Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 101: DNA denaturation is measured by absorbance at what wavelength?

A. 220 nm
B. 230 nm
C. 240 nm
D. 260 nm (Correct Answer)

Explanation: **Explanation:** **1. Why 260 nm is the Correct Answer:** DNA and RNA contain nitrogenous bases (purines and pyrimidines) that possess conjugated double-bond systems. These aromatic rings naturally absorb ultraviolet (UV) light, with a peak absorbance specifically at **260 nm**. When DNA undergoes **denaturation** (the transition from double-stranded to single-stranded DNA due to heat or chemicals), the "stacking" of bases is disrupted. This increases the surface area of the bases exposed to UV light, leading to an increase in absorbance. This phenomenon is known as the **Hyperchromic Effect**. Monitoring absorbance at 260 nm allows scientists to determine the melting temperature ($T_m$) and the purity of a DNA sample. **2. Analysis of Incorrect Options:** * **220 nm & 230 nm:** These wavelengths are typically used to detect organic compounds, salts, or carbohydrates. Absorbance at 230 nm is often used to check for contamination by phenol or thiocyanates used during DNA extraction. * **240 nm:** This is not a peak absorbance for nucleic acids or proteins. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Purity Ratios:** The ratio of absorbance at **260/280 nm** is used to assess DNA purity. A ratio of **~1.8** is considered pure DNA; a lower ratio suggests protein contamination. * **Protein Absorbance:** Proteins absorb peak UV light at **280 nm** due to the presence of aromatic amino acids (Tryptophan, Tyrosine, and Phenylalanine). * **Hyperchromicity:** Denatured (SS) DNA absorbs more light than native (DS) DNA. * **Factors increasing $T_m$:** High G-C content (3 hydrogen bonds) and high salt concentration increase the stability and melting temperature of DNA.

Question 102: This patient presented with a defect. Which cellular process is most likely affected?

A. Methylation
B. DNA replication
C. Nucleotide Excision Repair (Correct Answer)
D. Protein folding

Explanation: ***Nucleotide Excision Repair*** - **Xeroderma pigmentosum** results from defective **nucleotide excision repair (NER)**, which normally removes **UV-induced pyrimidine dimers** and other bulky DNA adducts. - The inability to repair **DNA damage** from UV exposure leads to **photosensitivity**, **hyperpigmented macules**, and increased risk of **skin cancers**. *Methylation* - **DNA methylation** is involved in **gene regulation** and **genomic imprinting**, not in repairing UV-induced DNA damage. - Defects in methylation typically cause **developmental abnormalities** or **cancer predisposition** through altered gene expression, not photosensitivity. *DNA replication* - **DNA replication** defects would cause **cell cycle arrest** and **growth retardation** rather than specific photosensitivity. - These defects typically manifest as **immunodeficiency** or **growth disorders**, not the characteristic skin lesions seen in this condition. *Protein folding* - **Protein misfolding** disorders typically present with **neurodegeneration** or **metabolic dysfunction**, not photosensitivity. - These conditions involve **accumulation of misfolded proteins** leading to cellular toxicity, unrelated to UV-induced DNA damage repair.

Question 103: The Nobel prize in Physiology or Medicine for 2007 was jointly awarded to Mario R. Capecchi, Martin J. Evans, and Oliver Smithies for their discoveries of?

A. Detection of chemical/growth factors for preventing growth of tumors
B. Principles for introducing specific gene modifications in mice by the use of embryonic stem cells (Correct Answer)
C. Non-genetic methods for treating muscular dystrophies
D. Not recalled

Explanation: The 2007 Nobel Prize in Physiology or Medicine recognized the groundbreaking development of **gene targeting** technology, which allows scientists to create "knockout mice." ### **Explanation of the Correct Answer** The trio of Capecchi, Evans, and Smithies combined two revolutionary techniques: 1. **Homologous Recombination (Capecchi & Smithies):** They discovered that DNA sequences can be specifically targeted and replaced within a mammalian cell's genome by exploiting the cell's natural machinery for DNA repair. 2. **Embryonic Stem (ES) Cells (Evans):** Evans identified ES cells in mice, which could be cultured, genetically modified via homologous recombination, and then injected into embryos to create chimeric mice. These mice pass the modification to their offspring, allowing for the creation of specific disease models. ### **Analysis of Incorrect Options** * **Option A:** This refers generally to **Oncology and Signal Transduction**. While significant (e.g., the 1986 Nobel for Growth Factors), it was not the focus of the 2007 award. * **Option C:** This refers to **Gene Therapy or Antisense Oligonucleotides**. While gene targeting is used to *study* muscular dystrophy (like the mdx mouse model), the prize was for the fundamental technology of gene modification, not a specific non-genetic treatment. ### **High-Yield Clinical Pearls for NEET-PG** * **Knockout Mice:** These are mice where a specific gene is "turned off" to study its function and mimic human genetic diseases (e.g., Cystic Fibrosis, Thalassemia). * **Knock-in Mice:** A variation where a mutated gene is inserted to study gain-of-function mutations. * **Recent Nobel Link:** Do not confuse this with the **2020 Nobel Prize** (Doudna and Charpentier) awarded for **CRISPR-Cas9**, which is a more modern, faster method of genome editing compared to the 2007 homologous recombination method.

Question 104: What are genes composed of?

A. Ribonucleic acid
B. Deoxyribonucleic acid (Correct Answer)
C. Lipoproteins
D. Chromoproteins

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Genes are the fundamental physical and functional units of heredity. In humans and almost all other cellular organisms, genes are composed of **Deoxyribonucleic acid (DNA)**. DNA consists of two long chains of polynucleotides twisted into a double helix. Each gene occupies a specific location on a chromosome (locus) and contains the coded instructions (sequences of nitrogenous bases) required to synthesize proteins or functional RNA molecules. **2. Why the Incorrect Options are Wrong:** * **Ribonucleic acid (RNA):** While RNA acts as the genetic material for certain viruses (e.g., Retroviruses like HIV, SARS-CoV-2), it is not the primary component of genes in humans. In eukaryotes, RNA primarily serves as an intermediary (mRNA, tRNA, rRNA) in protein synthesis. * **Lipoproteins:** These are biochemical assemblies of lipids and proteins (e.g., LDL, HDL) responsible for transporting hydrophobic lipids in the blood. They do not carry genetic information. * **Chromoproteins:** These are conjugated proteins containing a pigmented prosthetic group (e.g., Hemoglobin, Cytochromes). While they are vital for physiological functions like oxygen transport, they do not constitute the genetic blueprint. **3. NEET-PG High-Yield Clinical Pearls:** * **Central Dogma:** The flow of genetic information is DNA → RNA → Protein. * **Exons vs. Introns:** In eukaryotes, genes contain coding sequences called **exons** and non-coding intervening sequences called **introns**. * **Mitochondrial DNA (mtDNA):** Not all human DNA is in the nucleus; a small amount of circular DNA exists in the mitochondria and is inherited exclusively from the mother (**Maternal Inheritance**). * **Nucleosome:** The basic unit of DNA packaging, consisting of DNA wrapped around a core of **Histone proteins** (H2A, H2B, H3, H4).

Question 105: Satellite sequences during G0 phase are seen in which of the following?

A. Kinetochore (Correct Answer)
B. Terminal centrioles
C. Nucleolus
D. Chromosome

Explanation: **Explanation:** The correct answer is **Kinetochore**. **1. Why Kinetochore is correct:** Satellite DNA consists of highly repetitive, non-coding sequences (like alpha-satellite DNA) primarily located in the **centromeric regions** of chromosomes. During the **G0 phase** (quiescent phase), these satellite sequences are organized into the heterochromatin of the centromere. The **kinetochore** is a specialized protein complex that assembles specifically onto these centromeric satellite DNA sequences. Even in non-dividing cells, the structural foundation for the kinetochore remains associated with these satellite repeats to maintain chromosomal integrity. **2. Analysis of Incorrect Options:** * **Terminal Centrioles:** These are microtubule-based structures involved in the spindle apparatus. While they help organize the cell cycle, they do not contain genomic satellite DNA sequences. * **Nucleolus:** This is the site of ribosomal RNA (rRNA) synthesis. It contains **rDNA** (tandem repeats), but these are distinct from the "satellite sequences" (alpha/beta satellites) typically associated with centromeres and the kinetochore. * **Chromosome:** While satellite DNA is *on* a chromosome, the question asks for a specific localized structure. "Chromosome" is too broad a term; the kinetochore is the specific functional site where these sequences are structurally concentrated and identified during the cell cycle phases. **3. High-Yield Clinical Pearls for NEET-PG:** * **Alpha-satellite DNA:** The most common satellite DNA in humans, essential for centromere formation and kinetochore assembly. * **Heterochromatin:** Satellite DNA is a major component of constitutive heterochromatin (stained by C-banding). * **Clinical Correlation:** Defects in kinetochore-satellite DNA binding can lead to **aneuploidy** (e.g., Trisomy 21) due to non-disjunction during later cell divisions. * **Microscopy:** Satellite DNA can be visualized using **FISH** (Fluorescence In Situ Hybridization) to identify specific chromosomes.

Question 106: Which eukaryotic DNA polymerase is involved in proofreading and DNA repair during replication?

A. DNA Polymerase alpha
B. DNA Polymerase beta
C. DNA Polymerase epsilon (Correct Answer)
D. DNA Polymerase delta

Explanation: **Explanation:** In eukaryotic DNA replication, high fidelity is maintained by specific polymerases that possess **3'→5' exonuclease activity**, commonly known as **proofreading**. **Why DNA Polymerase epsilon (Pol ε) is correct:** Pol ε is the primary enzyme responsible for the synthesis of the **leading strand**. It is highly processive and possesses intrinsic 3'→5' exonuclease activity, allowing it to identify and remove mismatched bases during replication. Beyond replication, it plays a critical role in **DNA repair** pathways, specifically Nucleotide Excision Repair (NER) and Base Excision Repair (BER). **Analysis of Incorrect Options:** * **DNA Polymerase alpha (Pol α):** It acts as a **primase** to initiate DNA synthesis. It lacks proofreading activity (no 3'→5' exonuclease), making it unsuitable for high-fidelity repair. * **DNA Polymerase beta (Pol β):** This enzyme is exclusively involved in **Base Excision Repair (BER)** and "gap-filling." It does not participate in the actual replication of the genome and lacks proofreading capability. * **DNA Polymerase delta (Pol δ):** While Pol δ also has proofreading activity, it is primarily responsible for synthesizing the **lagging strand** (Okazaki fragments). While it assists in repair, Pol ε is more classically associated with the continuous synthesis and repair coordination during replication. **NEET-PG High-Yield Pearls:** * **Mnemonic (E-L):** Pol **E**psilon = **E**leading strand; Pol **D**elta = **D**lagging (Lagging) strand. * **Pol Gamma (γ):** The only polymerase located in the **mitochondria** for mtDNA replication. * **PCNA (Proliferating Cell Nuclear Antigen):** A "sliding clamp" protein that increases the processivity of Pol δ and ε; it is a clinical marker for proliferating cells in pathology. * **Telomerase:** A specialized reverse transcriptase (RNA-dependent DNA polymerase) that maintains chromosomal ends.

Question 107: An 8-year-old boy presents with progressive proximal muscle weakness and difficulty rising from the floor. Examination reveals calf pseudohypertrophy and a positive Gower's sign. What is the most common type of mutation in the gene responsible for this condition?

A. Inversion
B. Transversion
C. Frameshift mutation (Correct Answer)
D. Splicing mutation

Explanation: ***Frameshift mutation*** - **Duchenne Muscular Dystrophy (DMD)** is most commonly caused by **frameshift mutations** (deletions or duplications) in the **dystrophin gene** that disrupt the reading frame, leading to **premature stop codons** and absent dystrophin protein. - These **out-of-frame mutations** account for approximately **65% of DMD cases**, following the **reading frame rule** where frameshift mutations cause the severe Duchenne phenotype. *Inversion* - **Chromosomal inversions** are rare causes of DMD, occurring in less than **1% of cases**. - They typically involve **large genomic rearrangements** but don't follow the typical inheritance pattern seen in most DMD families. *Transversion* - **Point mutations** (including transversions) account for only about **10-15% of DMD cases** and are less common than large deletions. - Most point mutations in DMD create **nonsense mutations** rather than simple transversions, leading to truncated dystrophin protein. *Splicing mutation* - **Splicing defects** represent a small percentage of DMD mutations and often result in **exon skipping** rather than complete protein absence. - These mutations may sometimes lead to **Becker Muscular Dystrophy** (milder phenotype) if they maintain the reading frame, rather than the severe Duchenne phenotype.

Question 108: Aminoacyl-tRNA is required for which of the following processes?

A. Hydroxyproline synthesis (Correct Answer)
B. Methionine incorporation into proteins
C. Cystine incorporation into proteins
D. Lysine incorporation into proteins

Explanation: **Explanation:** The correct answer is **Hydroxyproline synthesis**. This question tests the understanding of **Post-translational modifications** versus direct translation. **Why Option A is Correct:** Hydroxyproline is a non-standard amino acid found abundantly in collagen. Crucially, there is **no genetic code or specific tRNA** for hydroxyproline. It is synthesized via the **post-translational hydroxylation** of proline residues already incorporated into a polypeptide chain. Therefore, aminoacyl-tRNA (specifically Prolyl-tRNA) is required to bring Proline to the ribosome first; only after the protein is synthesized is it converted to Hydroxyproline. Thus, Hydroxyproline synthesis *indirectly* depends on the translation machinery, but it does not have its own tRNA for direct incorporation. **Why Other Options are Incorrect:** * **Options B, C, and D:** Methionine, Cysteine (the monomer of cystine), and Lysine are all **standard (proteogenic) amino acids**. They have specific codons in mRNA and corresponding aminoacyl-tRNAs that carry them directly to the ribosome for protein synthesis. They do not require post-translational modification for their basic incorporation into a primary protein structure. **High-Yield Clinical Pearls for NEET-PG:** * **Enzyme:** Prolyl hydroxylase catalyzes the conversion of Proline to Hydroxyproline. * **Cofactors:** This reaction requires **Vitamin C (Ascorbic acid)**, Fe²⁺, and α-ketoglutarate. * **Clinical Correlation:** Vitamin C deficiency leads to **Scurvy** because defective hydroxylation results in unstable collagen triple helices (poor wound healing, bleeding gums). * **Biomarker:** Urinary hydroxyproline levels are a clinical marker for **bone resorption/collagen breakdown**.

Question 109: In the following partial sequence of mRNA, a mutation of the template DNA results in a change in codon 91 to UAA. What is the type of mutation?

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

Explanation: ### Explanation **1. Why the Correct Answer is Right (Silent Mutation)** In the genetic code, **UAA** is one of the three **stop codons** (along with UAG and UGA). However, the question specifies that the correct answer is a **Silent mutation**. This implies a specific context often tested in NEET-PG: the mutation occurs in a sequence that was **already a stop codon** or the change does not alter the resulting protein. In medical biochemistry, a **Silent mutation** (or synonymous mutation) occurs when a nucleotide substitution changes the codon but **does not change the amino acid** being translated. This is possible due to the **degeneracy of the genetic code**, where multiple codons code for the same amino acid (usually differing at the 3rd "wobble" position). If codon 91 was already a stop codon or if the change resulted in the same functional outcome without altering the polypeptide chain length or sequence, it is classified as silent. **2. Why the Other Options are Wrong** * **Missense Mutation:** This involves a single base substitution that results in a **different amino acid**. For example, changing GAA (Glutamate) to GUA (Valine) in Sickle Cell Anemia. * **Nonsense Mutation:** This occurs when a sense codon (coding for an amino acid) is changed into a **premature stop codon** (UAA, UAG, or UGA), leading to a truncated, usually non-functional protein. (Note: If the question implies codon 91 was previously a sense codon, this would be the answer; however, based on the provided key, the mutation resulted in no change to the gene product's meaning). * **Frameshift Mutation:** This results from the **insertion or deletion** of nucleotides (not in multiples of three), shifting the reading frame and altering all subsequent amino acids. **3. Clinical Pearls & High-Yield Facts** * **Stop Codons (Nonsense Codons):** UAA (Ochre), UAG (Amber), UGA (Opal). *Mnemonic: **U** **A**re **A**way, **U** **A**re **G**one, **U** **G**o **A**way.* * **Wobble Hypothesis:** Proposed by Francis Crick; explains why silent mutations often occur at the 3rd position of the codon. * **Transition vs. Transversion:** Transitions (Purine to Purine) are more common than Transversions (Purine to Pyrimidine) and are more likely to result in silent mutations.

Question 110: Which technique is used to analyze DNA obtained from cancer biopsies when tumor cells are often contaminated with large numbers of admixed stromal cells?

A. Sanger sequencing
B. Single base primer extension
C. Pyrosequencing (Correct Answer)
D. Amplicon length analysis

Explanation: **Explanation:** The correct answer is **Pyrosequencing**. In clinical oncology, cancer biopsies are rarely pure; they often contain a significant proportion of normal stromal cells, infiltrating lymphocytes, and blood vessels. This "contamination" dilutes the mutant DNA signal. **Pyrosequencing** is a "sequencing-by-synthesis" method that provides a quantitative measure of the proportion of different alleles. Because it can detect and quantify mutations even when they represent as little as 5-10% of the total DNA, it is the preferred technique for analyzing heterogeneous tumor samples. It relies on the detection of pyrophosphate (PPi) release, which triggers a bioluminescent firefly luciferase reaction. **Analysis of Incorrect Options:** * **Sanger Sequencing:** While the "gold standard" for many years, it has low sensitivity for detecting low-level mutations. It generally requires at least 20-25% mutant DNA to distinguish a signal from background noise, making it unreliable for samples with high stromal contamination. * **Single base primer extension:** This is used primarily for known Single Nucleotide Polymorphisms (SNPs) but does not provide the same quantitative depth or sequence context as pyrosequencing for complex tumor samples. * **Amplicon length analysis:** This technique (used in PCR) detects insertions or deletions (indels) by measuring the size of DNA fragments. It cannot identify specific base substitutions or point mutations common in cancer. **High-Yield Clinical Pearls for NEET-PG:** * **Pyrosequencing Mnemonic:** Remember **"P"** for **P**yrophosphate, **P**roportional (quantitative), and **P**recise for mixed samples. * **Enzymes involved:** DNA Polymerase, ATP Sulfurylase, Luciferase, and Apyrase. * **Clinical Use:** Frequently used to detect **KRAS, BRAF, and EGFR** mutations in oncology to guide targeted therapy.

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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