50 MCQs on Polymerase Chain Reaction (PCR)
1. What is the primary purpose of Polymerase Chain Reaction (PCR)?
Correct Answer: B. To amplify a specific DNA sequence
Explanation: PCR is an in vitro technique used to exponentially amplify a specific region of DNA. It can generate millions to billions of copies of a target sequence from a very small starting amount of DNA.
2. Who developed the Polymerase Chain Reaction?
Correct Answer: C. Kary Mullis
Explanation: Kary B. Mullis developed PCR in 1983 while working at Cetus Corporation. He was awarded the 1993 Nobel Prize in Chemistry for his contributions to the development of PCR.
3. Which enzyme is primarily used in conventional PCR?
Correct Answer: C. Taq DNA polymerase
Explanation: Taq DNA polymerase is a thermostable DNA polymerase isolated from Thermus aquaticus. Its heat stability allows it to remain active through repeated high-temperature denaturation steps.
4. Why is Taq DNA polymerase particularly suitable for PCR?
Correct Answer: B. It is resistant to high temperatures
Explanation: PCR involves repeated heating to approximately 94–98°C. Taq polymerase is thermostable and therefore survives these high-temperature cycles.
5. Which of the following is NOT an essential component of a conventional PCR reaction?
Correct Answer: D. Ribosomes
Explanation: PCR requires template DNA, primers, thermostable DNA polymerase, dNTPs, Mg²⁺, buffer, and water. Ribosomes are required for protein synthesis, not DNA amplification.
6. What is the role of primers in PCR?
Correct Answer: B. They provide a starting point for DNA synthesis
Explanation: DNA polymerase cannot initiate synthesis de novo. Primers provide a free 3′-OH group from which the polymerase can extend the new DNA strand.
7. How many primers are normally required for conventional PCR?
Correct Answer: B. Two
Explanation: Two primers are generally used: a forward primer and a reverse primer. They bind to opposite strands and flank the target DNA region.
8. Which type of nucleotides are incorporated during PCR?
Correct Answer: B. dATP, dGTP, dCTP and dTTP
Explanation: PCR uses the four deoxyribonucleoside triphosphates: dATP, dGTP, dCTP and dTTP, which serve as substrates for DNA synthesis.
9. Which step of PCR separates the two strands of DNA?
Correct Answer: C. Denaturation
Explanation: During denaturation, double-stranded DNA is heated, typically to approximately 94–98°C, causing hydrogen bonds between complementary strands to break.
10. During which PCR step do primers bind to the template DNA?
Correct Answer: B. Annealing
Explanation: During annealing, the temperature is lowered, allowing primers to hybridize with their complementary sequences on the template DNA.Setup
11. During which step does DNA polymerase synthesize the new DNA strand?
Correct Answer: C. Extension
Explanation: During extension, DNA polymerase adds dNTPs to the 3′ end of the primer, synthesizing a complementary DNA strand.
12. What is the typical extension temperature for Taq DNA polymerase?
Correct Answer: C. 72°C
Explanation: Taq DNA polymerase generally has optimal activity around 72°C, so this temperature is commonly used for the extension step.
13. What is the typical temperature range for DNA denaturation in PCR?
Correct Answer: D. 94–98°C
Explanation: High temperatures disrupt hydrogen bonds between complementary DNA strands and convert double-stranded DNA into single strands.
14. What determines the annealing temperature of PCR primers?
Correct Answer: A. Primer melting temperature (Tm)
Explanation: The annealing temperature is generally selected based on the melting temperatures of the primers. A common starting point is a few degrees below the lower primer Tm.
15. What is the approximate relationship between primer Tm and annealing temperature?
Correct Answer: A. Ta is generally several degrees below Tm
Explanation: Annealing temperature is typically set below primer Tm to allow stable and specific primer-template hybridization.
16. What is the main function of Mg²⁺ in PCR?
Correct Answer: B. It acts as a cofactor for DNA polymerase
Explanation: Mg²⁺ is an essential cofactor for DNA polymerase and participates in the chemistry of phosphodiester bond formation. Its concentration strongly affects PCR specificity and yield.
17. What happens if the Mg²⁺ concentration is excessively high?
Correct Answer: B. It may increase nonspecific amplification
Explanation: Excess Mg²⁺ can stabilize mismatched primer-template interactions and reduce PCR specificity, potentially producing nonspecific bands.
18. Which instrument is used to perform PCR?
Correct Answer: B. Thermal cycler
Explanation: A thermal cycler, or PCR machine, rapidly changes and maintains temperatures required for denaturation, annealing, and extension.
19. What is the major reason for using a thermostable DNA polymerase?
Correct Answer: B. To survive repeated denaturation temperatures
Explanation: Conventional DNA polymerases would be denatured during each high-temperature cycle. Thermostable polymerases can withstand these temperatures.
20. PCR amplification is described as:
Correct Answer: B. Exponential amplification
Explanation: Ideally, the amount of target DNA approximately doubles during each cycle. Thus, after n cycles, amplification approaches 2ⁿ-fold under ideal conditions.
21. Approximately how many copies of the target DNA could theoretically be generated after 30 ideal PCR cycles from one starting molecule?
Correct Answer: C. Approximately 1 billion
Explanation: The theoretical amplification is approximately 2³⁰, which is about 1.07 × 10⁹ copies.
22. Which PCR component determines the boundaries of the amplified product?
Correct Answer: B. Primers
Explanation: The primers define the start and end points of the region that will be amplified.
23. Which primer binds to the antisense/template strand?
Correct Answer: A. Forward primer
Explanation: The forward primer is complementary to the antisense strand and allows synthesis of the corresponding sense strand. The reverse primer binds the opposite strand.
24. DNA synthesis by DNA polymerase proceeds in which direction?
Correct Answer: B. 5′ → 3′
Explanation: DNA polymerases add nucleotides to the 3′-OH end of a growing DNA strand, so synthesis proceeds in the 5′ → 3′ direction.
25. What is the usual length of PCR primers?
Correct Answer: B. 18–30 nucleotides
Explanation: PCR primers are commonly around 18–30 nucleotides long, although the optimal length depends on the particular application and sequence.
26. What is the primary purpose of the initial denaturation step?
Correct Answer: B. To completely denature the template DNA
Explanation: An initial high-temperature treatment helps convert the starting double-stranded DNA into single-stranded templates before cycling begins.
27. What is the purpose of the final extension step?
Correct Answer: B. To allow incomplete DNA strands to finish extending
Explanation: A final extension, often around 72°C for several minutes, allows DNA polymerase to complete partially synthesized products.
28. Which PCR component provides the energy and building blocks for DNA synthesis?
Correct Answer: B. dNTPs
Explanation: dNTPs provide both the nucleotide building blocks and the chemical energy associated with their incorporation into DNA.
29. Which of the following is a common application of PCR?
Correct Answer: D. All of the above
Explanation: PCR has numerous applications, including cloning, diagnostics, forensic analysis, mutation detection, sequencing, and research.
30. Which PCR technique is used to amplify RNA-derived sequences?
Correct Answer: A. RT-PCR
Explanation: Reverse transcription PCR (RT-PCR) first converts RNA into complementary DNA (cDNA) using reverse transcriptase, followed by PCR amplification.
31. What is the first major step in RT-PCR?
Correct Answer: B. Reverse transcription
Explanation: Reverse transcriptase converts RNA into complementary DNA (cDNA), which can then serve as the template for PCR.
32. Which PCR method allows simultaneous amplification of several target sequences?
Correct Answer: B. Multiplex PCR
Explanation: Multiplex PCR uses multiple primer pairs in a single reaction to amplify several different DNA targets simultaneously.
33. What is the main purpose of nested PCR?
Correct Answer: A. Increase specificity
Explanation: Nested PCR uses two successive rounds of amplification with two sets of primers. The second primer pair targets a region within the first PCR product, improving specificity.
34. What is the primary advantage of hot-start PCR?
Correct Answer: B. It reduces nonspecific amplification
Explanation: In hot-start PCR, polymerase activity is inhibited at lower temperatures and activated after heating. This reduces nonspecific primer extension during reaction setup.
35. Which PCR technique is particularly useful for detecting very low-abundance targets?
Correct Answer: A. Nested PCR
Explanation: Nested PCR can substantially increase specificity and sensitivity by using a second set of primers within the initial amplification product.
36. What is real-time PCR commonly called?
Correct Answer: A. qPCR
Explanation: Quantitative PCR (qPCR) monitors DNA amplification in real time, usually using fluorescence, allowing quantification of the starting template.
37. Which molecule is commonly used as a fluorescent reporter in some qPCR assays?
Correct Answer: A. SYBR Green
Explanation: SYBR Green binds double-stranded DNA and fluoresces strongly when bound, allowing accumulation of PCR product to be monitored.
38. What is a major limitation of SYBR Green qPCR?
Correct Answer: B. It detects all double-stranded DNA, including nonspecific products
Explanation: Because SYBR Green binds double-stranded DNA nonspecifically, primer-dimers and nonspecific amplicons can contribute to the fluorescence signal.
39. Which qPCR chemistry provides greater target specificity than SYBR Green?
Correct Answer: A. TaqMan probe-based assay
Explanation: TaqMan assays use a sequence-specific fluorescent probe that hybridizes within the target region, providing additional specificity.
40. What does the Ct or Cq value in qPCR represent?
Correct Answer: B. The cycle at which fluorescence crosses a defined threshold
Explanation: Ct/Cq is the cycle number at which fluorescence becomes distinguishable from background according to a defined threshold. Generally, a lower Ct/Cq indicates a higher initial amount of target nucleic acid.
41. What is a major advantage of high-fidelity DNA polymerases over standard Taq polymerase?
Correct Answer: B. They generally have lower error rates
Explanation: Many high-fidelity polymerases possess 3′→5′ exonuclease proofreading activity, resulting in more accurate DNA synthesis than conventional Taq polymerase.
42. Which enzyme activity is generally absent in standard Taq DNA polymerase?
Correct Answer: C. 3′ → 5′ proofreading exonuclease activity
Explanation: Standard Taq polymerase lacks significant 3′→5′ proofreading activity, contributing to a higher error rate compared with many high-fidelity polymerases.
43. What is primer-dimer formation?
Correct Answer: B. Interaction between primers that results in unwanted amplification
Explanation: Primers can sometimes anneal to each other, especially through complementary 3′ sequences, producing short unwanted PCR products known as primer-dimers.
44. Which change can help increase PCR specificity when nonspecific bands are observed?
Correct Answer: B. Increasing annealing temperature appropriately
Explanation: Increasing the annealing temperature can reduce binding of primers to partially complementary nonspecific sites, thereby improving specificity.
45. What is the purpose of a negative control in PCR?
Correct Answer: B. To detect contamination
Explanation: A negative control contains all necessary reaction components except template DNA. If it produces an amplification product, contamination or nonspecific amplification may be present.
46. What is a positive control in PCR?
Correct Answer: A. A reaction known to contain the target template
Explanation: A positive control confirms that the PCR reagents, primers, and thermal-cycling conditions are capable of producing the expected amplification.
47. Why is PCR particularly vulnerable to contamination?
Correct Answer: B. Even tiny amounts of contaminating DNA can be amplified
Explanation: PCR is highly sensitive. A small amount of contaminating DNA can serve as a template and become exponentially amplified, producing false-positive results.
48. What is the usual method for visualizing conventional PCR products?
Correct Answer: B. Agarose gel electrophoresis
Explanation: PCR products are commonly separated according to size by agarose gel electrophoresis and visualized using an appropriate DNA stain.
49. What determines the migration of DNA fragments through an agarose gel?
Correct Answer: A. Primarily their size
Explanation: DNA has a relatively uniform negative charge-to-mass ratio, so its migration through agarose is primarily dependent on fragment size. Smaller fragments generally migrate faster than larger ones.
50. Which statement best describes PCR?
Correct Answer: B. It selectively amplifies a DNA region using primers and a thermostable DNA polymerase
Explanation: PCR is a powerful molecular biology technique that uses repeated cycles of denaturation, primer annealing, and extension to exponentially amplify a defined DNA sequence.
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