Molecular Biology Techniques in Microbiology — MCQs
On this page
What does Polymerase Chain Reaction (PCR) detect?
Acridine orange is a fluorescent dye used to bind which cellular components?
What is the best technique for initial viral load estimation?
A microbiologist is working with PCR and wishes to quantify the PCR by monitoring the amplification process while the PCR is ongoing. Which of the following types of PCR should be used?
Which of the following statements are true regarding Polymerase Chain Reaction (PCR)?
When compared to Western blot, the ELISA test is:
What is the best investigation for BCR-ABL?
Which of the following statements is not true for FISH technique?
What is the full form of CBNAAT?
Chromosomal mutations can be identified by all of the following methods except:
Molecular Biology Techniques in Microbiology Indian Medical PG Practice Questions and MCQs
Question 1: 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).
Question 2: Acridine orange is a fluorescent dye used to bind which cellular components?
- A. DNA and RNA (Correct Answer)
- B. Proteins
- C. Lipids
- D. Carbohydrates
Explanation: **Explanation:** **Acridine orange** is a fluorochrome dye that functions as a nucleic acid-selective stain. It has the unique property of **metachromasia**, meaning it can differentiate between double-stranded and single-stranded nucleic acids based on the wavelength of light emitted. 1. **Why A is Correct:** Acridine orange intercalates into **DNA** (double-stranded) and binds electrostatically to **RNA** (single-stranded). When excited by blue light (460 nm) under a fluorescence microscope, DNA-bound dye emits **green fluorescence**, while RNA-bound dye emits **orange-red fluorescence**. This makes it highly effective for detecting microorganisms in clinical specimens (like blood cultures or CSF) where bacteria/fungi appear bright against a dark background. 2. **Why Other Options are Incorrect:** * **B (Proteins):** Proteins are typically stained with dyes like Coomassie Brilliant Blue or Silver stain. * **C (Lipids):** Lipids are visualized using lipophilic stains such as Sudan Black or Oil Red O. * **D (Carbohydrates):** Carbohydrates (glycogen/mucin) are identified using the Periodic Acid-Schiff (PAS) stain. **High-Yield Clinical Pearls for NEET-PG:** * **Sensitivity:** Acridine orange is more sensitive than the Gram stain for detecting low concentrations of bacteria (e.g., in buffy coat smears or early positive blood cultures). * **Rapid Screening:** It is used for rapid screening of malaria parasites (QBC technique) and *Trichomonas vaginalis*. * **Cell Viability:** It can distinguish between live (green) and dead (red/orange) cells in certain laboratory assays.
Question 3: What is the best technique for initial viral load estimation?
- A. Real-time PCR (Correct Answer)
- B. Widal test
- C. Electrophoresis
- D. Immunofluorescence
Explanation: **Explanation:** **Real-time PCR (qPCR)** is the gold standard for viral load estimation because it allows for the **quantification** of nucleic acids in real-time during the amplification process. Unlike traditional PCR, which only provides a qualitative (yes/no) result at the end of the cycle, qPCR uses fluorescent dyes or probes (like TaqMan) to measure the amount of DNA/RNA as it accumulates. The "Cycle Threshold" (Ct) value obtained is inversely proportional to the viral load, making it the most sensitive and accurate method for monitoring diseases like HIV, Hepatitis B, and Hepatitis C. **Why other options are incorrect:** * **Widal Test:** This is a serological agglutination test used specifically for diagnosing Enteric (Typhoid) fever by detecting antibodies against *Salmonella typhi*. It cannot quantify viral particles. * **Electrophoresis:** This technique separates macromolecules (DNA, RNA, or proteins) based on size and charge. While it can visualize genetic material, it is not a primary tool for precise quantification of viral load in a clinical sample. * **Immunofluorescence (IF):** This technique uses fluorescently labeled antibodies to detect specific viral antigens in cells or tissues. While useful for rapid diagnosis (e.g., Rabies or RSV), it is qualitative or semi-quantitative and lacks the precision required for viral load estimation. **High-Yield Clinical Pearls for NEET-PG:** * **Reverse Transcriptase PCR (RT-PCR):** Used for RNA viruses (e.g., HIV, SARS-CoV-2) to convert RNA to cDNA before amplification. * **Ct Value:** A lower Ct value indicates a **higher** viral load. * **Viral Load Monitoring:** Essential for assessing the efficacy of Antiretroviral Therapy (ART) in HIV patients; a "detectable" viral load often indicates treatment failure or drug resistance.
Question 4: A microbiologist is working with PCR and wishes to quantify the PCR by monitoring the amplification process while the PCR is ongoing. Which of the following types of PCR should be used?
- A. Standard PCR
- B. Reverse Transcriptase PCR
- C. Real-Time PCR (qPCR) (Correct Answer)
- D. Multiplex PCR
Explanation: ### Explanation **Correct Answer: C. Real-Time PCR (qPCR)** **Why it is correct:** The hallmark of **Real-Time PCR (Quantitative PCR or qPCR)** is the ability to monitor the amplification of a targeted DNA molecule during the PCR (i.e., in "real-time"), rather than at the end. This is achieved by using fluorescent dyes (like SYBR Green) or sequence-specific fluorescent probes (like TaqMan) that emit a signal proportional to the amount of accumulated DNA product. Because the signal is measured during the exponential phase, it allows for the precise **quantification** of the starting amount of the template DNA. **Why the other options are incorrect:** * **A. Standard PCR:** This is a qualitative technique where the results are analyzed only after the reaction is complete (end-point analysis), usually via gel electrophoresis. It cannot provide real-time monitoring or accurate quantification. * **B. Reverse Transcriptase PCR (RT-PCR):** This technique is used to amplify **RNA** targets by first converting them into complementary DNA (cDNA) using the enzyme reverse transcriptase. While it can be combined with Real-Time PCR (qRT-PCR), the term "RT-PCR" alone refers to the conversion process, not the monitoring method. * **C. Multiplex PCR:** This involves using multiple sets of primers in a single reaction tube to amplify several different target sequences simultaneously. It is used for detecting multiple pathogens at once but does not inherently involve real-time monitoring. **High-Yield Clinical Pearls for NEET-PG:** * **Cycle Threshold (Ct) Value:** In qPCR, the Ct value is the number of cycles required for the fluorescent signal to cross a fixed threshold. **Lower Ct = Higher Viral/Bacterial Load.** * **Gold Standard:** qPCR is the gold standard for measuring **Viral Load** (e.g., HIV, Hepatitis B/C) and was the primary diagnostic tool for **SARS-CoV-2**. * **Nested PCR:** Uses two sets of primers in two successive runs to increase **sensitivity and specificity**.
Question 5: Which of the following statements are true regarding Polymerase Chain Reaction (PCR)?
- A. Protein amplification
- B. DNA amplification
- C. Same as western blot test (Correct Answer)
- D. Detection of infecting organisms
Explanation: **Explanation:** Polymerase Chain Reaction (PCR) is a revolutionary molecular biology technique used to amplify specific sequences of **DNA**. By using thermal cycling, primers, and DNA polymerase (Taq polymerase), a single copy of a DNA fragment can be multiplied into millions of copies. **Analysis of Options:** * **Correct Answer (B/C):** *Note: There appears to be a discrepancy in the provided key.* **DNA amplification (Option B)** is the fundamental definition of PCR. However, if the question implies a comparison of diagnostic utility, PCR and **Western Blot** are both confirmatory tests (e.g., in HIV diagnosis, PCR detects the proviral DNA/RNA, while Western Blot detects specific proteins/antibodies). * **Option A (Protein amplification):** This is incorrect. PCR only amplifies nucleic acids (DNA/RNA). Proteins are detected via methods like ELISA or Western Blot, but they cannot be "amplified" in the same enzymatic fashion as DNA. * **Option D (Detection of infecting organisms):** While PCR is used for this purpose, it is a *clinical application* rather than the definition of the technique itself. **Clinical Pearls for NEET-PG:** * **Components:** Needs template DNA, Primers, dNTPs, and **Taq Polymerase** (thermostable, derived from *Thermus aquaticus*). * **Steps:** 1. Denaturation (94°C), 2. Annealing (55°C), 3. Extension (72°C). * **RT-PCR:** Reverse Transcriptase PCR is used for **RNA viruses** (like HIV, SARS-CoV-2) to convert RNA into cDNA before amplification. * **Real-Time PCR (qPCR):** Used for **quantification** of viral load (e.g., HBV, HCV). * **Nested PCR:** Increases sensitivity and specificity by using two sets of primers in two successive runs.
Question 6: When compared to Western blot, the ELISA test is:
- A. Less sensitive and less specific
- B. Less sensitive and less specific
- C. Less sensitive and more specific
- D. More sensitive and less specific (Correct Answer)
Explanation: ### Explanation In clinical microbiology and immunology, diagnostic tests are often categorized into **Screening tests** and **Confirmatory tests**. **1. Why the Correct Answer (D) is Right:** * **ELISA (Enzyme-Linked Immunosorbent Assay)** is primarily used as a **screening tool**. Screening tests are designed to have high **sensitivity** to ensure that no true positive cases are missed (minimizing false negatives). However, this high sensitivity often comes at the cost of **specificity**, leading to more false positives due to cross-reactivity. * **Western Blot** is a **confirmatory test**. It involves the separation of proteins by molecular weight via electrophoresis before antibody binding. This added step of protein separation makes it highly **specific**, as it identifies antibodies against multiple specific viral proteins (e.g., gp120, gp41, and p24 in HIV), reducing false positives. **2. Why Other Options are Wrong:** * **Options A & B:** ELISA is generally more sensitive than Western Blot because it can detect lower concentrations of antibodies in a bulk format without the protein loss associated with the blotting process. * **Option C:** This is the opposite of the clinical reality. If ELISA were more specific but less sensitive, it would be used for confirmation rather than screening. **3. NEET-PG High-Yield Pearls:** * **HIV Testing Protocol:** The standard algorithm is **ELISA first** (Screening: High Sensitivity) followed by **Western Blot** (Confirmation: High Specificity). * **Sensitivity vs. Specificity:** Remember the mnemonics **SNOUT** (SeNsitivity rules OUT disease) and **SPIN** (SPecificity rules IN disease). * **Window Period:** Both tests can be negative during the "window period." In such cases, **p24 antigen detection** or **RT-PCR** (viral load) are the investigations of choice. * **Modern Shift:** In many current guidelines, the 4th generation ELISA (detecting both p24 antigen and antibodies) has replaced Western Blot in some diagnostic algorithms due to its extreme accuracy.
Question 7: What is the best investigation for BCR-ABL?
- A. Flow cytometry
- B. Fluorescent in situ hybridization (Correct Answer)
- C. ELISA
- D. Polymerase chain reaction
Explanation: **Explanation:** The **BCR-ABL** fusion gene is the molecular hallmark of the **Philadelphia chromosome [t(9;22)]**, primarily associated with Chronic Myeloid Leukemia (CML). **Why FISH is the correct answer:** **Fluorescent in situ hybridization (FISH)** is considered a gold standard for the initial diagnosis and detection of the BCR-ABL translocation. It uses fluorescently labeled DNA probes that bind to specific loci on chromosomes 9 and 22. When the translocation occurs, the signals overlap (co-localization), allowing for the visualization of the fusion gene within the interphase or metaphase nucleus. It is highly sensitive, specific, and can detect the translocation even in non-dividing cells. **Analysis of Incorrect Options:** * **Flow Cytometry:** This technique is used for **immunophenotyping** (detecting cell surface markers like CD34, CD19). It identifies cell lineages but cannot detect specific genetic translocations like BCR-ABL. * **ELISA:** This is a serological technique used to detect **antigens or antibodies** (e.g., HIV, HBsAg). It is not used for genetic or chromosomal analysis. * **Polymerase Chain Reaction (PCR):** While RT-PCR is excellent for **monitoring** minimal residual disease (MRD) and quantifying the transcript levels (quantitative PCR), FISH is traditionally preferred for the definitive structural identification of the translocation in a diagnostic setting. **High-Yield Clinical Pearls:** * **Philadelphia Chromosome:** t(9;22)(q34;q11). * **CML Treatment:** Targeted therapy with Tyrosine Kinase Inhibitors (TKIs) like **Imatinib**. * **Monitoring:** Quantitative RT-PCR is the most sensitive method to monitor the molecular response to TKI therapy. * **Cytogenetics:** Karyotyping can also detect the Philadelphia chromosome but has lower sensitivity compared to FISH.
Question 8: Which of the following statements is not true for FISH technique?
- A. It is a very rapid technique.
- B. Interface FISH needs rapidly multiplying cells. (Correct Answer)
- C. Even microdeletions can be identified.
- D. Can detect circulating tumor cells.
Explanation: **Explanation:** Fluorescence In Situ Hybridization (FISH) is a molecular cytogenetic technique that uses fluorescent probes to bind to specific DNA sequences on chromosomes. **1. Why Option B is the Correct Answer (The False Statement):** Unlike traditional karyotyping, which requires cells to be in the metaphase stage (requiring active cell culture and division), **Interphase FISH** can be performed on non-dividing cells. It does **not** require rapidly multiplying cells or cell culture. This allows for a much faster turnaround time because the clinician does not have to wait days for cells to grow in a lab. **2. Analysis of Other Options:** * **Option A (Rapid technique):** FISH is significantly faster than conventional cytogenetics. Results can often be obtained within 24–48 hours because the cell culture step is bypassed. * **Option C (Microdeletions):** FISH has high resolution. While standard karyotyping can only detect large structural changes (>5 Mb), FISH can identify submicroscopic **microdeletions** (e.g., 22q11.2 deletion in DiGeorge syndrome) by the absence of a fluorescent signal. * **Option D (Circulating tumor cells):** FISH is highly sensitive and can be used to detect and characterize rare **circulating tumor cells (CTCs)** in peripheral blood, aiding in cancer prognosis and monitoring. **Clinical Pearls for NEET-PG:** * **Gold Standard:** FISH is often the gold standard for detecting specific gene rearrangements (e.g., *BCR-ABL* in CML or *HER2/neu* amplification in breast cancer). * **Limitation:** You must know what you are looking for; FISH requires a specific probe for a suspected target. It cannot scan the entire genome blindly like a microarray. * **Key Applications:** Rapid prenatal diagnosis of trisomies (13, 18, 21) and identifying microdeletion syndromes (Prader-Willi, Williams, DiGeorge).
Question 9: What is the full form of CBNAAT?
- A. Cartridge based nucleic acid amplification test (Correct Answer)
- B. Cell based nucleic acid amplification test
- C. Cartridge based New allele test
- D. Cartridge based Newer Amino Acid test
Explanation: **Explanation:** **CBNAAT** stands for **Cartridge Based Nucleic Acid Amplification Test**. It is a revolutionary molecular diagnostic technique used for the rapid detection of infectious diseases, most notably Tuberculosis (TB). **Why Option A is Correct:** CBNAAT (commercially known as **GeneXpert**) is an automated, hemi-nested real-time PCR (Polymerase Chain Reaction) system. It is "Cartridge Based" because all necessary reagents are contained within a single-use plastic cartridge, minimizing manual handling and contamination risk. It is a "Nucleic Acid Amplification Test" (NAAT) because it amplifies specific DNA sequences (the *rpoB* gene in *M. tuberculosis*) to provide high sensitivity and specificity. **Why Other Options are Incorrect:** * **Option B:** While NAAT is correct, "Cell-based" is inaccurate. The test targets genetic material (DNA/RNA) extracted from the sample, not the intact cell structure. * **Options C & D:** These are distractor terms. "New allele" and "Newer Amino Acid" are not standard terminology associated with this diagnostic platform. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Use:** In India, under the National TB Elimination Program (NTEP), CBNAAT is the preferred first-line diagnostic tool for suspected TB and Rifampicin resistance. * **Target Gene:** It detects the **rpoB gene**, which is the marker for **Rifampicin resistance**. * **Turnaround Time:** Results are typically available within **2 hours**. * **Sample Type:** Most commonly used on sputum, but can also be used for extrapulmonary samples (e.g., CSF, lymph node aspirates). * **Advantage:** It can detect TB in "smear-negative" cases due to its low limit of detection (~131 cfu/ml).
Question 10: Chromosomal mutations can be identified by all of the following methods except:
- A. Single nucleotide polymorphism
- B. Agarose gel electrophoresis (Correct Answer)
- C. Denaturing gradient gel electrophoresis
- D. Dideoxynucleotide chain termination sequencing
Explanation: **Explanation:** The core concept here is the **resolution power** of molecular techniques. Chromosomal mutations (specifically point mutations or small indels) involve changes at the single-base level, which require high-resolution detection methods. **Why Agarose Gel Electrophoresis (AGE) is the correct answer:** AGE is a technique used to separate DNA fragments based on **size and charge**. While it is excellent for visualizing large DNA fragments (e.g., PCR products or plasmids), it lacks the resolution to detect a single nucleotide change (mutation) within a fragment. Two DNA strands of the same length—one wild-type and one with a point mutation—will migrate to the exact same position on an agarose gel. Therefore, AGE cannot identify mutations unless they involve large insertions or deletions. **Analysis of other options:** * **Single Nucleotide Polymorphism (SNP) analysis:** Specifically designed to detect variations at a single base pair position using microarrays or specialized PCR. * **Denaturing Gradient Gel Electrophoresis (DGGE):** A powerful screening tool that separates DNA based on its **melting properties**. A single base mutation changes the stability (melting temperature) of the DNA duplex, causing it to stop at different positions in a denaturing gradient, thus identifying the mutation. * **Dideoxynucleotide chain termination sequencing (Sanger Sequencing):** The "Gold Standard" for mutation identification. It determines the exact linear sequence of nucleotides, allowing for the definitive identification of any chromosomal mutation. **Clinical Pearls for NEET-PG:** * **Pulse Field Gel Electrophoresis (PFGE):** Often confused with AGE; it is the "Gold Standard" for **molecular epidemiology** (outbreak typing) as it separates very large genomic fragments. * **Sanger Sequencing** uses **ddNTPs** (which lack a 3'-OH group) to terminate chain elongation. * To detect **structural** chromosomal abnormalities (like translocations), **FISH** or **Karyotyping** are preferred over basic electrophoresis.
Practice by Chapter
Nucleic Acid Extraction Methods
Practice Questions
Polymerase Chain Reaction
Practice Questions
Real-Time PCR
Practice Questions
DNA Sequencing Techniques
Practice Questions
Next-Generation Sequencing in Microbiology
Practice Questions
Pulsed-Field Gel Electrophoresis
Practice Questions
Whole Genome Sequencing
Practice Questions
MALDI-TOF Mass Spectrometry
Practice Questions
Microarrays in Microbial Diagnostics
Practice Questions
Molecular Typing Methods
Practice Questions
Bioinformatics Tools in Microbiology
Practice Questions
Metagenomics
Practice Questions
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free