Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 371: Which type of mutation typically involves a change in a gene coding for which of the following molecules?

A. Silent mutation
B. Nonsense mutation
C. Missense mutation
D. Nonsense suppressor mutation (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Nonsense suppressor mutation** The core concept here is the **site of the mutation**. While most mutations occur in the structural gene coding for a protein, a **nonsense suppressor mutation** occurs in a gene coding for **tRNA molecules**. In a nonsense mutation, a codon is changed to a stop codon (UAG, UAA, UGA), leading to premature protein termination. A suppressor mutation is a second mutation that "undoes" the effect of the first. In this specific case, a mutation occurs in the **anticodon loop of a tRNA molecule**, allowing it to recognize and bind to a stop codon and insert an amino acid instead of terminating translation. This allows the full-length protein to be synthesized despite the primary nonsense mutation. **Why the other options are incorrect:** * **A, B, and C (Silent, Nonsense, Missense):** These are types of point mutations that occur within the **mRNA/DNA sequence of the structural protein** itself. They describe the *effect* on the protein product (no change, premature stop, or single amino acid substitution) rather than a mutation in a secondary regulatory molecule like tRNA. **High-Yield Clinical Pearls for NEET-PG:** * **Nonsense Mutation:** Results in a truncated, usually non-functional protein (e.g., many cases of β-thalassemia or Duchenne Muscular Dystrophy). * **Frameshift Mutation:** Caused by insertion or deletion of nucleotides not divisible by three; usually more devastating than point mutations. * **Transition vs. Transversion:** Transitions (Purine to Purine) are more common than Transversions (Purine to Pyrimidine). * **Wobble Hypothesis:** Explains why multiple codons can code for a single amino acid, primarily due to flexibility in the 3rd base of the codon.

Question 372: Which enzyme is responsible for DNA supercoiling?

A. DNA polymerase I
B. DNA polymerase II
C. DNA topoisomerase
D. DNA gyrase (Correct Answer)

Explanation: **Explanation:** The correct answer is **DNA gyrase**. DNA supercoiling is a critical process that manages the topological stress induced during the unwinding of the DNA double helix. **1. Why DNA Gyrase is Correct:** DNA gyrase is a specialized **Type II Topoisomerase** found in prokaryotes. It is unique because it is the only enzyme capable of actively introducing **negative supercoils** into DNA using energy derived from ATP hydrolysis. This process neutralizes the positive supercoiling (overwinding) that occurs ahead of the replication fork, allowing DNA replication and transcription to proceed smoothly. **2. Why the Other Options are Incorrect:** * **DNA Polymerase I:** Primarily involved in DNA repair and the removal of RNA primers (via its 5' to 3' exonuclease activity) during lagging strand synthesis. * **DNA Polymerase II:** Mainly functions in DNA repair mechanisms when the replication fork stalls. * **DNA Topoisomerase:** While DNA gyrase is a *type* of topoisomerase, "DNA Topoisomerase" (specifically Type I) generally relaxes supercoils by cutting a single strand without requiring ATP. In the context of NEET-PG, if "DNA gyrase" is an option, it is the most specific and correct answer for the active introduction of supercoils. **3. Clinical Pearls & High-Yield Facts:** * **Pharmacology Link:** DNA gyrase is the primary target of **Quinolones and Fluoroquinolones** (e.g., Ciprofloxacin). These drugs inhibit the enzyme, leading to double-strand breaks and bacterial cell death. * **Eukaryotic Equivalent:** Eukaryotes do not have DNA gyrase; they use Topoisomerase II to relax supercoils, which is targeted by anticancer drugs like **Etoposide**. * **Energy Requirement:** Remember that Type I Topoisomerases are ATP-independent, whereas Type II (including Gyrase) are **ATP-dependent**.

Question 373: A four-year-old child is diagnosed with Duchenne muscular dystrophy, an X-linked recessive disorder. Genetic analysis shows that the patient's gene for the muscle protein dystrophin contains a mutation in its promoter region. What would be the most likely effect of this mutation?

A. Tailing of dystrophin mRNA will be defective.
B. Capping of dystrophin mRNA will be defective.
C. Termination of dystrophin transcription will be deficient.
D. Initiation of dystrophin transcription will be deficient. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** The **promoter region** of a gene is a specific DNA sequence (located upstream of the transcription start site) where **RNA polymerase II** and general transcription factors bind to initiate the synthesis of mRNA. If a mutation occurs in the promoter (such as the TATA box or CAAT box), the affinity for these transcription factors decreases, leading to a failure or significant reduction in the **initiation of transcription**. In Duchenne Muscular Dystrophy (DMD), while most cases are due to large deletions, a promoter mutation results in insufficient production of dystrophin mRNA, leading to the clinical phenotype. **2. Why Other Options are Incorrect:** * **Options A & B (Tailing and Capping):** These are **post-transcriptional modifications**. Capping occurs at the 5' end and tailing (polyadenylation) occurs at the 3' end of the pre-mRNA. These processes are governed by specific sequences within the transcribed region (e.g., the polyadenylation signal AAUAAA), not the promoter. * **Option C (Termination):** Transcription termination involves specific sequences at the end of the gene that signal RNA polymerase to stop. The promoter is exclusively involved in the "start" phase, not the "stop" phase. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **DMD Genetics:** Most common cause is **out-of-frame deletions** (frameshift) in the *DMD* gene (the largest known human gene). Promoter mutations are rarer but high-yield for understanding gene expression. * **Promoter Elements:** The **TATA box** (Hogness box) is the most common eukaryotic promoter element, typically located at -25 bp. * **Enhancers vs. Promoters:** Promoters are position-dependent (must be near the start site), whereas enhancers can be thousands of base pairs away and increase the *rate* of transcription. * **Dystrophin Function:** It anchors the cytoskeleton of muscle fibers to the extracellular matrix; its absence leads to muscle fiber necrosis and pseudohypertrophy.

Question 374: A 10-year-old child presents with short stature, external ophthalmoplegia, gait abnormality, and sensorineural hearing loss. Lab findings reveal elevated fasting blood glucose levels. What is true about the DNA involved in this condition?

A. Circular (Correct Answer)
B. Double-helix
C. Bounded by nuclear envelope
D. Packed into chromatin

Explanation: **Explanation:** The clinical presentation of short stature, external ophthalmoplegia, gait abnormality (ataxia), sensorineural hearing loss, and diabetes mellitus (elevated glucose) is characteristic of **Kearns-Sayre Syndrome (KSS)**. KSS is a mitochondrial myopathy caused by large-scale deletions in **Mitochondrial DNA (mtDNA)**. **1. Why the Correct Answer is Right:** Mitochondrial DNA differs significantly from nuclear DNA. It is **circular**, double-stranded, and lacks introns. Unlike nuclear DNA, it is not enclosed within a nucleus but resides in the mitochondrial matrix. Because KSS is a mitochondrial disorder, the DNA involved is circular. **2. Why Incorrect Options are Wrong:** * **Double-helix (Option B):** While mtDNA is double-stranded, "Circular" is the more specific defining characteristic used to differentiate it from the linear structure of nuclear DNA in medical exams. * **Bounded by nuclear envelope (Option C):** This describes nuclear DNA. mtDNA is located in the mitochondria, reflecting its endosymbiotic evolutionary origin. * **Packed into chromatin (Option D):** Nuclear DNA is wrapped around histone proteins to form chromatin. mtDNA is "naked" (lacks histones) and is instead organized into structures called nucleoids. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial Inheritance:** Transmitted only through the mother (maternal inheritance). All children of an affected mother are at risk, but an affected father cannot pass it on. * **Heteroplasmy:** The coexistence of mutated and wild-type mtDNA within a single cell, explaining the clinical variability in mitochondrial diseases. * **Mitochondrial DNA Facts:** Encodes 13 polypeptides (oxidative phosphorylation subunits), 22 tRNAs, and 2 rRNAs. It has a higher mutation rate than nuclear DNA due to a lack of robust repair mechanisms and proximity to free radicals.

Question 375: The CG region is involved in which of the following processes?

A. Acetylation
B. Methylation (Correct Answer)
C. Phosphorylation
D. DNA Replication

Explanation: **Explanation:** The correct answer is **Methylation**. In eukaryotic genomes, DNA methylation occurs predominantly at **CpG islands**—regions where a Cytosine nucleotide is followed by a Guanine nucleotide (connected by a phosphate group). **1. Why Methylation is Correct:** DNA methylation is a key epigenetic mechanism catalyzed by **DNA Methyltransferases (DNMTs)**. The enzyme adds a methyl group to the 5th carbon of the cytosine ring, forming **5-methylcytosine**. CpG islands are typically located in or near promoter regions of genes. When these regions are methylated, it generally leads to **gene silencing** (transcriptional repression) by preventing the binding of transcription factors or recruiting chromatin-remodeling proteins. **2. Why Other Options are Incorrect:** * **Acetylation:** This primarily occurs on **lysine residues of histone proteins**, not directly on the DNA sequence. Histone acetylation (by HATs) usually relaxes chromatin (euchromatin) and activates transcription. * **Phosphorylation:** This is a post-translational modification of proteins (e.g., enzymes or histones) or a step in nucleotide metabolism, but it is not the primary regulatory modification of CG regions in DNA. * **DNA Replication:** While CG-rich regions can influence the stability of the DNA helix due to triple hydrogen bonding, "CG regions" as a regulatory motif are specifically associated with the biochemical process of methylation. **Clinical Pearls for NEET-PG:** * **Genomic Imprinting:** Methylation of CpG islands is the basis for imprinting (e.g., **Prader-Willi** and **Angelman syndromes**). * **Fragile X Syndrome:** Characterized by CGG triplet repeat expansion leading to hypermethylation of the FMR1 gene and subsequent silencing. * **Cancer:** Hypermethylation of tumor suppressor genes (like *p16*) is a common finding in various malignancies. * **SAM (S-adenosylmethionine):** Acts as the universal methyl donor for this reaction.

Question 376: Xeroderma pigmentosum is caused by a group of closely related abnormalities in which DNA repair pathway?

A. Mismatch repair
B. Base excision repair
C. Nucleotide excision repair (Correct Answer)
D. Phagosomes

Explanation: **Explanation:** **Xeroderma Pigmentosum (XP)** is an autosomal recessive genetic disorder characterized by extreme sensitivity to ultraviolet (UV) radiation. **Why Nucleotide Excision Repair (NER) is correct:** The primary mechanism to repair DNA damage caused by UV light (specifically **pyrimidine dimers** like thymine dimers) is **Nucleotide Excision Repair**. In XP, there is a deficiency in the specific **UV-specific endonucleases** (excinucleases) required to recognize and excise these bulky lesions. Failure to repair these dimers leads to mutations in proto-oncogenes and tumor suppressor genes, resulting in a 1000-fold increased risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). **Why other options are incorrect:** * **Mismatch Repair (MMR):** This pathway corrects errors that escape proofreading during DNA replication (e.g., C-T mismatches). Defects in MMR lead to **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). * **Base Excision Repair (BER):** This pathway repairs "small" non-bulky lesions like deaminated bases (e.g., Cytosine to Uracil) or oxidized bases. It utilizes **DNA Glycosylases**. * **Phagosomes:** These are cytoplasmic vesicles used by immune cells for engulfing and digesting pathogens; they have no role in DNA repair. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Severe sunburn on minimal sun exposure, "parchment-like" skin, excessive freckling, and early-onset skin cancers. * **Key Enzyme:** The most common defect is in the **XP-A to XP-G** genes, which code for proteins involved in the NER pathway. * **Associated Condition:** **Cockayne Syndrome** also involves defects in transcription-coupled NER but presents with "progeroid" (premature aging) features and "Mickey Mouse" facies rather than a high risk of skin cancer.

Question 377: What is a null mutation?

A. A mutation occurring in a non-coding region.
B. A mutation that does not change the amino acid or end product.
C. A mutation that codes for a change in progeny without any chromosomal change.
D. A mutation that leads to no functional gene product. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** A **null mutation** (also known as an **amorph**) is a type of mutation that results in the complete loss of function of a gene. This can occur through several mechanisms: * **Total deletion** of the gene locus. * **Nonsense mutations** that create a premature stop codon, leading to truncated, non-functional proteins. * **Frameshift mutations** that garble the genetic code. * **Splice-site mutations** that prevent mature mRNA formation. The hallmark of a null mutation is that the gene product is either not synthesized at all or is synthesized in a form that is biochemically inert. **2. Why Other Options are Incorrect:** * **Option A:** Mutations in non-coding regions (like introns or intergenic regions) may be silent or may affect regulation, but they are not defined as "null" unless they result in zero functional protein. * **Option B:** This describes a **Silent Mutation** (Synonymous mutation), where the genetic code changes but the amino acid remains the same due to the degeneracy of the genetic code. * **Option C:** This is a vague description that does not align with standard genetic terminology. Mutations, by definition, involve a change in the DNA sequence (genotype). **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Loss-of-Function (LOF):** Null mutations are the most extreme form of LOF mutations. They are typically **recessive** because the wild-type allele on the homologous chromosome can often compensate (haplosufficiency). * **Haploinsufficiency:** If a null mutation in one allele leads to a disease phenotype despite a normal second allele, it is called haploinsufficiency (e.g., **Familial Hypercholesterolemia**). * **Hypomorph:** A mutation that leads to a partial loss of function (reduced activity) rather than a total loss. * **Dominant Negative (Antimorph):** A mutation where the altered gene product antagonizes the function of the wild-type product (e.g., **Osteogenesis Imperfecta**).

Question 378: What is the function of helicase?

A. Reannealing of DNA
B. Unwinding of DNA (Correct Answer)
C. Synthesis of RNA primer
D. DNA polymerization

Explanation: **Explanation:** **DNA Helicase** is a crucial enzyme in DNA replication that functions by breaking the hydrogen bonds between complementary nitrogenous bases (Adenine-Thymine and Cytosine-Guanine). This process, known as **unwinding of the DNA double helix**, creates the replication fork and provides the single-stranded DNA templates necessary for DNA polymerase to act upon. In prokaryotes (E. coli), this function is primarily performed by **DnaB**. **Analysis of Incorrect Options:** * **Option A (Reannealing):** This is the opposite of helicase's function. Reannealing is prevented during replication by **Single-Stranded Binding (SSB) proteins**, which stabilize the unwound strands. * **Option C (Synthesis of RNA primer):** This is the function of **Primase** (DnaG in prokaryotes). A primer is essential because DNA polymerase cannot initiate synthesis *de novo*. * **Option D (DNA polymerization):** This is the function of **DNA Polymerases** (e.g., Pol III in prokaryotes, Pol $\delta$ and $\epsilon$ in eukaryotes), which add nucleotides to the growing DNA strand. **High-Yield Clinical Pearls for NEET-PG:** * **Energy Requirement:** Helicase is an ATP-dependent enzyme; it utilizes the energy from ATP hydrolysis to move along the phosphodiester backbone. * **Clinical Correlation:** Mutations in the **RecQ helicase family** lead to genomic instability syndromes characterized by premature aging and cancer predisposition, such as **Bloom Syndrome** (BLM gene), **Werner Syndrome** (WRN gene), and **Rothmund-Thomson Syndrome**. * **Directionality:** Helicases can be 5'→3' or 3'→5' depending on the specific enzyme and organism.

Question 379: What is telomerase?

A. DNA dependent RNA polymerase
B. RNA dependent DNA polymerase (Correct Answer)
C. RNA dependent RNA polymerase
D. DNA dependent DNA polymerase

Explanation: **Explanation:** **Telomerase** is a specialized enzyme responsible for maintaining the length of telomeres (the repetitive TTAGGG sequences at the ends of eukaryotic chromosomes). 1. **Why Option B is Correct:** Telomerase is a **ribonucleoprotein** complex. It contains an internal RNA template (known as TERC) that it uses to synthesize complementary DNA sequences. Because it uses an **RNA template** to synthesize **DNA**, it is functionally an **RNA-dependent DNA polymerase** (a type of reverse transcriptase). This activity prevents the "end-replication problem," where chromosomes shorten with each cell division. 2. **Why Other Options are Incorrect:** * **Option A (DNA-dependent RNA polymerase):** These enzymes (like RNA Polymerase I, II, and III) use DNA as a template to synthesize RNA (transcription). * **Option C (RNA-dependent RNA polymerase):** These are primarily found in RNA viruses (e.g., Poliovirus, SARS-CoV-2) to replicate their RNA genomes; they are not naturally occurring in human telomere maintenance. * **Option D (DNA-dependent DNA polymerase):** These are standard DNA polymerases (e.g., Pol $\alpha, \delta, \epsilon$) used in semi-conservative DNA replication. **High-Yield Clinical Pearls for NEET-PG:** * **Cellular Aging:** Telomerase is active in germ cells, stem cells, and cancer cells, but is **absent or low in most somatic cells**, leading to cellular senescence (the Hayflick limit). * **Cancer:** Approximately 85-90% of cancer cells upregulate telomerase to achieve **replicative immortality**. * **Components:** It consists of **TERT** (Telomerase Reverse Transcriptase - the catalytic protein) and **TERC** (Telomerase RNA component - the template). * **Clinical Correlation:** Mutations in telomerase components can lead to **Dyskeratosis Congenita**, characterized by bone marrow failure and premature aging.

Question 380: The same amino acid can be coded by multiple codons due to which phenomenon?

A. Degeneracy (Correct Answer)
B. Frame-shift mutation
C. Transcription
D. Mutation

Explanation: **Explanation:** The correct answer is **Degeneracy**. In the genetic code, there are 64 possible codons but only 20 standard amino acids. This redundancy, where multiple different codons specify the same single amino acid, is termed "Degeneracy." This phenomenon primarily occurs due to the **Wobble Hypothesis**, which states that the third base of the codon (the 3' end) has less stringent base-pairing requirements with the anticodon of tRNA. For example, Leucine, Serine, and Arginine are each coded by six different codons. **Analysis of Incorrect Options:** * **B. Frame-shift mutation:** This occurs when the addition or deletion of nucleotides (not in multiples of three) shifts the reading frame of the mRNA, usually resulting in a completely different protein sequence or a premature stop codon. * **C. Transcription:** This is the biological process of copying a segment of DNA into RNA by the enzyme RNA polymerase. It is a step in gene expression, not a property of the genetic code itself. * **D. Mutation:** This is a general term for any permanent alteration in the DNA sequence. While mutations can lead to changes in codons, they do not define the inherent redundancy of the genetic code. **High-Yield Clinical Pearls for NEET-PG:** * **Universality:** The genetic code is the same in almost all organisms (Exceptions: Mitochondrial DNA, where UGA codes for Tryptophan instead of a Stop codon). * **Non-overlapping & Commaless:** The code is read sequentially, three bases at a time, without skipping any bases. * **Unambiguous:** While one amino acid can have many codons (Degeneracy), **one specific codon always codes for only one amino acid.** * **Initiation Codon:** AUG (Methionine); **Stop Codons:** UAA, UAG, UGA.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

Practice Questions

Epigenetics and DNA Methylation

Practice Questions

RNA Processing and Splicing

Practice Questions

miRNA and RNA Interference

Practice Questions

Protein Synthesis and Post-Translational Modifications

Practice Questions

Genomics and Human Genome Project

Practice Questions

Single Nucleotide Polymorphisms

Practice Questions

Gene Therapy Approaches

Practice Questions

CRISPR-Cas9 and Genome Editing

Practice Questions

DNA Fingerprinting and Forensics

Practice Questions

Molecular Basis of Genetic Diseases

Practice Questions

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