PCR Technology — MCQs
Steps of PCR in sequence are?
Which of the following is a primarily RNA based technique?
Which one of the following enzymes is obtained from Thermus aquaticus bacterium that is heat stable and used in PCR at high temperature?
DNA amplification is done by all, except:
The Takayama test is primarily used for what purpose?
What is a confirmatory test for species of origin?
Which of the following is the best method for paternity testing?
Which test is performed to determine if a biological stain is of human origin?
What is the purpose of using a dried blood stain's crust in forensic analysis?
What is the most informative test for parental identification?
PCR Technology Indian Medical PG Practice Questions and MCQs
Question 1: Steps of PCR in sequence are?
- A. Denature DNA, Extend DNA, Anneal Primers
- B. Anneal Primers, Extend DNA, Denature DNA
- C. Extend DNA, Anneal Primers, Denature DNA
- D. Denature DNA, Anneal Primers, Extend DNA (Correct Answer)
Explanation: ***Denature DNA, Anneal Primers, Extend DNA*** - This sequence represents the three fundamental steps of each PCR cycle, ensuring accurate and efficient **DNA amplification**. - **Denaturation** separates the double-stranded DNA template, **annealing** allows primers to bind to specific sequences, and **extension** synthesizes new DNA strands. *Denature DNA, Extend DNA, Anneal Primers* - This order is incorrect because **primer annealing** must occur before DNA extension can begin. - Primers provide the necessary starting points for the **DNA polymerase** to synthesize the new strands. *Anneal Primers, Extend DNA, Denature DNA* - This sequence is incorrect as the **template DNA** must first be denatured to separate the strands before primers can anneal to them. - If the DNA is not denatured, the primers cannot access their target sequences. *Extend DNA, Anneal Primers, Denature DNA* - This order is incorrect because **DNA extension** is the final step, occurring only after denaturation and primer annealing. - The polymerase requires both a denatured template and bound primers to initiate synthesis.
Question 2: Which of the following is a primarily RNA based technique?
- A. Western blotting
- B. Northern blotting (Correct Answer)
- C. Southern blotting
- D. Sanger's technique
Explanation: ***Northern blotting*** - **Northern blotting** is a molecular biology technique used to study **gene expression** by detecting specific **RNA molecules** (mRNA) in a sample. - It involves separating RNA fragments by **gel electrophoresis**, transferring them to a membrane, and then detecting specific sequences using **labeled probes**. *Western blotting* - **Western blotting** is a technique used to detect specific **proteins** in a sample. - It involves separating proteins by **gel electrophoresis**, transferring them to a membrane, and then detecting specific proteins using labeled **antibodies**. *Southern blotting* - **Southern blotting** is a molecular biology method used for the detection of **specific DNA sequences** in DNA samples. - It involves separating **DNA fragments** by **gel electrophoresis**, transferring them to a membrane, and then hybridizing with a labeled probe. *Sanger's technique* - **Sanger sequencing**, or the **dideoxy chain-termination method**, is a widely used method for **DNA sequencing**. - It uses **dideoxynucleotides** to terminate DNA synthesis at specific bases, allowing the determination of the **DNA sequence**.
Question 3: Which one of the following enzymes is obtained from Thermus aquaticus bacterium that is heat stable and used in PCR at high temperature?
- A. DNA gyrase
- B. DNA polymerase III
- C. Taq polymerase (Correct Answer)
- D. Endonuclease
Explanation: ***Taq polymerase*** - This **heat-stable DNA polymerase** is isolated from the thermophilic bacterium *Thermus aquaticus*. - Its ability to withstand high temperatures makes it ideal for the **polymerase chain reaction (PCR)**, where DNA denaturation steps occur at elevated temperatures. *DNA gyrase* - **DNA gyrase** is a type II topoisomerase that introduces negative supercoils into DNA, which is important for DNA replication and transcription. - It is not heat-stable and is not directly used for DNA amplification in PCR. *DNA polymerase III* - **DNA polymerase III** is the primary enzyme responsible for DNA replication in *E. coli* and other bacteria. - It rapidly synthesizes DNA but is **not heat-stable** and would denature at the temperatures required for PCR. *Endonuclease* - **Endonucleases** are enzymes that cleave phosphodiester bonds within a polynucleotide chain. - While essential for processes like DNA repair and restriction mapping, they are not primarily involved in and are not heat-stable for DNA synthesis in PCR.
Question 4: DNA amplification is done by all, except:
- A. DNA sequencing (Correct Answer)
- B. Loop-mediated isothermal amplification (LAMP)
- C. Ligase chain reaction
- D. Polymerase chain reaction
Explanation: ***DNA sequencing*** - **DNA sequencing** determines the **nucleotide base order** in a DNA molecule but does not increase the amount of DNA. - While requiring a DNA template, it is an **analytical technique** rather than an amplification method. *Loop-mediated isothermal amplification (LAMP)* - **LAMP** is an **isothermal DNA amplification** technique that amplifies target DNA sequences at a constant temperature (60-65°C). - It uses a DNA polymerase with strand displacement activity and 4-6 primers to produce large amounts of DNA rapidly. *Ligase chain reaction* - **LCR** is an amplification method that detects specific **DNA sequences** by ligating adjacent probes. - It amplifies the signal from a target DNA sequence rather than the DNA itself by creating many copies of joined probes. *Polymerase chain reaction* - **PCR** is a widely used technique for **amplifying** a specific segment of DNA to produce many copies. - It involves cycles of **denaturation**, **annealing**, and **extension** using a DNA polymerase.
Question 5: The Takayama test is primarily used for what purpose?
- A. To determine the crystalline structure of a stain. (Correct Answer)
- B. To identify the species of origin of a stain.
- C. To perform blood grouping.
- D. None of the above.
Explanation: The **Takayama test** (also known as the **Haemochromogen crystal test**) is a microcrystalline test used for the confirmatory identification of blood. ### Why Option A is Correct The test involves treating a suspected bloodstain with Takayama reagent (containing glucose, sodium hydroxide, and pyridine). When heated, the pyridine reacts with the heme group of hemoglobin to form **pink, feathery, needle-shaped crystals** of pyridine haemochromogen. Therefore, the test's primary mechanism is the formation and observation of a specific **crystalline structure**, confirming the presence of hemoglobin. ### Why Other Options are Incorrect * **Option B:** Species of origin is determined using **serological tests** like the **Precipitin test** or Electrophoresis (e.g., Counter-immunoelectrophoresis), which rely on antigen-antibody reactions. * **Option C:** Blood grouping is typically performed using the **Absorption-Elution method** (Siracusa method) or the **Lattes Crust method** to identify ABO antigens or antibodies in dried stains. ### High-Yield Pearls for NEET-PG * **Confirmatory vs. Screening:** The Takayama test and Teichmann test are **confirmatory** for blood. Benzidine, Phenolphthalein (Kastle-Meyer), and Luminol are **presumptive/screening** tests. * **Takayama vs. Teichmann:** The Takayama test is generally preferred over the Teichmann (Haemin) test because it is more reliable, works better on old/weathered stains, and requires less heat. * **Reagent Components:** Remember the "Pyridine" in Takayama reagent; it is the key chemical that produces the characteristic pink crystals.
Question 6: What is a confirmatory test for species of origin?
- A. Origin Test
- B. Ouchterlony Test (Correct Answer)
- C. Olympus Test
- D. SOO Test
Explanation: ### Explanation The **Ouchterlony Test** (Double Immunodiffusion) is the gold standard confirmatory test for determining the **species of origin** in forensic biology. **1. Why Ouchterlony Test is Correct:** When a biological sample (like a bloodstain) is found at a crime scene, forensic experts must determine if it is human or animal. The Ouchterlony test uses the principle of **antigen-antibody reaction** in an agar gel. An extract of the unknown sample (antigen) and specific anti-sera (antibodies, e.g., anti-human serum) are placed in separate wells. If the sample belongs to that species, the antigens and antibodies diffuse toward each other, meeting to form a visible **precipitin line**. This confirms the species identity with high specificity. **2. Analysis of Incorrect Options:** * **A. Origin Test:** This is a generic term and not a specific forensic laboratory technique. * **C. Olympus Test:** This is a distractor; Olympus is a well-known brand of microscopes and optical equipment, not a serological test. * **D. SOO Test:** This is an invented acronym (likely standing for "Species of Origin") used to confuse candidates; no such standardized test exists in forensic practice. **3. High-Yield Clinical Pearls for NEET-PG:** * **Precipitin Test:** The broader category of tests for species origin; Ouchterlony is the most common variant. * **Kastle-Meyer Test:** A common screening (presumptive) test for the *presence* of blood (turns pink), but it does not determine species. * **Teichmann and Takayama Tests:** Confirmatory tests for the *presence* of blood (crystal tests), not species. * **Species Identification:** Essential in hit-and-run cases or animal poaching investigations to differentiate human blood from animal blood.
Question 7: Which of the following is the best method for paternity testing?
- A. Karyotyping
- B. Microsatellite analysis (Correct Answer)
- C. Northern blot analysis
- D. All of the above
Explanation: **Explanation:** **Microsatellite analysis**, also known as **Short Tandem Repeat (STR) analysis**, is the gold standard for paternity testing. STRs are small sequences of DNA (2–6 base pairs) that repeat multiple times at specific loci. Because the number of repeats is highly polymorphic (variable) among individuals and inherited in a Mendelian fashion (50% from each parent), comparing these loci provides a "DNA fingerprint" with a probability of paternity often exceeding 99.99%. **Analysis of Incorrect Options:** * **Karyotyping (A):** This involves visualizing the entire set of chromosomes to detect numerical or structural abnormalities (e.g., Trisomy 21). It lacks the resolution to distinguish between individuals for parentage, as all healthy humans have the same basic karyotype (46,XX or 46,XY). * **Northern blot analysis (C):** This technique is used to study **RNA** expression levels, not DNA. Since paternity is based on inherited genomic DNA, RNA analysis is irrelevant for this purpose. * **All of the above (D):** Incorrect, as only STR analysis provides the necessary genetic resolution for individual identification. **High-Yield Facts for NEET-PG:** * **CODIS (Combined DNA Index System):** The standard database uses 13–20 core STR loci for forensic identification. * **Mitochondrial DNA (mtDNA):** Useful for tracing **maternal** lineage only (all children of one mother have identical mtDNA). * **Y-STR Analysis:** Useful for tracing **paternal** lineage in male offspring. * **RFLP (Restriction Fragment Length Polymorphism):** The older method of DNA profiling; it is accurate but requires large samples of undegraded DNA, making STR (which uses PCR) the modern preference.
Question 8: Which test is performed to determine if a biological stain is of human origin?
- A. Florence test
- B. Takayama test
- C. Precipitant test (Correct Answer)
- D. Barberio's test
Explanation: **Explanation:** In forensic investigations, the examination of a biological stain (like blood or semen) follows a three-step hierarchy: **Preliminary/Presumptive tests** (is it blood?), **Confirmatory tests** (is it definitely blood?), and **Species-origin tests** (is it human?). **1. Why the Precipitin Test is Correct:** The **Precipitin test** (also known as the Uhlenhuth test) is the standard method used to determine the species of origin. It is an antigen-antibody reaction. When a sample containing human proteins is reacted against "anti-human serum" (produced in rabbits), a visible precipitate forms if the sample is of human origin. Modern variations include the **Crossover Electrophoresis** technique. **2. Analysis of Incorrect Options:** * **Florence Test (Option A):** This is a preliminary chemical test for **semen**. It detects the presence of choline. It is not species-specific and can give false positives with other biological fluids. * **Takayama Test (Option B):** Also known as the Hemochromogen crystal test. It is a **confirmatory test for blood**. It produces salmon-pink, rhomboid crystals but does *not* distinguish between human and animal blood. * **Barberio’s Test (Option D):** This is a preliminary chemical test for **semen** that detects spermine. It produces yellow, needle-shaped crystals of spermine picrate. **3. High-Yield Clinical Pearls for NEET-PG:** * **Teichmann Test:** Another confirmatory test for blood (Haemin crystal test); produces dark brown, rhombic crystals. * **Kastle-Meyer Test:** The most common preliminary/screening test for blood (uses Phenolphthalein); gives a pink color. * **Acid Phosphatase Test:** The best screening test for semen. * **Species Origin:** If the Precipitin test is negative for humans, forensic labs use specific antisera for other animals (e.g., anti-bovine, anti-canine) to identify the source.
Question 9: What is the purpose of using a dried blood stain's crust in forensic analysis?
- A. Detection of the species of origin
- B. Determination of secretor status
- C. Characterization of the nature of the stain
- D. Determination of the blood group (Correct Answer)
Explanation: **Explanation:** The correct answer is **D. Determination of the blood group.** In forensic serology, a dried blood crust is particularly valuable for determining the ABO blood group. This is achieved using the **Absorption-Elution technique** (Siracusa’s method) or the **Absorption-Inhibition technique**. These methods rely on the fact that A and B antigens on the surface of red blood cell membranes are remarkably stable and can remain detectable in dried stains for long periods, even when the cells themselves have lysed. **Analysis of Options:** * **A. Detection of the species of origin:** This is typically determined using the **Precipitin test** or the **Coombs’ antihuman globulin consumption test**. While a crust can be used, the primary diagnostic utility of a concentrated crust in forensic protocols is specifically for grouping. * **B. Determination of secretor status:** Secretor status refers to the presence of blood group antigens in body fluids like saliva, semen, or sweat. It is determined by analyzing these fluids, not by analyzing a blood crust itself. * **C. Characterization of the nature of the stain:** This refers to confirming if a stain is actually blood. This is done via **presumptive tests** (e.g., Kastle-Meyer) or **confirmatory tests** (e.g., Teichmann or Takayama crystal tests), which require only a small scrap or extract, not necessarily the "crust" specifically for grouping purposes. **High-Yield Facts for NEET-PG:** * **Absorption-Elution Test:** The most sensitive and common method for grouping dried bloodstains. * **Lattes Crust Method:** Used to detect **antibodies** (agglutinins) in a dried stain, whereas Absorption-Elution detects **antigens**. * **Species Identification:** The Precipitin test is based on the principle of antigen-antibody reaction (formation of a white ring). * **Stability:** Antigens in dried stains are more stable than the enzymes (like PGM) used in older electrophoretic methods.
Question 10: What is the most informative test for parental identification?
- A. Human Leukocyte Antigen (HLA)
- B. Parental likeness assessment
- C. Assessment of developmental defects
- D. DNA fingerprinting (Correct Answer)
Explanation: **Explanation:** **DNA Fingerprinting (DNA Profiling)** is the gold standard and most informative test for parental identification because it analyzes specific regions of DNA called **Short Tandem Repeats (STRs)** or Variable Number Tandem Repeats (VNTRs). Since an individual inherits exactly 50% of their nuclear DNA from each biological parent, comparing these genetic markers provides a statistical certainty of over 99.9% for inclusion and 100% for exclusion of paternity/maternity. **Analysis of Incorrect Options:** * **Human Leukocyte Antigen (HLA):** While HLA typing was used historically for paternity testing, it is significantly less specific than DNA profiling. It relies on protein markers on white blood cells, which have limited polymorphism compared to the vast variability found in non-coding DNA. * **Parental Likeness Assessment:** This is a subjective, phenotypic observation (e.g., facial features, eye color). It is scientifically unreliable and inadmissible as primary evidence in modern forensics due to the complexities of polygenic inheritance and environmental factors. * **Assessment of Developmental Defects:** Congenital anomalies or hereditary defects are not unique identifiers. While they may suggest a genetic link, they do not provide the definitive molecular proof required for legal parental identification. **High-Yield Facts for NEET-PG:** * **Alec Jeffreys (1984):** Developed the first DNA fingerprinting technique. * **Lalji Singh:** Known as the "Father of Indian DNA Fingerprinting." * **Specimen of Choice:** Blood (EDTA) is preferred, but any nucleated cell (semen, hair follicle, buccal swab) can be used. * **Mitochondrial DNA (mtDNA):** Used specifically for maternal lineage (matrilineal inheritance) as it is passed only from the mother to all her children. * **Y-STR Analysis:** Used to trace paternal lineage (patrilineal inheritance) in male offspring.
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