Whole Genome Sequencing — MCQs
Which of the following statements is NOT applicable to bacterial genomes?
What is the best investigation for identifying malaria species?
Disease not under integrated disease surveillance project is?
Pulsed gel electrophoresis is used for:
Which of the following diseases is classified under category-B of bioterrorism?
The molecular basis of penicillin resistance in *S. pneumoniae* is:
Best method for the detection of mutations with low allele frequency is:
Restriction fragment length polymorphism is used for:
An outbreak of staphylococcal infection involving umbilical cords of seven newborn babies was reported in the nursery. Bacteriological survey reveals that two nurses have a large number of Staphylococcus aureus in the nasopharynx. What test should be performed to determine whether these nurses may have been responsible for the outbreak?
What does Polymerase Chain Reaction (PCR) detect?
Whole Genome Sequencing Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following statements is NOT applicable to bacterial genomes?
- A. It is composed of DNA
- B. It does not contain histones
- C. It is circular
- D. Its DNA has both introns and exons (Correct Answer)
Explanation: ***Its DNA has both introns and exons*** - **Bacterial genomes** are typically organized as continuous coding sequences and **lack introns** (non-coding regions) that are characteristic of eukaryotic genes. - The presence of introns and their subsequent splicing is a hallmark of **eukaryotic gene expression**, not prokaryotic. *It is composed of DNA* - The genetic material of bacteria, like all cellular life forms, is primarily composed of **DNA (deoxyribonucleic acid)**. - DNA serves as the blueprint for all cellular processes and hereditary information. *It does not contain histones* - **Bacterial DNA** is typically compacted by various DNA-binding proteins, but these are not the **histone proteins** found in eukaryotes. - Histones are fundamental for packaging DNA into **chromatin** in eukaryotic cells. *It is circular* - The main chromosome in most bacteria is **covalently closed** and **circular**, unlike the linear chromosomes found in eukaryotes. - This circular structure aids in replication and stability within the bacterial cell.
Question 2: What is the best investigation for identifying malaria species?
- A. Thick smear
- B. Thin smear with Giemsa (Correct Answer)
- C. QBC
- D. Thin smear with acridine orange
Explanation: ***Thin smear with Giemsa*** - A **thin smear** allows for the visualization of **parasite morphology** within red blood cells, which is crucial for distinguishing between species of *Plasmodium*. - **Giemsa stain** provides optimal contrast for identifying characteristic features such as **merozoites**, **trophozoites**, **schizonts**, and **gametocytes** of different malaria species. *Thick smear* - A **thick smear** is primarily used for **detecting the presence of malaria parasites** and for quantifying parasite density due to its higher sensitivity. - However, because red blood cells are lysed, it **does not preserve parasite morphology** well, making species identification difficult. *QBC* - **Quantitative Buffy Coat (QBC) analysis** is a rapid method for detecting malaria parasites based on their fluorescence under UV light. - While sensitive for detection, it generally **does not allow for precise species identification** due to the lack of clear morphological detail. *Thin smear with acridine orange* - A **thin smear stained with acridine orange** is used for rapid detection of parasites by fluorescence microscopy. - Similar to QBC, it is **less effective for detailed morphological examination** and specific species identification compared to Giemsa-stained thin smears.
Question 3: Disease not under integrated disease surveillance project is?
- A. Herpes Zoster (Correct Answer)
- B. Meningoencephalitis
- C. TB
- D. Cholera
Explanation: ***Herpes Zoster*** - **Herpes Zoster**, also known as shingles, is a viral disease that is typically not included in the list of diseases under routine surveillance by the Integrated Disease Surveillance Project (IDSP) in many regions. - The IDSP primarily focuses on diseases with **epidemic potential** or high public health impact for early detection and rapid response. *Meningoencephalitis* - **Meningoencephalitis** (inflammation of the brain and meninges) is a serious condition with epidemic potential, making it a key disease for surveillance under projects like IDSP. - Early detection of clusters can help prevent widespread outbreaks and manage severe neurological outcomes. *TB* - **Tuberculosis (TB)** is a major public health concern due to its high prevalence, chronic nature, and potential for transmission, especially drug-resistant forms. - It is consistently included in surveillance programs like IDSP for consistent monitoring, case finding, and treatment adherence. *Cholera* - **Cholera** is an acute diarrheal disease with high epidemic potential due to rapid transmission, particularly in areas with poor sanitation. - It is a critical disease for surveillance to enable quick identification of outbreaks, implementation of control measures, and prevention of mass fatalities.
Question 4: Pulsed gel electrophoresis is used for:
- A. RNA
- B. Protein
- C. Ribosome
- D. DNA (Correct Answer)
Explanation: ***DNA*** - **Pulsed-field gel electrophoresis (PFGE)** is a specialized technique used to separate **large DNA molecules** that are difficult to resolve using conventional gel electrophoresis. - It involves periodically changing the direction of the electric field, which forces large DNA molecules to reorient themselves, allowing for better separation based on size up to **10 Mb**. *RNA* - While conventional gel electrophoresis (e.g., agarose or polyacrylamide gels) can be used to separate **RNA molecules**, PFGE is not typically employed for RNA. - RNA molecules are generally much smaller than the large DNA fragments for which PFGE is designed, and their secondary structures can interfere with pulsed-field separation. *Protein* - **Proteins** are separated using different types of electrophoresis, such as **SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis)**, which separates proteins primarily by size. - PFGE is specifically designed for nucleic acids, particularly very large DNA, and is not suitable for protein separation. *Ribosome* - **Ribosomes** are large macromolecular complexes composed of ribosomal RNA and proteins. - Techniques like **sucrose gradient centrifugation** or specialized gel electrophoresis (e.g., non-denaturing agarose gels for intact ribosomes) are used to separate ribosomes, not PFGE.
Question 5: Which of the following diseases is classified under category-B of bioterrorism?
- A. Anthrax
- B. Plague
- C. Botulism
- D. Cholera (Correct Answer)
Explanation: ***Cholera*** - **Cholera** is classified under **Category B** agents due to its moderate ease of dissemination, moderate morbidity rates, and low mortality rates. - While it can cause severe diarrheal disease, its treatment is relatively straightforward with **rehydration therapy**, and it poses a lower risk of mass casualties compared to Category A agents. *Anthrax* - **Anthrax** is a **Category A** bioterrorism agent, characterized by its high mortality rate, ease of dissemination, and potential for major public health impact. - It poses a significant threat due to its ability to form **spores** that are highly resistant and can cause severe lung infection. *Plague* - **Plague** is designated as a **Category A** agent because of its high potential for mass dissemination, high mortality if untreated, and potential to cause widespread panic. - It can be spread via **aerosols** and can lead to severe systemic illness. *Botulism* - **Botulism** is classified as a **Category A** agent due to the extreme potency of the **botulinum toxin**, even in minute quantities, which can cause severe flaccid paralysis and death. - It has a high potential for causing severe public health impact and requires complex medical interventions.
Question 6: The molecular basis of penicillin resistance in *S. pneumoniae* is:
- A. Alteration in cell membranes
- B. Alteration in the cell wall
- C. Alteration of penicillin-binding protein (Correct Answer)
- D. Production of beta-lactamase enzyme
Explanation: ***Alteration of penicillin-binding protein*** - The primary mechanism of **penicillin resistance in *Streptococcus pneumoniae*** involves modifications to its **penicillin-binding proteins (PBPs)**, which are the targets of penicillin. - These alterations reduce the affinity of PBPs for penicillin, allowing cell wall synthesis to continue even in the presence of the antibiotic. *Alteration in cell membranes* - Changes in cell membranes are not the primary mechanism of **penicillin resistance** in bacteria. - **Penicillins** primarily target the **bacterial cell wall**, not the cell membrane. *Alteration in the cell wall* - While penicillin resistance *involves* the cell wall, the direct "alteration in the cell wall" itself is not the molecular basis. - The key is the change in the **PBPs** located within or associated with the cell wall, not a general change in the cell wall structure. *Production of beta-lactamase enzyme* - **Beta-lactamase production** is a common mechanism of penicillin resistance in many bacteria (e.g., *Staphylococcus aureus*, *Haemophilus influenzae*), but it is **not the primary mechanism for penicillin resistance in *Streptococcus pneumoniae***. - *S. pneumoniae* primarily relies on **altered PBPs** to evade penicillin, rather than enzymatic degradation of the antibiotic.
Question 7: Best method for the detection of mutations with low allele frequency is:
- A. FISH
- B. Droplet digital PCR (Correct Answer)
- C. Sanger sequencing
- D. Nested PCR
Explanation: ***Droplet digital PCR*** - **Droplet digital PCR (ddPCR)** offers superior sensitivity for detecting **low allele frequency mutations** by partitioning the sample into thousands of individual reactions. - This compartmentalization allows for the direct quantification of target DNA molecules without relying on a standard curve, making it highly accurate for rare mutation detection. *FISH* - **Fluorescence in situ hybridization (FISH)** primarily detects **chromosomal abnormalities** like translocations, deletions, or amplifications, rather than single-nucleotide variants or small indels with low allele frequencies [2]. - It visualizes genetic changes at a **cytogenetic level** on an intracellular basis, not typically for quantifying rare DNA mutations in a heterogeneous sample. *Sanger sequencing* - **Sanger sequencing** is the gold standard for **sequencing individual DNA fragments** but has a detection limit of around 15-20% for allele frequency, making it unsuitable for very low allele frequency mutations [1]. - It struggles to reliably detect minor alleles when they are present in a small proportion of the total DNA pool. *Nested PCR* - **Nested PCR** increases the sensitivity and specificity of amplification by using two sets of primers in a sequential manner but does not inherently provide the **quantification capability** or the same level of **low allele frequency detection** as ddPCR processes. - While sensitive for detecting target sequences, it is not designed for precise quantification of rare mutations in a background of wild-type sequences. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, p. 185. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 185-186.
Question 8: Restriction fragment length polymorphism is used for:
- A. Detection of gene mutations
- B. Genetic mapping and identification (Correct Answer)
- C. Paternity testing
- D. Forensic analysis
Explanation: ***Genetic mapping and identification*** - **Restriction fragment length polymorphism (RFLP)** exploits variations in DNA sequences that create or abolish **restriction enzyme recognition sites**, leading to fragments of different lengths. - These polymorphic fragments serve as **genetic markers** to map genes on chromosomes and identify specific genes or genetic regions. *Detection of gene mutations* - While RFLP can detect some mutations by altering restriction sites, it is not the primary or most efficient method for general **gene mutation detection**. - Techniques like **DNA sequencing** or **PCR-based assays** are typically more sensitive and comprehensive for direct mutation analysis. *Paternity testing* - RFLP was historically used for **paternity testing** by comparing inheritance patterns of polymorphic markers between child and alleged father. - However, it has largely been replaced by more advanced and faster methods like **short tandem repeat (STR) analysis** due to higher discriminatory power and lower DNA requirements. *Forensic analysis* - Similar to paternity testing, RFLP was an early technique employed in **forensic analysis** for DNA fingerprinting to identify individuals. - Modern forensic DNA analysis predominantly uses **STR profiling**, which offers greater resolution, speed, and requires smaller, less degraded samples.
Question 9: An outbreak of staphylococcal infection involving umbilical cords of seven newborn babies was reported in the nursery. Bacteriological survey reveals that two nurses have a large number of Staphylococcus aureus in the nasopharynx. What test should be performed to determine whether these nurses may have been responsible for the outbreak?
- A. Coagulase testing
- B. Nasopharyngeal culture on mannitol salt agar
- C. Bacteriophage typing (Correct Answer)
- D. Protein A typing
Explanation: ***Bacteriophage typing*** - **Bacteriophage typing** involves using specific **bacteriophages** to identify different strains within a bacterial species based on their susceptibility to lysis by these phages. - This method helps determine if the specific strain of **Staphylococcus aureus** found in the nurses' nasopharynx matches the strain causing the outbreak in the newborns' umbilical cords, thereby establishing an epidemiological link. - This is the **classical method** for *S. aureus* strain typing in outbreak investigations. Modern molecular methods like PFGE, MLST, and whole genome sequencing have largely replaced bacteriophage typing, but it remains a fundamental concept tested in medical examinations. *Coagulase testing* - **Coagulase testing** differentiates **Staphylococcus aureus** (coagulase-positive) from other coagulase-negative staphylococci. - While it identifies the species, it does not provide the **strain-level differentiation** needed to link a specific individual to an outbreak. *Nasopharyngeal culture on mannitol salt agar* - **Mannitol salt agar** is a selective and differential medium used to isolate and identify **Staphylococcus aureus** from mixed cultures due to its ability to ferment mannitol and tolerate high salt concentrations. - This test would confirm the presence of **Staphylococcus aureus** in the nasopharynx but would not provide the detailed **strain-specific information** required to trace the source of the outbreak. *Protein A typing* - **Protein A** is a common cell wall component of **Staphylococcus aureus** that binds to the Fc region of immunoglobulins. - While its presence is characteristic of **Staphylococcus aureus**, **Protein A typing** does not offer the necessary **strain-specific resolution** to epidemiologically link an individual carrier to a specific outbreak strain.
Question 10: What does Polymerase Chain Reaction (PCR) detect?
- A. Antigen
- B. Antibody
- C. Nucleic acid (Correct Answer)
- D. All of the above
Explanation: **Explanation:** **Why Nucleic Acid is the Correct Answer:** Polymerase Chain Reaction (PCR) is a molecular technique used to **amplify specific sequences of DNA**. It utilizes a heat-stable DNA polymerase (like *Taq* polymerase) to create millions of copies of a target genetic sequence. In microbiology, PCR is used to detect the **nucleic acid** (DNA or RNA) of a pathogen. For RNA viruses (like HIV or SARS-CoV-2), a variation called Reverse Transcription-PCR (RT-PCR) is used to first convert RNA into complementary DNA (cDNA) before amplification. **Why Other Options are Incorrect:** * **Antigens (Option A):** These are proteins or polysaccharides on the surface of a pathogen. They are detected using immunological assays like **ELISA** (Enzyme-Linked Immunosorbent Assay) or Lateral Flow Assays (Rapid Antigen Tests), not PCR. * **Antibodies (Option B):** These are host proteins produced by B-cells in response to an infection. They are detected via **Serology** (e.g., ELISA, Western Blot, or Agglutination tests) to identify past or current exposure, whereas PCR identifies the presence of the organism itself. **High-Yield Clinical Pearls for NEET-PG:** * **Steps of PCR:** Denaturation (94-96°C) → Annealing (50-65°C) → Extension (72°C). * **Real-Time PCR (qPCR):** Allows for **quantification** of the microbial load (e.g., Viral Load in Hepatitis C or HIV). * **Multiplex PCR:** Can detect multiple different pathogens in a single clinical sample simultaneously using different primers. * **Sensitivity:** PCR is highly sensitive, making it the "Gold Standard" for diagnosing organisms that are difficult to culture (e.g., *M. tuberculosis*, *Chlamydia*, or viral infections).
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