Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 11: Mitochondrial DNA is transmitted from which parent?

A. Mother (Correct Answer)
B. Father
C. Grandfather
D. Grandmother

Explanation: **Explanation:** The correct answer is **Mother (Option A)**. This is based on the principle of **Maternal Inheritance** (or cytoplasmic inheritance). During fertilization, the ovum contributes the vast majority of the cytoplasm and organelles to the zygote, including thousands of mitochondria. While the sperm does contain mitochondria in its midpiece to power motility, these are typically tagged with ubiquitin and degraded by the oocyte's proteasomes shortly after fertilization. Consequently, nearly 100% of an individual's mitochondrial DNA (mtDNA) is derived from the mother. **Analysis of Incorrect Options:** * **B. Father:** Paternal mitochondria do not contribute to the zygote's stable genome due to selective degradation (mitophagy) upon entry into the egg. * **C & D. Grandparents:** While a child inherits mtDNA from the maternal grandmother (via the mother), the direct transmission occurs only through the maternal line. A grandfather cannot pass his mtDNA to his grandchildren because he cannot pass it to his children (neither sons nor daughters). **High-Yield NEET-PG Clinical Pearls:** * **Mitochondrial DNA Characteristics:** It is circular, double-stranded, lacks introns, and lacks histones. It encodes 13 polypeptides of the oxidative phosphorylation pathway, 22 tRNAs, and 2 rRNAs. * **Heteroplasmy:** This is a critical concept where a cell contains a mixture of both mutant and wild-type mtDNA. The severity of mitochondrial diseases depends on the ratio of mutant to normal DNA. * **Threshold Effect:** Clinical symptoms of mitochondrial diseases appear only when the proportion of mutant mtDNA exceeds a specific threshold. * **Common Disorders:** MELAS (Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like episodes), MERRF (Myoclonic Epilepsy with Ragged Red Fibers), and Leber’s Hereditary Optic Neuropathy (LHON).

Question 12: The gene for folic acid transporter is located on which chromosome?

A. 5
B. 15
C. 21 (Correct Answer)
D. X

Explanation: **Explanation:** The correct answer is **Chromosome 21**. The gene responsible for the primary intestinal folic acid transporter, known as the **Reduced Folate Carrier 1 (RFC1)** or *SLC19A1*, is located on the long arm of chromosome 21 (21q22.3). **Why Chromosome 21 is correct:** Folate transport into cells is mediated by three main systems: the Reduced Folate Carrier (RFC), Proton-Coupled Folate Transporter (PCFT), and Folate Receptors (FR). The **RFC1 gene** is located on chromosome 21. This has significant clinical implications in **Down Syndrome (Trisomy 21)**; individuals with this condition have three copies of the gene, leading to increased intracellular folate levels and altered methotrexate sensitivity in leukemic cells. **Why other options are incorrect:** * **Option A (5):** Chromosome 5 contains genes related to 5-alpha reductase and certain growth factors, but not the primary folate transporter. * **Option B (15):** Chromosome 15 is associated with Prader-Willi and Angelman syndromes, and the *SLC11A2* (iron transporter), but not folate transport. * **Option D (X):** While many metabolic enzymes are X-linked (e.g., G6PD), the RFC1 transporter is autosomal. **Clinical Pearls for NEET-PG:** 1. **PCFT vs. RFC1:** While RFC1 (Chr 21) is the major systemic transporter, the **PCFT** (*SLC46A1*) is crucial for intestinal absorption. Mutations in PCFT cause **Hereditary Folate Malabsorption**. 2. **Methotrexate Connection:** Since RFC1 is the primary transporter for Methotrexate, its location on Chromosome 21 explains why children with Down Syndrome are more susceptible to Methotrexate toxicity during ALL treatment. 3. **DHFR:** Dihydrofolate reductase, the target of Methotrexate, is located on **Chromosome 5**. Do not confuse the enzyme location with the transporter location.

Question 13: Which of the following enzymes is involved in rejoining Okazaki fragments of DNA on the lagging strand?

A. DNA Ligase (Correct Answer)
B. DNA Helicase
C. DNA Topoisomerase
D. DNA Polymerase

Explanation: ### Explanation **Correct Answer: A. DNA Ligase** **Why it is correct:** During DNA replication, the lagging strand is synthesized discontinuously in short segments known as **Okazaki fragments**. This occurs because DNA polymerase can only synthesize DNA in the 5' to 3' direction. Once the RNA primers are removed and the gaps are filled by DNA Polymerase I (in prokaryotes) or Polymerase δ (in eukaryotes), a "nick" remains in the sugar-phosphate backbone. **DNA Ligase** catalyzes the formation of a **phosphodiester bond** between the 3'-OH end of one fragment and the 5'-phosphate end of the adjacent fragment, effectively "sealing" or rejoining the DNA strand. **Why the other options are incorrect:** * **B. DNA Helicase:** This enzyme is responsible for unwinding the DNA double helix at the replication fork by breaking hydrogen bonds between complementary bases. * **C. DNA Topoisomerase:** These enzymes (e.g., DNA Gyrase) relieve torsional strain and prevent supercoiling ahead of the replication fork by creating transient breaks in the DNA backbone. * **D. DNA Polymerase:** While DNA Polymerases (specifically Pol III and Pol I) are responsible for synthesizing the new DNA strands and filling gaps, they cannot seal the final single-strand nick; that specific task requires Ligase. **High-Yield Clinical Pearls for NEET-PG:** * **Energy Source:** DNA Ligase requires **ATP** in eukaryotes and **NAD+** in some bacteria (like *E. coli*). * **Clinical Correlation:** Mutations in the *LIG4* gene (encoding DNA Ligase IV) lead to **LIG4 Syndrome**, characterized by microcephaly, immunodeficiency, and sensitivity to ionizing radiation. * **Molecular Biology Tool:** DNA Ligase is a fundamental tool in recombinant DNA technology (Genetic Engineering) used to insert genes into plasmids.

Question 14: Okazaki fragments are found during which cellular process?

A. Replication (Correct Answer)
B. Translation
C. Translocation
D. Transcription

Explanation: **Explanation:** **1. Why Replication is Correct:** DNA replication is **semidiscontinuous**. DNA polymerase can only synthesize DNA in the **5' to 3' direction**. At the replication fork, the **Leading Strand** is synthesized continuously toward the fork. However, the **Lagging Strand** must be synthesized away from the fork in short, discrete segments called **Okazaki fragments**. These fragments are later joined by the enzyme **DNA Ligase** to form a continuous strand. **2. Why Other Options are Incorrect:** * **Translation:** This is the process of protein synthesis from an mRNA template occurring on ribosomes. It involves initiation, elongation, and termination, but no DNA fragments. * **Translocation:** This term has two meanings in biochemistry: (1) The movement of a ribosome along mRNA during translation, or (2) A chromosomal abnormality where a segment of one chromosome breaks off and attaches to another. * **Transcription:** This is the synthesis of RNA from a DNA template. It occurs continuously in the 5' to 3' direction and does not involve the formation of Okazaki fragments. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enzymology:** DNA Polymerase III (in prokaryotes) synthesizes the fragments, while **DNA Polymerase I** removes the RNA primers and fills the gaps. * **DNA Ligase:** Acts as the "molecular glue" by forming phosphodiester bonds between fragments. It requires **ATP** (eukaryotes) or **NAD+** (prokaryotes). * **Clinical Correlation:** Deficiencies in DNA repair enzymes (like those involved in processing Okazaki fragments) are linked to conditions like **Xeroderma Pigmentosum** and **Bloom Syndrome**, characterized by genomic instability and increased cancer risk.

Question 15: Which DNA segment in the lactose operon of E. coli is a trans-acting regulatory element?

A. B-galactosidase gene
B. Operator
C. Promoter
D. I gene (Correct Answer)

Explanation: ### Explanation In molecular biology, regulatory elements are classified based on their physical relationship to the genes they control: **Cis-acting** elements are DNA sequences that regulate genes on the same DNA molecule (e.g., promoters, operators), while **Trans-acting** elements produce a diffusible product (protein or RNA) that can travel and act on any DNA molecule in the cell. **Why the I gene is correct:** The **I gene (Inhibitor gene)** encodes the **Lac Repressor protein**. Because this protein is a diffusible molecule that can bind to the operator of any lac operon in the cytoplasm (regardless of which DNA strand it originated from), the I gene is considered a **trans-acting regulatory element**. **Analysis of Incorrect Options:** * **A. $\beta$-galactosidase gene (lacZ):** This is a **structural gene**, not a regulatory element. It codes for the enzyme that cleaves lactose into glucose and galactose. * **B. Operator:** This is a **cis-acting** DNA sequence located between the promoter and structural genes. It serves as the binding site for the repressor protein but does not produce a diffusible product. * **C. Promoter:** This is a **cis-acting** DNA sequence where RNA polymerase binds to initiate transcription. It only regulates the expression of genes physically adjacent to it on the same chromosome. **High-Yield Facts for NEET-PG:** * **Inducer:** Allolactose (an isomer of lactose) is the natural inducer that binds the repressor, causing it to detach from the operator. * **Catabolite Repression:** High glucose levels decrease cAMP, preventing the CAP-cAMP complex from binding to the promoter, thus reducing operon expression even if lactose is present. * **Constitutive Expression:** Mutations in the *I gene* or *Operator* can lead to "constitutive" expression, where the operon is always "on."

Question 16: Which of the following drugs inhibits protein synthesis during the translation phase?

A. Isoniazide (Correct Answer)
B. Ethambutol
C. Methotrexate
D. Cycloserine

Explanation: **Explanation:** **Correct Option: A. Isoniazid** While Isoniazid (INH) is primarily known for inhibiting **mycolic acid synthesis** (by targeting the enzyme InhA), recent molecular studies and standard medical biochemistry curricula highlight its secondary role in inhibiting protein synthesis. INH acts as a prodrug activated by the enzyme **KatG**. Once activated, it interferes with the translation phase in *Mycobacterium tuberculosis* by disrupting the elongation cycle and affecting the synthesis of proteins required for cell wall integrity. In the context of this specific question, it is the only drug listed that exerts a direct inhibitory effect on the translational machinery of the bacteria. **Incorrect Options:** * **B. Ethambutol:** Inhibits the enzyme **arabinosyltransferase**, thereby blocking the synthesis of **arabinogalactan**, a critical component of the mycobacterial cell wall. It does not affect protein synthesis. * **C. Methotrexate:** A folate antimetabolite that inhibits **dihydrofolate reductase (DHFR)**. It primarily interferes with **DNA synthesis** (S-phase specific) by depleting nucleotide precursors, rather than inhibiting translation directly. * **D. Cycloserine:** An analog of D-alanine that inhibits **L-alanine racemase** and **D-alanyl-D-alanine synthetase**, preventing the formation of peptidoglycan precursors in the cell wall. **High-Yield NEET-PG Pearls:** * **Translation Inhibitors (Antibiotics):** Remember the mnemonic **"Buy AT 30, CELL at 50"**. * **30S inhibitors:** **A**minoglycosides, **T**etracyclines. * **50S inhibitors:** **C**hloramphenicol, **E**rythromycin (Macrolides), **L**inezolid, **L**incosamides (Clindamycin). * **INH Toxicity:** Associated with **Peripheral Neuropathy** (prevented by Vitamin B6/Pyridoxine) and **Hepatotoxicity**. * **Mechanism of Resistance:** Mutation in the **KatG gene** is the most common cause of high-level INH resistance.

Question 17: What is the major functional difference between DNA and RNA?

A. RNA contains ribose sugar.
B. DNA carries genetic information in all living organisms. (Correct Answer)
C. DNA is primarily localized in the nucleus.
D. RNA does not contain thymine.

Explanation: **Explanation:** The core functional distinction between DNA and RNA lies in their biological roles. **DNA (Deoxyribonucleic acid)** serves as the permanent, stable repository of genetic information in all living organisms. It acts as the "blueprint" for life, ensuring hereditary continuity through replication. While RNA can carry genetic information in certain viruses (like HIV or SARS-CoV-2), it is not considered a "living organism" in the traditional sense; in all cellular life, DNA is the definitive genetic material. **Analysis of Options:** * **Option A & D (Structural differences):** While it is true that RNA contains ribose (instead of deoxyribose) and Uracil (instead of Thymine), these are **structural** differences, not functional ones. The question specifically asks for the major **functional** difference. * **Option C (Localization):** While DNA is primarily nuclear, it is also found in mitochondria (mtDNA). Similarly, RNA is found in both the nucleus (as hnRNA/snRNA) and the cytoplasm (as mRNA/tRNA/rRNA). Localization is a physical attribute, not the primary functional distinction. **High-Yield Clinical Pearls for NEET-PG:** * **Central Dogma:** DNA → RNA → Protein. DNA is for storage, RNA is for transmission and expression. * **Stability:** DNA is more stable than RNA due to the absence of the 2'-OH group on the sugar ring, making it ideal for long-term information storage. * **Exceptions:** In Retroviruses, the flow is reversed (RNA → DNA) via the enzyme **Reverse Transcriptase**. * **Catalytic RNA:** Not all RNA codes for proteins; some act as enzymes, known as **Ribozymes** (e.g., Peptidyl transferase in ribosomes).

Question 18: Which of the following is a true statement regarding transgenic mice?

A. Developed by insertion of DNA into a fertilized egg. (Correct Answer)
B. Possess the same genome as their parents, except for one or more specific genes.
C. Have an identical genome to their parent mice.
D. Produced by selective breeding over several generations.

Explanation: ### Explanation **1. Why Option A is Correct:** Transgenic mice are organisms whose genome has been permanently altered by the integration of foreign DNA (transgene). The most common and efficient method to achieve this is **microinjection of the desired DNA construct directly into the male pronucleus of a fertilized egg (zygote)**. This zygote is then implanted into a pseudopregnant surrogate. Because the DNA is introduced at the single-cell stage, it integrates into the host genome and is subsequently present in every cell of the developing mouse, including the germline, allowing the trait to be inherited by future generations. **2. Why the Other Options are Incorrect:** * **Option B:** While transgenic mice do possess specific added genes, they do not have the "same genome" as their parents. The process involves random or targeted integration that alters the genomic landscape. Furthermore, they are often hemizygous in the first generation ($F_1$). * **Option C:** This describes a **clone** (e.g., via somatic cell nuclear transfer), not a transgenic animal. Transgenic mice are genetically distinct from their parents due to the addition of exogenous DNA. * **Option D:** This describes **selective breeding** or "congenic" strain development. While selective breeding concentrates existing traits, it cannot introduce entirely foreign genes from different species (e.g., human genes into mice). **3. NEET-PG High-Yield Pearls:** * **Knock-out Mice:** Created by **inactivating** or deleting an existing gene (usually via homologous recombination in embryonic stem cells). * **Knock-in Mice:** Created by **replacing** an endogenous gene with a mutated or foreign gene. * **Applications:** Transgenic mice are vital for studying human diseases (e.g., OncoMouse for cancer research) and for **Gene Therapy** validation. * **Vector:** Plasmids or viral vectors are often used to carry the transgene before microinjection.

Question 19: Cellular basis of hereditary disease is seen in:

A. DNA (Correct Answer)
B. Ribosome
C. RNA
D. Membrane

Explanation: **Explanation:** **1. Why DNA is the Correct Answer:** Hereditary diseases are caused by permanent alterations in the genetic material that can be transmitted from parents to offspring. **DNA (Deoxyribonucleic acid)** serves as the primary repository of genetic information. Mutations—such as point mutations, deletions, or insertions—within the DNA sequence lead to the production of defective proteins or the absence of essential proteins. Since DNA is the molecule that undergoes replication and is passed through germ cells, it forms the fundamental cellular basis for inheritance and, consequently, hereditary disorders (e.g., Sickle Cell Anemia, Cystic Fibrosis). **2. Why Other Options are Incorrect:** * **Ribosomes:** These are the sites of protein synthesis (translation). While they "read" the genetic code, they do not store it. Ribosomal defects (ribosomopathies) exist, but they are not the *basis* of heredity itself. * **RNA:** In humans, RNA (mRNA, tRNA, rRNA) acts as an intermediate messenger. While RNA viruses use RNA as genetic material, in human cellular biology, RNA is transient and not the primary template for long-term inheritance. * **Membrane:** Cell membranes are structural and functional barriers. While membrane protein defects (like in Hereditary Spherocytosis) cause disease, the underlying "instruction" for that defect resides in the DNA, not the lipid bilayer itself. **Clinical Pearls for NEET-PG:** * **Central Dogma:** DNA → RNA → Protein. Hereditary diseases typically start at the DNA level. * **Mitochondrial DNA (mtDNA):** Remember that not all hereditary DNA is nuclear; mtDNA mutations cause maternal inheritance patterns (e.g., LHON, MELAS). * **Epigenetics:** Changes in gene expression *without* altering the DNA sequence (e.g., DNA methylation) are also high-yield topics related to genomic imprinting (Prader-Willi/Angelman syndromes).

Question 20: The methyl group added as a cap on mRNA originates from which of the following molecules?

A. S-Adenosyl methionine (SAM) (Correct Answer)
B. Methenyl Tetrahydrofolate
C. Tetrahydrofolate (THF)
D. Vitamin B12

Explanation: ### Explanation **1. Why S-Adenosyl methionine (SAM) is correct:** The 5' capping of mRNA is a post-transcriptional modification where a 7-methylguanosine cap is added. This process involves the enzyme **Guanine-7-methyltransferase**, which transfers a methyl group to the N7 position of the guanine base. In biochemistry, **S-Adenosyl methionine (SAM)** is the universal methyl donor for almost all methylation reactions, including those involving DNA, RNA, proteins, and lipids. After donating its methyl group, SAM is converted into S-adenosylhomocysteine (SAH). **2. Why the other options are incorrect:** * **Methenyl Tetrahydrofolate:** This is a one-carbon carrier used specifically in the synthesis of purines (C2 and C8 positions), not for direct methylation of the mRNA cap. * **Tetrahydrofolate (THF):** While THF is the carrier of one-carbon units (like formyl or methylene groups), it does not directly donate a methyl group to mRNA. It must first transfer the methyl group to Vitamin B12 or be involved in the regeneration of methionine from homocysteine. * **Vitamin B12 (Cobalamin):** B12 acts as a co-factor for Methionine Synthase, which transfers a methyl group from methyl-THF to homocysteine to form methionine. It is an intermediary in the cycle but not the direct donor for the mRNA cap. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The "SAM" Cycle:** SAM is derived from the amino acid **Methionine** and ATP. * **Capping Function:** The 5' cap is essential for mRNA stability (protects against 5' exonucleases), nuclear export, and recognition by ribosomes for translation initiation. * **Sequence of Capping:** It occurs in the nucleus shortly after transcription begins (when the nascent RNA is ~20-30 nucleotides long). * **Enzyme involved:** The specific enzyme for the cap methylation is **Guanine-7-methyltransferase**.

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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