Polymerase Chain Reaction (PCR) — MCQs
Which PCR technique is best suited for identifying a syndrome with multiple causative agents?
Steps of PCR in sequence are?
Which of the following techniques is primarily used for RNA analysis?
What is meant by the melting of double-stranded DNA?
Which of the following is a primarily RNA based technique?
Which of the following statements about Taq DNA polymerase is correct?
DNA amplification is done by all, except:
Which one of the following enzymes is obtained from Thermus aquaticus bacterium that is heat stable and used in PCR at high temperature?
Gene amplification is achieved through
According to IUB system, hydrolases belong to which class?
Polymerase Chain Reaction (PCR) Indian Medical PG Practice Questions and MCQs
Question 1: Which PCR technique is best suited for identifying a syndrome with multiple causative agents?
- A. RT-PCR
- B. Multiplex PCR (Correct Answer)
- C. Nested PCR
- D. Conventional PCR
Explanation: ***Multiplex PCR*** - **Multiplex PCR** allows for the simultaneous amplification of **multiple DNA targets** in a single reaction, making it ideal for identifying syndromes with numerous potential causative agents. - This method uses **multiple primer pairs** in one reaction tube, each designed to amplify a specific target sequence, thus efficiently detecting various pathogens or genetic markers. *RT-PCR* - **Reverse Transcription PCR (RT-PCR)** is used to detect **RNA targets** by first converting RNA into cDNA, which is then amplified. - While useful for RNA viruses or gene expression studies, it is not primarily designed for simultaneous detection of multiple diverse causative agents in the same way as multiplex PCR. *Nested PCR* - **Nested PCR** uses two sets of primers in sequential reactions to **increase sensitivity and specificity** by reducing non-specific binding. - This technique is generally employed to detect very low copies of a specific target or to overcome issues with non-specific amplification, rather than for identifying multiple different agents concurrently. *Conventional PCR* - **Conventional PCR** amplifies a **single specific DNA target** using one pair of primers per reaction. [1] - It requires separate reactions for each potential causative agent, making it inefficient and labor-intensive when testing for a syndrome with multiple etiologies. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 56-57.
Question 2: 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 3: Which of the following techniques is primarily used for RNA analysis?
- A. Sanger's technique
- B. Western blot
- C. Next generation sequencing (Correct Answer)
- D. PCR
Explanation: ***Next generation sequencing*** - **Next-generation sequencing (NGS)**, particularly RNA-Seq, is widely used for **transcriptome analysis** to quantify and discover RNA molecules. - RNA-Seq allows for the precise measurement of **gene expression levels**, identification of **novel transcripts**, and detection of **splicing variants**. *Sanger's technique* - **Sanger sequencing** is primarily used for **DNA sequencing** to determine the exact order of nucleotides in a DNA molecule. - While it can be applied to cDNA (synthesized from RNA), it is not directly used for **RNA analysis** itself. *Western blot* - **Western blot** is a laboratory technique used to detect specific **proteins** in a sample. - It involves separating proteins by size using gel electrophoresis and then transferring them to a membrane for antibody-based detection, making it unsuitable for direct **RNA analysis**. *PCR* - **Polymerase Chain Reaction (PCR)** is used to amplify specific **DNA sequences**. - While **Reverse Transcription PCR (RT-PCR)** can quantify RNA by first converting it to cDNA, PCR itself does not directly analyze the RNA molecule.
Question 4: What is meant by the melting of double-stranded DNA?
- A. Splitting of double strands into single strands (Correct Answer)
- B. Splitting of DNA into fragments
- C. Formation of triple helix
- D. Separation of double-stranded bases
Explanation: ***Splitting of double strands into single strands*** * **DNA melting**, also known as **DNA denaturation**, refers to the process where the two complementary strands of a **double-stranded DNA** molecule separate to form two individual single strands. * This process involves the breaking of the **hydrogen bonds** between the paired bases (**A-T and G-C**) due to increased temperature or changes in pH. * The temperature at which 50% of the DNA is denatured is called the **melting temperature (Tm)**, which depends on GC content (higher GC = higher Tm due to three hydrogen bonds vs. two in AT pairs). *Splitting of DNA into fragments* * The splitting of DNA into fragments is referred to as **DNA fragmentation**, which typically occurs due to processes like **restriction enzyme digestion**, mechanical shearing, or programmed cell death (apoptosis). * This process involves the breaking of the **phosphodiester bonds** within the DNA backbone, not just the hydrogen bonds between strands. *Formation of triple helix* * The formation of a **triple helix** (triplex DNA) is a less common DNA structure where a third oligonucleotide strand binds into the major groove of a **B-form DNA duplex**. * This process is distinct from DNA melting, which involves the *separation* of existing double strands rather than the *addition* of a third strand. *Separation of double-stranded bases* * The term "double-stranded bases" is imprecise terminology; bases are paired (e.g., A with T, G with C) within the double helix structure. * While the separation of base pairs does occur during melting, the more accurate description is the **separation of the entire double helix into two single strands**, not just the individual bases.
Question 5: 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 6: Which of the following statements about Taq DNA polymerase is correct?
- A. Optimum temperature for chain elongation is 75°C (Correct Answer)
- B. Denatures at high temperatures
- C. Provides high fidelity during DNA synthesis
- D. Exhibits 3' to 5' exonuclease activity
Explanation: ***Optimum temperature for chain elongation is 75°C*** - **Taq polymerase** is a **thermostable enzyme** isolated from *Thermus aquaticus*, functioning optimally at high temperatures. - The optimal temperature for the **elongation step** in PCR, where Taq polymerase synthesizes new DNA strands, is typically around **72-78°C**, with 75°C falling within this optimal range. *Denatures at high temperatures* - While all proteins will eventually denature at extremely high temperatures, Taq polymerase is specifically known for its **thermostability** and **resistance to denaturation** at temperatures required for DNA strand separation in PCR (typically 94-98°C). - Its ability to withstand these high temperatures without significant loss of activity is its key advantage for use in **Polymerase Chain Reaction (PCR)**. *Provides high fidelity during DNA synthesis* - **Taq polymerase** is known for its relatively **low fidelity** due to the lack of 3' to 5' exonuclease activity (proofreading). - This low fidelity results in a higher error rate during DNA synthesis compared to other polymerases with proofreading capabilities, leading to more **mutations** during PCR. *Exhibits 3' to 5' exonuclease activity* - **Taq polymerase** typically **lacks 3' to 5' exonuclease activity**, meaning it does not have the ability to proofread and remove incorrectly incorporated nucleotides. - This absence of proofreading contributes to its relatively **lower fidelity** during DNA replication compared to other polymerases that possess this activity.
Question 7: 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 8: 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 9: Gene amplification is achieved through
- A. Polymerase Chain Reaction (Correct Answer)
- B. DNA strand hybridization
- C. In situ DNA hybridization
- D. Ligase chain reaction (LCR)
Explanation: ***Polymerase Chain Reaction*** - **PCR** is the **gold standard** molecular biology technique that generates **millions to billions of copies** of a specific DNA segment over a short period. - It utilizes a cyclical process of **denaturation**, **annealing**, and **extension** with **thermostable DNA polymerase** to achieve exponential amplification. - **Most widely used** method for gene amplification in research and diagnostics. *DNA strand hybridization* - **DNA strand hybridization** is the process where two complementary single-stranded DNA molecules bind together to form a **double-stranded molecule**. - This process is fundamental to many molecular techniques but does not, in itself, achieve **amplification**; rather, it is a **binding event**. *In situ DNA hybridization* - **In situ hybridization** is a technique that localizes and detects specific **nucleic acid sequences** (DNA or RNA) within cells or tissues directly on a slide. - While it uses **hybridization**, its primary purpose is **detection and localization**, not the **amplification** of DNA sequences. *Ligase chain reaction (LCR)* - **LCR** is a molecular technique that does amplify DNA sequences exponentially using **DNA ligase** to join adjacent oligonucleotide probes. - However, it is **less commonly used** than PCR, has more **stringent requirements** (requires knowledge of both strands), and is primarily used for detecting **known point mutations** rather than general gene amplification. - **PCR remains the standard** technique when the question refers to gene amplification without additional qualifiers.
Question 10: According to IUB system, hydrolases belong to which class?
- A. EC-1
- B. EC-2
- C. EC-3 (Correct Answer)
- D. EC-4
Explanation: ***EC-3*** - **Hydrolases** catalyze the **hydrolysis** of chemical bonds, which involves the addition of water to break the bond. - This class includes enzymes like **esterases**, **peptidases**, and **glycosidases**, all of which use water to cleave molecules. *EC-1* - **EC-1** refers to **oxidoreductases**, which catalyze **oxidation-reduction reactions**. - These enzymes are involved in the transfer of electrons or hydrogen atoms, not the hydrolysis of bonds. *EC-2* - **EC-2** represents **transferases**, enzymes that catalyze the **transfer of a functional group** from one molecule to another. - Examples include **kinases** and **transaminases**, which are distinct from hydrolytic enzymes. *EC-4* - **EC-4** encompasses **lyases**, which catalyze the **cleavage of various bonds** by means other than hydrolysis or oxidation, often forming double bonds. - This class includes enzymes like **decarboxylases** and **aldolases**, which are not primarily involved in breaking bonds with water.
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