followed by Trisomy 21, 18, and 13. **Analysis of Options:** * **Nuchal Translucency (NT):** While an increased NT is a classic screening marker for Trisomy 21, its association with aneuploidy (approx. 20-30% if significantly thickened) is statistically lower than that of a true cystic hygroma. * **Choroid Plexus Cyst (CPC):** These are often transient and "soft markers." While associated with Trisomy 18, the majority of isolated CPCs occur in chromosomally normal fetuses. * **Single Umbilical Artery (SUA):** This is the most common umbilical cord anomaly. Although it increases the risk for renal and cardiac issues, its isolated association with aneuploidy is relatively low (approx. <10%). **High-Yield Clinical Pearls for NEET-PG:** * **Turner Syndrome:** Most common cause of cystic hygroma in the second trimester. * **Hydrops Fetalis:** Cystic hygroma is a frequent precursor to non-immune hydrops. * **Soft Markers vs. Structural Defects:** Cystic hygroma is considered a major structural defect, whereas NT and CPC are screening markers. * **Management:** If a cystic hygroma is detected, the next best step is definitive diagnostic testing via **CVS or Amniocentesis** for karyotyping/microarray.

Question 53: DNA transfer in bacteria by phage is:

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

Explanation: ***Transduction*** - **Transduction** is the process by which foreign DNA is introduced into a cell by a virus or viral vector, specifically a **bacteriophage** in bacteria. - During transduction, the phage infects a bacterial cell, and its genetic material becomes incorporated into the host cell's DNA, or it may package bacterial DNA into new phage particles, which then infect other bacteria. *Transformation* - **Transformation** is the process where bacteria take up **naked DNA** from their environment. - This process does not involve a bacteriophage or any other viral vector. *Conjugation* - **Conjugation** is a process of genetic transfer between bacterial cells involving direct cell-to-cell contact through a **pilus**. - It typically involves the transfer of plasmids or other genetic elements, not mediated by a bacteriophage. *Translation* - **Translation** is the process by which messenger RNA (mRNA) is decoded to produce a specific **amino acid sequence** (protein). - This is a fundamental step in gene expression and does not involve the transfer of DNA between bacteria via a phage.

Question 54: What is the mode of transmission of genetic material to bacteria through bacteriophage?

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

Explanation: ***Transduction*** - **Transduction** is the process by which foreign DNA is introduced into a bacterium by a **bacteriophage** (a virus that infects bacteria). - This occurs when a bacteriophage carries bacterial DNA from one bacterium to another, often as a result of errors during viral replication. *Transformation* - **Transformation** involves the uptake of **naked DNA** from the environment by a bacterial cell. - This process does not involve a bacteriophage as an intermediary for genetic transfer. *Conjugation* - **Conjugation** is the transfer of genetic material between bacterial cells by direct cell-to-cell contact through a **pilus**. - This process requires physical contact between two bacteria and does not involve bacteriophages. *Translation* - **Translation** is the process by which messenger RNA (mRNA) is decoded to produce a specific protein. - This is a fundamental step in gene expression and is distinctly different from genetic material transmission between organisms.

Question 55: Which statement concerning plasmids is true?

A. All plasmids possess the information for their own transfer by conjugation
B. R plasmids cannot be transferred to other bacterial cells
C. Much of the information coded in the plasmid is essential to the survival of the bacteria cell
D. R plasmids carry genes for antibiotic resistance (Correct Answer)

Explanation: ***R plasmids carry genes for antibiotic resistance*** - **R plasmids** are extrachromosomal DNA molecules found in bacteria that commonly encode **resistance genes** against various antibiotics. - This genetic information allows bacteria to survive in the presence of antibiotics, contributing to issues like multi-drug resistance. *All plasmids possess the information for their own transfer by conjugation* - Only **conjugative plasmids** contain the necessary genes (**tra genes**) to mediate their own transfer between bacteria via conjugation. - **Non-conjugative plasmids** require the presence of a conjugative plasmid in the same cell to be co-transferred. *R plasmids cannot be transferred to other bacterial cells* - Many **R plasmids are conjugative**, meaning they can be readily transferred between bacterial cells, even across different species, via conjugation. - This transfer mechanism is a significant factor in the rapid spread of **antibiotic resistance** in bacterial populations. *Much of the information coded in the plasmid is essential to the survival of the bacteria cell* - Plasmids typically carry genes that provide **selective advantages** under specific conditions (e.g., antibiotic resistance, virulence factors), but these are generally **not essential** for basic cell survival and growth in routine environments. - The essential genes for bacterial survival are located on the **bacterial chromosome**.

Question 56: False regarding bacterial plasmids is:

A. Extrachromosomal
B. Transmission to different species
C. Eliminated by treating with radiation
D. Can cause lysogenic conversion (Correct Answer)

Explanation: ***Can cause lysogenic conversion*** - **Lysogenic conversion** is a phenomenon caused by **bacteriophages**, which are viruses that infect bacteria, not by plasmids directly. - It involves the integration of a **phage genome** into the bacterial chromosome, altering the bacterium's phenotype (e.g., toxin production). *Extrachromosomal* - Bacterial plasmids are indeed **extrachromosomal DNA molecules**, meaning they exist independently of the bacterial chromosome. - This characteristic allows them to be easily transferred between bacteria. *Transmission of different species* - Plasmids can be transmitted horizontally between bacteria, even across **different species**, through mechanisms like **conjugation**, **transformation**, or **transduction**. - This interspecies transmission is a major factor in the spread of **antibiotic resistance**. *Eliminated by treating with radiation* - Plasmids, like all DNA, can be eliminated or degraded by treatments such as **radiation** (e.g., UV) or certain chemicals. - Such treatments disrupt the plasmid DNA structure, preventing its replication or function.

Question 57: Which of the following is the most clinically significant function of plasmids in antimicrobial resistance?

A. Involved in multidrug resistance transfer (Correct Answer)
B. Involved in conjugation
C. Imparts capsule formation
D. Imparts pili formation

Explanation: ***Involved in multidrug resistance transfer*** - **R-plasmids (resistance plasmids)** carry genes that provide bacteria with resistance to multiple antibiotics simultaneously. - These resistance genes can be transferred horizontally between bacteria through conjugation, transformation, or transduction, contributing to the rapid spread of **multidrug-resistant infections**. - This is the most **clinically significant** function of plasmids in medical microbiology. *Involved in conjugation* - While **F-plasmids (fertility plasmids)** encode the machinery for bacterial conjugation, this is a specific function of fertility plasmids rather than the primary clinical concern. - Conjugation is the mechanism by which plasmids (including R-plasmids) transfer between bacteria, but the question asks about the function most relevant to antimicrobial resistance. *Imparts capsule formation* - **Capsule formation** is typically encoded by genes on the **bacterial chromosome**, not plasmids. - Capsule genes are part of the core bacterial genome and represent a stable chromosomal trait. *Imparts pili formation* - While **sex pili** involved in conjugation can be plasmid-encoded (by F-plasmids), most **common pili** (fimbriae) involved in adhesion are chromosomally encoded. - This is not the primary clinically significant function of plasmids in the context of antimicrobial resistance.

Question 58: F factor integrates with bacterial chromosome to form -

A. F-
B. Hfr (Correct Answer)
C. F'
D. F

Explanation: ***Hfr*** - When the **F plasmid (fertility factor)** integrates into the bacterial chromosome, it forms a **high-frequency recombination (Hfr) strain**. - This integration allows for the transfer of chromosomal DNA during **conjugation**. *F-* - An **F- bacterium** is a recipient cell that **lacks the F plasmid** entirely. - It cannot initiate conjugation as a donor and receives genetic material from F+ or Hfr cells. *F'* - An **F' plasmid** is a variation of the F plasmid that has excised from the bacterial chromosome, carrying with it a **small segment of bacterial chromosomal DNA**. - It differs from simple F plasmids by containing host genes. *F* - The **F factor (fertility factor)** is a type of plasmid that carries genes for pilus formation and DNA transfer. - An F+ bacterium contains this plasmid as an **extrachromosomal element**, but it is not integrated into the chromosome.

Question 59: The mechanism of direct transfer of free DNA involves _____

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

Explanation: ***Transformation*** - **Transformation** is a process of horizontal gene transfer where bacteria take up **free DNA** from their environment. - This DNA can originate from dead bacterial cells and be integrated into the recipient bacterium's genome. *Mutation* - A **mutation** is a spontaneous change in the nucleotide sequence of an organism's genome. - It does not involve the transfer of DNA from one organism to another, but rather an alteration within an existing gene. *Conjugation* - **Conjugation** involves the direct transfer of genetic material between bacterial cells through physical contact via a **pilus**. - This mechanism typically transfers large pieces of DNA, often plasmids, not "free DNA" from the environment. *Transduction* - **Transduction** is the process by which DNA is transferred from one bacterium to another by a **bacteriophage** (a virus that infects bacteria). - This mechanism requires a viral vector to carry the genetic material, distinguishing it from the direct uptake of free DNA.

Question 60: Which of the following organisms is a natural transformer?

A. Escherichia coli
B. Plasmodium vivax
C. Neisseria gonorrhoeae (Correct Answer)
D. Pseudomonas aeruginosa

Explanation: ***Neisseria gonorrhoeae*** - *Neisseria gonorrhoeae* is known as a **natural transformer** because it can actively take up exogenous DNA from its environment. - This capability is significant for its **genetic variability** and antibiotic resistance development through horizontal gene transfer. *Escherichia coli* - *E. coli* is not naturally competent; it requires **artificial methods** such as heat shock or electroporation to take up foreign DNA in laboratories. - While widely used in genetic engineering, its transformation is not a **natural physiological process**. *Plasmodium vivax* - *Plasmodium vivax* is a **eukaryotic parasite** responsible for malaria, and it does not undergo bacterial transformation. - Its genetic exchange mechanisms are complex and involve **sexual reproduction** within the mosquito vector, not direct DNA uptake from the environment. *Pseudomonas aeruginosa* - *Pseudomonas aeruginosa* is a bacterium that can exhibit some level of natural competence, but it is not as highly recognized for it as *Neisseria* species. - Its ability to take up DNA is often **inducible** and less constitutive compared to primary natural transformers.

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

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Regulation of Gene Expression

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CRISPR-Cas Systems

Practice Questions

Bacterial Stress Responses

Practice Questions

Genetics of Antimicrobial Resistance

Practice Questions

Genetic Basis of Bacterial Virulence

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