Single Nucleotide Polymorphisms Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Single Nucleotide Polymorphisms. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Single Nucleotide Polymorphisms Indian Medical PG Question 1: Gene not involved in SCID:
- A. BTK (Correct Answer)
- B. ZAP70
- C. IL2RG
- D. JAK3
Single Nucleotide Polymorphisms Explanation: ***BTK***
- **Bruton's tyrosine kinase (BTK)** is associated with **X-linked agammaglobulinemia (XLA)**, a primary immunodeficiency characterized by the absence of mature B cells and significantly reduced antibody production. While it causes severe immune deficiency, it is not a direct cause of **SCID**.
- XLA results in recurrent bacterial infections due to an inability to produce antibodies, rather than the severe combined T and B cell dysfunction seen in SCID.
*ZAP70*
- **ZAP70** deficiency is a cause of **SCID**. It leads to impaired T-cell receptor signaling, resulting in profound functional T-cell lymphopenia.
- Patients with ZAP70 deficiency have normal numbers of CD4 T cells but very low or absent CD8 T cells, and their T cells are functionally impaired, leading to severe immunodeficiency.
*IL2RG*
- The **IL2RG** gene encodes the common gamma chain (γc), a crucial component of several **interleukin receptors (IL-2, IL-4, IL-7, IL-9, IL-15, IL-21)**. [1]
- Mutations in IL2RG cause **X-linked SCID (X-SCID)**, the most common form of SCID, leading to a block in T-cell and NK-cell development due to defective cytokine signaling. [1]
*JAK3*
- **Janus kinase 3 (JAK3)** is a tyrosine kinase that associates with the **common gamma chain (γc)** and is essential for cytokine signaling downstream of the γc-containing receptors. [1]
- **JAK3 deficiency** results in an **autosomal recessive form of SCID**, clinically indistinguishable from X-SCID, with impaired T-cell and NK-cell development due to defective cytokine signaling. [1]
**References:**
[1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 247-248.
Single Nucleotide Polymorphisms Indian Medical PG Question 2: Which of the following statements accurately describes a gene library?
- A. A physical library containing books about genetics.
- B. A digital database of genetic sequences.
- C. A collection of cloned DNA fragments that represent an organism's genome. (Correct Answer)
- D. A collection of DNA sequences or fragments.
Single Nucleotide Polymorphisms Explanation: ***A collection of cloned DNA fragments that represent an organism's genome.***
- A **gene library** (also called a DNA library) is a collection of various **DNA fragments** from a genome, each carried by a vector and cloned in a host cell, so that the entire genome is represented and can be accessed.
- This systematic collection ensures that essentially all the organism's **genetic material** is available for study, screening, and sequencing.
*A physical library containing books about genetics.*
- This statement describes a traditional **resource for genetic information**, not the molecular construct itself.
- A physical library contains **printed materials** (books, journals) for reading and research, distinct from biological samples.
*A digital database of genetic sequences.*
- While digital databases store genetic sequence information (e.g., GenBank), they are **in silico** representations and not physical collections of DNA.
- These databases are used to **analyze and retrieve genetic information**, but they are not the gene libraries themselves.
*A collection of DNA sequences or fragments.*
- This definition is too broad, as a gene library specifically refers to **cloned DNA fragments**.
- Without the cloning and representation of an entire genome, it's merely a collection, not a structured "library" in the molecular biology sense.
Single Nucleotide Polymorphisms Indian Medical PG Question 3: Which of the following is used to detect abnormal gene sequences EXCEPT?
- A. RFLP analysis
- B. Pyrosequencing
- C. Flow cytometry (Correct Answer)
- D. FISH
Single Nucleotide Polymorphisms 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.
Single Nucleotide Polymorphisms Indian Medical PG Question 4: Which type of mutation can act as a suppressor to restore the wild-type phenotype in organisms carrying a mutant gene?
- A. Frameshift mutation of coding gene
- B. Mutation of tRNA (Correct Answer)
- C. Deletion of mutant gene
- D. Addition of another normal gene
Single Nucleotide Polymorphisms Explanation: ***Mutation of tRNA***
- A **tRNA suppressor mutation** can alter its anticodon, allowing it to recognize a **stop codon** (nonsense suppressor) or a missense codon, and insert an amino acid, thereby suppressing the original mutation.
- This is a classic example of an **intergenic suppressor mutation** that acts at a different genetic locus from the original mutation.
- These suppressors are particularly effective for **nonsense mutations** (premature stop codons) and certain missense mutations by correcting the decoding error during translation.
*Frameshift mutation of coding gene*
- A single frameshift mutation causes a shift in the **reading frame**, leading to a completely different protein sequence downstream and often a premature stop codon, which would worsen the phenotype.
- While a **second compensating frameshift** mutation in the same gene could theoretically restore the reading frame (acting as an intragenic suppressor), this is context-dependent and less reliable than tRNA suppressors.
- The question asks for mutations that "can act as a suppressor," and **tRNA mutations are the more universally recognized and reliable suppressor mechanism** in classical genetics.
*Deletion of mutant gene*
- **Deleting the mutant gene** removes the genetic information entirely but does not restore wild-type function; instead, it typically results in **loss of function** or complete absence of the protein.
- This would lead to a **null phenotype** rather than restoration of wild-type phenotype, especially if the gene is essential.
*Addition of another normal gene*
- The **addition of another normal (wild-type) gene copy** provides a functional protein that can compensate for the mutant gene's deficiency.
- While this can restore a wild-type phenotype, it represents **gene complementation** or gene therapy, not a true suppressor mutation that modifies the interpretation or expression of the existing mutant allele.
Single Nucleotide Polymorphisms Indian Medical PG Question 5: Which of the following statements about polymorphism is true?
- A. Single phenotype linked to a single locus with multiple abnormal alleles.
- B. Single locus with multiple normal alleles. (Correct Answer)
- C. Single locus with multiple abnormal alleles, not linked to a specific phenotype.
- D. Single phenotype linked to a single locus with both normal and abnormal alleles.
Single Nucleotide Polymorphisms Explanation: ***Single locus with multiple normal alleles.***
- **Polymorphism** refers to the existence of multiple alleles at a **single genetic locus** within a population.
- For a variant to be considered a polymorphism, the most common allele must have a frequency of **less than 99%**, meaning at least two alleles are common.
*Single locus with multiple abnormal alleles, not linked to a specific phenotype.*
- While polymorphism involves multiple alleles at a single locus, classifying them as "abnormal" is misleading, as polymorphism often refers to **variations that are common** in the population and not necessarily disease-causing or abnormal.
- The definition emphasizes the presence of multiple alleles, not their clinical implications, and many polymorphisms have **no overt phenotypic effect**.
*Single phenotype linked to a single locus with both normal and abnormal alleles.*
- Polymorphism primarily describes **genetic variation (alleles)**, not direct links to a single phenotype. A single locus can influence **multiple phenotypes**, and a single phenotype can be influenced by multiple loci.
- Grouping alleles as "normal" and "abnormal" oversimplifies the concept; **many polymorphisms are neutral** or beneficial, and some "normal" alleles can become "abnormal" in certain contexts.
*Single phenotype linked to a single locus with multiple abnormal alleles.*
- This option incorrectly narrows the definition by focusing on a **single phenotype** and exclusively "abnormal" alleles. Polymorphism encompasses any common variation, regardless of its phenotypic effect or whether the alleles are considered abnormal.
- Many polymorphic variations are **silent mutations** or variations that do not result in overt phenotypic changes or disease.
Single Nucleotide Polymorphisms Indian Medical PG Question 6: Which of the following techniques can be used to detect single base pair substitutions?
- A. FISH
- B. Southern blot
- C. PCR (Correct Answer)
- D. Restriction Fragment Length Polymorphism (RFLP)
Single Nucleotide Polymorphisms Explanation: ***PCR (with sequencing or allele-specific methods)***
- **PCR-based techniques** are the most versatile methods for detecting single base pair substitutions (point mutations)
- **Allele-specific PCR** can directly detect known point mutations by using primers specific to mutant or wild-type alleles
- **PCR followed by Sanger sequencing** is the gold standard for identifying any single base pair substitution
- **High-resolution melting (HRM) analysis** after PCR can detect mutations based on melting curve differences
- PCR amplification is the foundation that enables these detection methods
*FISH (Fluorescence in situ hybridization)*
- FISH detects **large chromosomal abnormalities** such as aneuploidy, translocations, large deletions, and duplications
- It visualizes chromosomal-level changes using fluorescent probes
- **Not sensitive enough** to detect single base pair changes, as these are too small to visualize cytogenetically
*Southern blot*
- Southern blot detects **large DNA rearrangements**, insertions, deletions, or copy number variations
- Analyzes restriction enzyme fragments separated by gel electrophoresis
- **Generally cannot detect** single base pair substitutions unless they create or abolish a restriction enzyme recognition site
- Even when applicable, PCR-based methods are more efficient and sensitive
*Restriction Fragment Length Polymorphism (RFLP)*
- RFLP can detect single base pair substitutions **only if** they create or abolish a **restriction enzyme recognition site**
- Classic example: **Sickle cell mutation** (GAG→GTG in β-globin gene) abolishes an MstII restriction site
- **Limited applicability** - can only detect the subset of point mutations that affect restriction sites
- PCR-based methods are preferred as they can detect **any** single base pair substitution, not just those affecting restriction sites
Single Nucleotide Polymorphisms Indian Medical PG Question 7: What is the term for a single mutation in a nucleotide base pair that results in a termination codon?
- A. Missense mutation
- B. Nonsense mutation (Correct Answer)
- C. Termination mutation
- D. Silent mutation
Single Nucleotide Polymorphisms Explanation: ***Nonsense mutation***
- A **nonsense mutation** occurs when a single nucleotide base pair change leads to the formation of a **premature stop codon**, which results in a truncated and often non-functional protein.
- The term "nonsense" refers to the fact that the new codon signals an early termination of protein synthesis.
*Missense mutation*
- A **missense mutation** involves a single nucleotide change that results in a codon coding for a **different amino acid**, potentially altering protein function but not necessarily terminating it.
- This type of mutation can have varying effects on protein function, from benign to severe, depending on the amino acid substitution.
*Termination mutation*
- While a nonsense mutation does result in **premature termination**, "termination mutation" is not the standard or most precise scientific term used to describe this specific type of genetic alteration.
- The more accurate and widely accepted terminology is **nonsense mutation** for a change leading to a stop codon.
*Silent mutation*
- A **silent mutation** is a type of point mutation that changes a single nucleotide, but does not change the amino acid sequence of the protein due to the **degeneracy of the genetic code**.
- These mutations have **no observable effect** on the organism's phenotype as the protein produced remains unchanged.
Single Nucleotide Polymorphisms Indian Medical PG Question 8: Which of the following is a primarily RNA based technique?
- A. Sanger's technique
- B. Western blotting
- C. Next generation sequencing (Correct Answer)
- D. PCR
Single Nucleotide Polymorphisms Explanation: ***Next generation sequencing***
- NGS, particularly **RNA-Seq (RNA sequencing)**, is the most advanced and comprehensive technique for studying RNA among the given options.
- RNA-Seq directly sequences **RNA transcripts** to analyze gene expression, identify splice variants, detect novel transcripts, and quantify RNA abundance genome-wide.
- While it involves converting RNA to cDNA for sequencing, **RNA is the primary starting material and target** of analysis, making it the most RNA-focused technique listed.
- Superior to other methods for comprehensive transcriptome analysis.
*Sanger's technique*
- **Sanger sequencing** is a **DNA sequencing method** that determines the nucleotide sequence of DNA molecules.
- Primarily designed for and used with DNA templates.
- Not used for direct RNA analysis in routine practice.
*Western blotting*
- **Western blotting** detects and analyzes **proteins**, not nucleic acids.
- Uses antibodies to identify specific proteins after electrophoretic separation.
- Not related to RNA techniques.
*PCR*
- Standard **PCR amplifies DNA sequences** only.
- While **RT-PCR** (reverse transcriptase PCR) starts with RNA, it immediately converts RNA to cDNA, and the actual amplification is DNA-based.
- Less comprehensive for RNA analysis compared to RNA-Seq.
Single Nucleotide Polymorphisms Indian Medical PG Question 9: All are added to PCR, except:
- A. Thermostable DNA polymerase
- B. Template DNA
- C. Deoxynucleotide
- D. Dideoxynucleotide (Correct Answer)
Single Nucleotide Polymorphisms 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.
Single Nucleotide Polymorphisms Indian Medical PG Question 10: Transcription is inhibited by which of the following agents?
- A. Actinomycin D (Correct Answer)
- B. Amanitin
- C. Chloramphenicol
- D. Streptomycin
Single Nucleotide Polymorphisms Explanation: **Explanation:**
Transcription is the process of synthesizing RNA from a DNA template. **Actinomycin D (Dactinomycin)** is a potent inhibitor of transcription in both prokaryotes and eukaryotes. It works by **intercalating between cytosine-guanine (C-G) base pairs** of the DNA template, creating a stable complex that physically blocks the movement of RNA polymerase, thereby preventing RNA chain elongation.
**Analysis of Options:**
* **A. Actinomycin D (Correct):** As described, it inhibits transcription by DNA intercalation. Clinically, it is used as a chemotherapy agent (e.g., for Wilms tumor and Ewing sarcoma).
* **B. Amanitin (α-Amanitin):** While this also inhibits transcription (specifically **RNA Polymerase II** in eukaryotes), it is derived from the *Amanita phalloides* mushroom. In many MCQ contexts, if both are present, Actinomycin D is the classic general inhibitor cited for its action on the DNA template itself. (Note: Some sources consider both correct; however, Actinomycin D is the universal inhibitor for both cell types).
* **C. Chloramphenicol:** This is a **translation (protein synthesis) inhibitor**. It binds to the **50S ribosomal subunit** of bacteria, inhibiting peptidyl transferase.
* **D. Streptomycin:** This is an aminoglycoside that inhibits **translation** by binding to the **30S ribosomal subunit**, causing misreading of mRNA and inhibiting the initiation of protein synthesis.
**High-Yield Clinical Pearls for NEET-PG:**
* **Rifampicin:** Inhibits bacterial transcription by binding to the **beta-subunit of DNA-dependent RNA polymerase**.
* **α-Amanitin:** Found in "Death Cap" mushrooms; causes severe hepatotoxicity by halting mRNA synthesis.
* **Prokaryotic vs. Eukaryotic Inhibition:** Always distinguish if a drug acts on the 30S/50S (Prokaryotic translation) or 40S/60S (Eukaryotic translation) subunits.
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