Molecular Biology and Genomics Practice Questions

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

Nonsense suppressor mutation. ### 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.

Q: Which enzyme is responsible for DNA supercoiling?

DNA gyrase. **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**.

Q: 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?

Circular. **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.

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

Methylation. **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.

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

Nucleotide excision repair. **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.

Q: What is the function of helicase?

Unwinding of DNA. **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.

Q: What is telomerase?

RNA dependent DNA polymerase. **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.

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

Degeneracy. **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.

Question 1

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

Accepted Answer

Nonsense suppressor mutation

Answer

Silent mutation

Answer

Nonsense mutation

Answer

Missense mutation

Question 2

Which enzyme is responsible for DNA supercoiling?

Accepted Answer

DNA gyrase

Answer

DNA polymerase I

Answer

DNA polymerase II

Answer

DNA topoisomerase

Question 3

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?

Accepted Answer

Initiation of dystrophin transcription will be deficient.

Answer

Tailing of dystrophin mRNA will be defective.

Answer

Capping of dystrophin mRNA will be defective.

Answer

Termination of dystrophin transcription will be deficient.

Question 4

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?

Accepted Answer

Circular

Answer

Double-helix

Answer

Bounded by nuclear envelope

Answer

Packed into chromatin

Question 5

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

Accepted Answer

Methylation

Answer

Acetylation

Answer

Phosphorylation

Answer

DNA Replication

Question 6

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

Accepted Answer

Nucleotide excision repair

Answer

Mismatch repair

Answer

Base excision repair

Answer

Phagosomes

Question 7

What is a null mutation?

Accepted Answer

A mutation that leads to no functional gene product.

Answer

A mutation occurring in a non-coding region.

Answer

A mutation that does not change the amino acid or end product.

Answer

A mutation that codes for a change in progeny without any chromosomal change.

Question 8

What is the function of helicase?

Accepted Answer

Unwinding of DNA

Answer

Reannealing of DNA

Answer

Synthesis of RNA primer

Answer

DNA polymerization

Question 9

What is telomerase?

Accepted Answer

RNA dependent DNA polymerase

Answer

DNA dependent RNA polymerase

Answer

RNA dependent RNA polymerase

Answer

DNA dependent DNA polymerase

Question 10

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

Accepted Answer

Degeneracy

Answer

Frame-shift mutation

Answer

Transcription

Answer

Mutation

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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