Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 511: Which of the following is an example of post-translational modification?

A. Methylation
B. Oxidation
C. Phosphorylation (Correct Answer)
D. Splicing

Explanation: **Explanation:** **Post-translational modification (PTM)** refers to the covalent and generally enzymatic modification of proteins following protein biosynthesis. These modifications occur after the polypeptide chain has been synthesized on the ribosome and are crucial for protein folding, stability, localization, and biological activity. **Why Phosphorylation is Correct:** **Phosphorylation** is the most common PTM. It involves the addition of a phosphate group (usually to Serine, Threonine, or Tyrosine residues) by enzymes called **kinases** and its removal by **phosphatases**. This process acts as a molecular "on/off" switch, regulating enzyme activity and signal transduction pathways (e.g., the Insulin signaling pathway). **Analysis of Incorrect Options:** * **Methylation:** While it can occur on proteins (like histones), in the context of standard NEET-PG biochemistry, it is more frequently discussed as a **pre-translational/epigenetic** modification of DNA (Cytosine residues) to regulate gene expression. * **Oxidation:** This is generally a form of **protein damage** caused by reactive oxygen species (ROS) rather than a regulated, functional post-translational modification. * **Splicing:** This is a **post-transcriptional** modification. It involves the removal of introns and joining of exons from the primary RNA transcript (hnRNA) to form mature mRNA. **High-Yield Clinical Pearls for NEET-PG:** * **Zymogen Activation:** Proteolysis (e.g., Trypsinogen to Trypsin) is an irreversible PTM. * **Gamma-carboxylation:** Occurs on Glutamate residues of Clotting Factors II, VII, IX, and X; it requires **Vitamin K** as a cofactor. * **Hydroxylation:** Proline and Lysine residues in collagen require **Vitamin C** for hydroxylation; deficiency leads to Scurvy. * **Glycosylation:** Occurs in the ER and Golgi apparatus; it is essential for membrane protein function.

Question 512: Which of the following statements about DNA is true?

A. The two strands are held together by peptide bonds.
B. The sugar-phosphate backbone is stabilized by non-covalent bonds.
C. The most common form of DNA is Z-DNA.
D. The melting point of DNA is closely related to its cytosine and guanine content. (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** The melting point ($T_m$) of DNA is the temperature at which 50% of the double-stranded DNA denatures into single strands. This is directly proportional to the **Guanine-Cytosine (G-C) content**. G-C pairs are held together by **three hydrogen bonds**, whereas Adenine-Thymine (A-T) pairs have only two. Therefore, DNA with higher G-C content requires more thermal energy to break these additional bonds, resulting in a higher $T_m$. **2. Why the Other Options are Incorrect:** * **Option A:** The two strands are held together by **hydrogen bonds** between nitrogenous bases, not peptide bonds (which are found in proteins). * **Option B:** The sugar-phosphate backbone is held together by **3'–5' phosphodiester bonds**, which are strong **covalent bonds**. Non-covalent interactions (like hydrophobic stacking and H-bonds) stabilize the double helix structure, but not the backbone itself. * **Option C:** The most common physiological form of DNA is **B-DNA** (Right-handed helix). Z-DNA is a rare, left-handed helix associated with specific gene expression patterns. **3. High-Yield Clinical Pearls for NEET-PG:** * **Chargaff’s Rule:** In double-stranded DNA, A=T and G=C; therefore, Purines (A+G) = Pyrimidines (T+C). * **Hyperchromicity:** When DNA denatures (melts), its absorbance of UV light at **260 nm increases**. This is used to monitor the melting process. * **DNA Forms:** * **B-DNA:** Right-handed, 10.5 base pairs per turn (Standard). * **A-DNA:** Right-handed, seen in DNA-RNA hybrids. * **Z-DNA:** Left-handed, zig-zag backbone. * **Renaturation (Annealing):** The process of two strands coming back together, which is the basis for PCR and Southern Blotting.

Question 513: "Proofreading" is the role of?

A. DNA primase
B. DNA Polymerase (Correct Answer)
C. Exonuclease I
D. Restriction endonuclease

Explanation: **Explanation:** **Why DNA Polymerase is correct:** Proofreading is the essential mechanism by which a cell ensures high fidelity during DNA replication. This function is performed by **DNA Polymerases** (specifically DNA Pol I, II, and III in prokaryotes; Pol $\delta$ and $\epsilon$ in eukaryotes). These enzymes possess **3' to 5' exonuclease activity**, which allows them to "backspace" and remove a mismatched nucleotide immediately after it is incorrectly incorporated. Once the error is excised, the polymerase resumes its 5' to 3' synthetic activity to insert the correct base. **Why the other options are incorrect:** * **DNA Primase:** This is an RNA polymerase that synthesizes short RNA primers required to initiate DNA synthesis. It lacks proofreading capabilities. * **Exonuclease I:** While it is an exonuclease, its primary role in *E. coli* is the degradation of single-stranded DNA from the 3' end. It is not the primary enzyme responsible for the co-replicational proofreading process. * **Restriction Endonuclease:** These are "molecular scissors" used by bacteria to cleave double-stranded DNA at specific palindromic sequences as a defense against viral DNA. They are not involved in replication fidelity. **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** Remember, DNA synthesis occurs **5' $\rightarrow$ 3'**, but proofreading (exonuclease activity) occurs **3' $\rightarrow$ 5'**. * **Mismatch Repair (MMR):** If DNA polymerase fails to catch an error, the MMR system takes over. Mutations in MMR genes (like *MSH2*, *MLH1*) lead to **Hereditary Non-Polyposis Colorectal Cancer (HNPCC/Lynch Syndrome)**. * **Xeroderma Pigmentosum:** Caused by a defect in **Nucleotide Excision Repair (NER)**, which fixes pyrimidine dimers caused by UV light.

Question 514: Which organelle is primarily responsible for the catabolism of hydrogen peroxide (H2O2)?

A. Peroxisomes (Correct Answer)
B. Mitochondria
C. Endoplasmic reticulum
D. Lysosomes

Explanation: **Explanation:** **1. Why Peroxisomes are correct:** Peroxisomes (also known as microbodies) are membrane-bound organelles specialized for oxidative reactions. They contain high concentrations of **Catalase**, the primary enzyme responsible for the catabolism of hydrogen peroxide ($H_2O_2$) into water and oxygen ($2H_2O_2 \rightarrow 2H_2O + O_2$). This process is vital for protecting the cell from oxidative damage caused by reactive oxygen species (ROS) generated during the $\beta$-oxidation of very-long-chain fatty acids (VLCFA). **2. Why other options are incorrect:** * **Mitochondria:** While mitochondria produce $H_2O_2$ as a byproduct of the electron transport chain and contain Glutathione peroxidase, their primary role is ATP production via oxidative phosphorylation, not the specialized catabolism of $H_2O_2$. * **Endoplasmic Reticulum (ER):** The ER is primarily involved in protein synthesis (RER), lipid synthesis, and detoxification via the Cytochrome P450 system (SER), but it lacks the high catalase activity characteristic of peroxisomes. * **Lysosomes:** These are the "suicide bags" of the cell, containing acid hydrolases for the degradation of macromolecules (proteins, lipids, polysaccharides) at an acidic pH. They do not play a role in peroxide metabolism. **3. High-Yield Clinical Pearls for NEET-PG:** * **Zellweger Syndrome:** A rare autosomal recessive disorder caused by a deficiency in peroxisome biogenesis (PEX gene mutations), leading to the accumulation of VLCFAs. * **Adrenoleukodystrophy (X-linked):** Defective transport of VLCFAs into peroxisomes, causing demyelination and adrenal insufficiency. * **Marker Enzyme:** Catalase is the gold-standard biochemical marker for identifying peroxisomes in cell biology.

Question 515: A frameshift mutation does not affect the complete amino acid sequence if it occurs in multiples of which number?

A. 1
B. 2
C. 3 (Correct Answer)
D. None of the above

Explanation: ### Explanation The genetic code is **triplet** in nature, meaning that a sequence of three nucleotides (a **codon**) codes for a single amino acid. During translation, the mRNA is read sequentially in a specific **reading frame**. **Why 3 is the Correct Answer:** A **frameshift mutation** occurs when nucleotides are inserted or deleted in a number that is not a multiple of three. This shifts the entire reading frame downstream of the mutation, altering every subsequent codon and usually resulting in a premature stop codon (nonsense mutation). However, if an insertion or deletion occurs in a **multiple of 3** (e.g., 3, 6, 9 nucleotides), it results in the addition or loss of whole amino acids without shifting the reading frame for the rest of the protein. This is technically an **in-frame mutation**, preserving the integrity of the remaining amino acid sequence. **Analysis of Incorrect Options:** * **Options A (1) and B (2):** Inserting or deleting 1 or 2 nucleotides disrupts the triplet grouping. For example, deleting 1 nucleotide shifts the frame "+1," changing every codon thereafter. These are the classic causes of devastating frameshift mutations. * **Option D:** Incorrect, as the triplet nature of the codon dictates the rule of three. **Clinical Pearls for NEET-PG:** * **Cystic Fibrosis:** The most common mutation (**ΔF508**) is an **in-frame deletion** of 3 nucleotides (one codon), leading to the loss of Phenylalanine. * **Duchenne vs. Becker Muscular Dystrophy:** * **Duchenne (DMD):** Usually caused by **frameshift mutations** (deletions not in multiples of 3), leading to a truncated, non-functional dystrophin protein (Severe). * **Becker (BMD):** Usually caused by **in-frame mutations** (deletions in multiples of 3), leading to a shorter but partially functional protein (Milder).

Question 516: Which statement is TRUE regarding prophase of meiosis I?

A. Chromosomes separate
B. The resultant cell is diploid (Correct Answer)
C. The resultant cell is haploid
D. Sister chromatids replicate

Explanation: ### Explanation **Correct Option: B. The resultant cell is diploid** In the context of the cell cycle, **Prophase I** is the first stage of Meiosis I. At this stage, the cell has already undergone DNA replication during the S-phase but has not yet completed its first division. Therefore, the cell still contains the full **diploid (2n)** complement of chromosomes (46 chromosomes in humans), although each chromosome consists of two sister chromatids (4n DNA content). The reduction from diploid to haploid only occurs *after* the completion of Meiosis I (Telophase I/Cytokinesis). --- ### Analysis of Incorrect Options: * **A. Chromosomes separate:** This occurs during **Anaphase**. In Anaphase I, homologous chromosomes separate; in Anaphase II (and Mitosis), sister chromatids separate. Prophase is characterized by condensation, not separation. * **C. The resultant cell is haploid:** This is the outcome of the **entire Meiosis I division**, not Prophase I. The transition to haploidy (n) happens when homologous pairs are pulled to opposite poles and the cell divides. * **D. Sister chromatids replicate:** DNA replication occurs during the **S-phase of Interphase**, which precedes Meiosis. No DNA replication occurs during any phase of Prophase. --- ### NEET-PG High-Yield Pearls: 1. **Sub-stages of Prophase I:** Remember the mnemonic **"Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis"** (L-Z-P-D-D). 2. **Pachytene:** This is the most high-yield sub-stage where **crossing over** (genetic recombination) occurs via the enzyme **recombinase**. 3. **Diplotene:** This is where **Chiasmata** become visible. In females, primary oocytes are arrested in the Diplotene stage (Dictyotene) from birth until ovulation. 4. **DNA vs. Chromosome Count:** In Prophase I, the cell is **2n** (diploid) but **4C** (four times the haploid DNA content).

Question 517: Klinefelter syndrome is diagnosed by?

A. Karyotyping (Correct Answer)
B. USG abdomen
C. Triple test
D. Echocardiography

Explanation: **Explanation:** **Klinefelter syndrome (47, XXY)** is a chromosomal aneuploidy characterized by the presence of one or more extra X chromosomes in a male phenotype. 1. **Why Karyotyping is the Correct Answer:** Karyotyping is the **gold standard** for diagnosing chromosomal abnormalities. It involves visualizing the complete set of chromosomes in a cell (usually from peripheral blood lymphocytes) during metaphase. In Klinefelter syndrome, karyotyping reveals the classic **47, XXY** pattern (or mosaics like 46, XY/47, XXY), confirming the numerical chromosomal aberration that defines the disorder. 2. **Why Other Options are Incorrect:** * **USG Abdomen:** While it may be used to look for undescended testes or assess pelvic structures, it cannot identify the genetic cause. * **Triple Test:** This is a prenatal screening tool (measuring AFP, hCG, and estriol) used to assess the risk of Down syndrome or neural tube defects; it is not diagnostic for Klinefelter syndrome. * **Echocardiography:** Used to detect structural heart defects (like Mitral Valve Prolapse, which can occur in Klinefelter), but it is a supportive investigation, not a diagnostic one. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Features:** Tall stature, gynecomastia, small firm testes (testicular dysgenesis), and infertility (azoospermia). * **Biochemical Profile:** Hypergonadotropic hypogonadism (↑ FSH, ↑ LH, ↓ Testosterone) due to Leydig cell dysfunction and seminiferous tubule hyalinization. * **Barr Body:** Positive (unlike normal males) due to the extra X chromosome. * **Risk:** Increased risk of male breast cancer and extragonadal germ cell tumors.

Question 518: Gene amplification is achieved by which technique?

A. Polymerase Chain Reaction (Correct Answer)
B. Ligase chain reaction
C. DNA hybridization
D. In situ hybridization

Explanation: **Explanation:** **1. Why Polymerase Chain Reaction (PCR) is Correct:** PCR is an *in vitro* enzymatic method used to produce millions of copies of a specific DNA segment from a minute amount of starting material. It is the gold standard for **gene amplification**. The process involves three repetitive steps: **Denaturation** (separating strands at high heat), **Annealing** (primers binding to target sequences), and **Extension** (Taq polymerase synthesizing new DNA). This results in exponential amplification ($2^n$ copies, where $n$ is the number of cycles). **2. Why the Other Options are Incorrect:** * **Ligase Chain Reaction (LCR):** While it also amplifies nucleic acids, it primarily uses DNA ligase to join two oligonucleotides. It is used more for detecting specific **point mutations** rather than general gene amplification. * **DNA Hybridization:** This is a technique where a labeled probe binds to a complementary DNA sequence. It is used for **detection and identification** (e.g., Southern Blotting), not for increasing the quantity of the gene. * **In situ Hybridization (ISH):** This technique (like FISH) uses probes to locate specific DNA/RNA sequences directly within a tissue section or cell. It is a **localization** tool, not an amplification tool. **Clinical Pearls for NEET-PG:** * **Taq Polymerase:** Derived from *Thermus aquaticus*; it is heat-stable, which is essential for the high temperatures of PCR. * **RT-PCR:** Used for amplifying RNA (e.g., diagnosing **SARS-CoV-2** or HIV viral load) by first converting RNA to cDNA using Reverse Transcriptase. * **Real-Time PCR (qPCR):** Allows for the quantification of DNA as the amplification occurs. * **Gene Amplification in Cancer:** Naturally occurring gene amplification (e.g., **N-myc** in Neuroblastoma or **HER2/neu** in Breast Cancer) is a hallmark of many malignancies.

Question 519: Nonsense mutation is seen in which of the following conditions?

A. AIHA
B. Thalassemia (Correct Answer)
C. Sickle cell anemia
D. Hemophilia

Explanation: ### Explanation **Correct Answer: B. Thalassemia** **1. Why Thalassemia is correct:** A **nonsense mutation** occurs when a single nucleotide substitution results in a premature stop codon (UAG, UAA, or UGA). This leads to the production of a truncated, non-functional protein. In **$\beta$-Thalassemia**, nonsense mutations are a common cause of the $\beta^0$ phenotype (total absence of $\beta$-globin chain production). For example, a mutation at codon 39 ($CAG \to UAG$) transforms a Glutamine codon into a stop codon, halting translation prematurely and leading to severe disease. **2. Analysis of Incorrect Options:** * **A. AIHA (Autoimmune Hemolytic Anemia):** This is an acquired condition caused by antibodies against RBC antigens, not a primary genetic mutation of the globin chain. * **C. Sickle Cell Anemia:** This is the classic example of a **missense mutation**. A single base substitution ($GAG \to GTG$) at the 6th position of the $\beta$-globin gene replaces Glutamic acid with Valine. * **D. Hemophilia:** While Hemophilia A and B can involve various mutations (including nonsense), the most characteristic genetic defect in severe Hemophilia A is an **intron inversion** (specifically Intron 22 inversion), not primarily a nonsense mutation in the context of standard comparative biochemistry questions. **3. High-Yield Clinical Pearls for NEET-PG:** * **Point Mutations:** * **Silent:** Same amino acid (due to degeneracy of genetic code). * **Missense:** Different amino acid (e.g., Sickle Cell, HbC). * **Nonsense:** Premature stop codon (e.g., $\beta$-Thalassemia). * **Frameshift Mutation:** Insertion or deletion of nucleotides (not in multiples of 3), seen in **Tay-Sachs disease** and certain forms of Thalassemia. * **Trinucleotide Repeat Expansion:** Seen in Huntington’s (CAG) and Fragile X (CGG).

Question 520: Fluorescence in situ hybridization (FISH) is used for which of the following applications?

A. Gene mapping
B. Study of 3D chromosome organization in interphase nuclei
C. Monitoring the success of bone marrow transplantation
D. All the above (Correct Answer)

Explanation: **Fluorescence in situ hybridization (FISH)** is a cytogenetic technique that uses fluorescent probes that bind to only those parts of a nucleic acid sequence with a high degree of sequence complementarity. It bridges the gap between conventional cytogenetics (karyotyping) and molecular biology. ### **Explanation of Options:** * **Gene Mapping:** FISH is a fundamental tool for physical mapping of the genome. By using sequence-specific probes, scientists can visualize the exact chromosomal location (locus) of a specific gene. * **3D Chromosome Organization:** Unlike traditional karyotyping which requires metaphase cells, FISH can be performed on **interphase nuclei**. This allows researchers to study "chromosome territories" and how chromatin is spatially organized within the nucleus, which is crucial for understanding gene regulation. * **Monitoring Bone Marrow Transplantation:** In sex-mismatched transplants (e.g., female donor to male recipient), FISH for X and Y chromosomes is used to calculate the percentage of donor versus recipient cells (chimerism). This helps in assessing graft take or detecting early relapse. Since all the applications listed are valid uses of the technology, **Option D** is the correct answer. ### **High-Yield Clinical Pearls for NEET-PG:** * **Speed:** FISH is faster than karyotyping because it does not always require cell culture (can be done on interphase cells). * **Common Clinical Uses:** * **Aneuploidy screening:** Rapid detection of Trisomy 21, 18, 13. * **Microdeletion syndromes:** Best test for **Prader-Willi, Angelman, and DiGeorge syndromes** (where the deletion is too small for karyotyping). * **Cancer Genetics:** Detecting the **BCR-ABL** translocation in CML or **HER2/neu** amplification in breast cancer. * **Limitation:** You must know what you are looking for (must have a specific probe); it is not a "global" screening tool like Chromosomal Microarray (CMA).

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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