DNA Sequencing Techniques — MCQs
Which of the following doesn't occur in 5' to 3' direction?
Which of the following statements is NOT applicable to bacterial genomes?
Which PCR technique is best suited for identifying a syndrome with multiple causative agents?
All are added to PCR, except:
Which of the following techniques is primarily used for RNA analysis?
PCR is primarily a
Which of the following is used to detect abnormal gene sequences EXCEPT?
By which method foreign DNA is introduced into a cell by a virus or viral vector?
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?
Influenza virus causes new epidemic by (3-5 yrs)-
DNA Sequencing Techniques Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following doesn't occur in 5' to 3' direction?
- A. DNA repair
- B. Transcription
- C. DNA replication
- D. RNA editing (Correct Answer)
Explanation: ***RNA editing*** - **RNA editing** involves modifications to **RNA molecules** after transcription, such as base insertions, deletions, or substitutions. - This process does not follow a 5' to 3' synthesis direction, unlike DNA or RNA synthesis. *DNA repair* - **DNA repair mechanisms**, such as **excision repair**, involve synthesizing new DNA to replace damaged sections. - This synthesis occurs in the **5' to 3' direction** by **DNA polymerases**. *Transcription* - **Transcription** is the process where **RNA polymerase** synthesizes an **RNA molecule** from a **DNA template**. - This synthesis always occurs in the **5' to 3' direction**, adding nucleotides to the 3' end of the growing RNA strand. *DNA replication* - **DNA replication** involves the synthesis of new **DNA strands** from a **template strand**. - **DNA polymerase** adds nucleotides exclusively in the **5' to 3' direction**, requiring a primer for initiation.
Question 2: 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 3: 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 4: All are added to PCR, except:
- A. Thermostable DNA polymerase
- B. Template DNA
- C. Deoxynucleotide
- D. Dideoxynucleotide (Correct Answer)
Explanation: ***Dideoxynucleotide*** - **Dideoxynucleotides (ddNTPs)** are chain-terminating nucleotides that lack a 3'-hydroxyl group, preventing further phosphodiester bond formation and DNA strand elongation. They are primarily used in **Sanger sequencing**, not standard PCR. - In PCR, the goal is to amplify DNA segments, which requires continued strand synthesis, making ddNTPs unsuitable as they would halt the amplification process. *Thermostable DNA polymerase* - **Thermostable DNA polymerase** (e.g., Taq polymerase) is a crucial component of PCR, responsible for synthesizing new DNA strands during the extension phase. - Its thermostability allows it to withstand the high temperatures used during the denaturation step in each cycle without losing activity. *Template DNA* - **Template DNA** is the specific DNA sequence that needs to be amplified, serving as the blueprint for the PCR reaction. - The primers anneal to the template DNA, dictating the region that will be copied. *Deoxynucleotide* - **Deoxynucleotides (dNTPs)** are the basic building blocks of DNA (dATP, dCTP, dGTP, dTTP) that are incorporated by DNA polymerase to synthesize new DNA strands. - They provide the raw materials for the "extension" phase of PCR, where the DNA polymerase adds nucleotides complementary to the template strand.
Question 5: 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 6: PCR is primarily a
- A. DNA sequencing technique
- B. All of these
- C. DNA degradation technique
- D. DNA amplification technique (Correct Answer)
Explanation: ***Correct: DNA amplification technique*** - **Polymerase Chain Reaction (PCR)** is a laboratory technique used to make millions of copies of a specific DNA segment - This amplification allows for easier detection, analysis, and manipulation of even very small amounts of DNA - PCR uses repeated cycles of heating and cooling with DNA polymerase to exponentially amplify target DNA sequences *Incorrect: DNA sequencing technique* - **DNA sequencing** determines the exact order of nucleotides within a DNA molecule, which is different from PCR's primary function - While PCR products can be sequenced afterward, PCR itself does not determine the nucleotide sequence - Sequencing is a separate technique (e.g., Sanger sequencing, Next-generation sequencing) *Incorrect: DNA degradation technique* - **DNA degradation** involves the breakdown of DNA molecules, typically by nucleases or chemical/physical processes - PCR's purpose is to **synthesize and increase** the amount of DNA, not to break it down - This is the opposite of what PCR does *Incorrect: All of these* - PCR has a specific primary function: **DNA amplification** - It is not a combination of amplification, sequencing, and degradation techniques - While PCR can be part of a workflow that includes sequencing, its primary role is amplification only
Question 7: Which of the following is used to detect abnormal gene sequences EXCEPT?
- A. RFLP analysis
- B. Pyrosequencing
- C. Flow cytometry (Correct Answer)
- D. FISH
Explanation: ***Flow cytometry*** - **Flow cytometry** is primarily used to analyze **cell populations** based on their physical and biochemical characteristics (e.g., size, granularity, and protein expression) by passing them single file through a laser beam, not for direct gene sequencing. - It detects and quantifies cells labeled with **fluorescent antibodies**, making it useful for immunophenotyping, cell sorting, and DNA content analysis, but not for identifying specific gene sequences or mutations. *RFLP analysis* - **Restriction fragment length polymorphism (RFLP) analysis** detects variations in **DNA sequences** by using **restriction enzymes** to cut DNA at specific sites. - Differences in fragment lengths indicate **polymorphisms** or **mutations** within the recognition sites, thereby identifying abnormal gene sequences. *Pyrosequencing* - **Pyrosequencing** is a method of **DNA sequencing** that determines the sequence of nucleotides by detecting the release of pyrophosphate during DNA synthesis. - It is used to identify **single nucleotide polymorphisms (SNPs)** and **short genetic variations**, making it suitable for detecting abnormal gene sequences. *FISH* - **Fluorescence in situ hybridization (FISH)** uses **fluorescently labeled DNA probes** that bind to specific complementary **DNA sequences** on chromosomes. - It is a powerful cytogenetic technique for detecting **chromosomal abnormalities**, such as deletions, translocations, and amplifications, thereby identifying abnormal gene sequences.
Question 8: By which method foreign DNA is introduced into a cell by a virus or viral vector?
- A. Transduction (Correct Answer)
- B. Transcription
- C. Lysogenic conversion
- D. Transformation
Explanation: ***Transduction*** - **Transduction** is the process by which foreign DNA is introduced into a cell by a **virus** or **viral vector**. - This method is commonly used in **gene therapy** to deliver specific genes into target cells for therapeutic purposes. *Transcription* - **Transcription** is the process of synthesizing an **RNA molecule** from a **DNA template**. - It involves the copying of genetic information from DNA to RNA and is not a method for introducing foreign DNA into a cell. *Lysogenic conversion* - **Lysogenic conversion** is a process where the presence of a **prophage** (a bacteriophage genome inserted into the host bacterium's DNA) alters the **phenotype** of the bacterium. - This is a specific type of genetic change caused by a bacteriophage in bacteria, not a general method for introducing foreign DNA into any cell type via a virus. *Transformation* - **Transformation** in biology refers to the uptake of **naked foreign DNA** from the environment by a cell. - Unlike transduction, it does not involve a **viral vector** as an intermediary for DNA delivery.
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: Influenza virus causes new epidemic by (3-5 yrs)-
- A. Cycle trends
- B. Antigenic drift (Correct Answer)
- C. Mosaicism
- D. Antigenic shift
Explanation: ***Antigenic drift*** - **Antigenic drift** involves minor changes in the **hemagglutinin (HA)** and **neuraminidase (NA)** surface proteins of the influenza virus due to point mutations. - These minor changes allow the virus to slightly evade the host's immune system, leading to **seasonal epidemics** (typically every 2-3 years) as pre-existing immunity is less effective. *Cycle trends* - This term is too general and does not specifically describe the ** virological mechanism** responsible for influenza epidemics. - While influenza does exhibit cyclical patterns, "cycle trends" doesn't explain the underlying biological process of viral evolution. *Mosaicism* - **Mosaicism** refers to the presence of two or more populations of cells with different genotypes within a single individual. - This genetic phenomenon is completely unrelated to how influenza viruses cause new epidemics. *Antigenic shift* - **Antigenic shift** involves abrupt, major changes in the HA or NA proteins, usually through **gene reassortment** when two different influenza viruses co-infect the same cell. - This leads to entirely **new viral subtypes** that can cause global **pandemics** (less frequently, perhaps every 10-40 years), not the more regular 3-5 year epidemics.
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