Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 111: Which of the following statement is false regarding mitochondria?

A. Guanine rich strand is referred to as the heavy strand
B. Each DNA molecule consists of 15,000-17,000 base pairs
C. Single stranded straight DNA (Correct Answer)
D. Transmitted by maternal nonmendelian inheritance

Explanation: **Explanation:** The correct answer is **C (Single stranded straight DNA)** because mitochondrial DNA (mtDNA) is actually **double-stranded and circular**. Unlike nuclear DNA, which is linear and organized into chromosomes, mtDNA resembles bacterial DNA, supporting the endosymbiotic theory. **Analysis of Options:** * **Option A (Heavy Strand):** This is **true**. mtDNA consists of two strands with different buoyant densities. The **Heavy (H) strand** is rich in Guanine, while the Light (L) strand is rich in Cytosine. Most mitochondrial genes are encoded on the H-strand. * **Option B (15,000-17,000 bp):** This is **true**. Human mtDNA is approximately **16,569 base pairs** long. It is highly compact, containing 37 genes (13 polypeptides, 22 tRNAs, and 2 rRNAs) with very little non-coding "junk" DNA. * **Option D (Maternal Inheritance):** This is **true**. Mitochondria are transmitted via the oocyte's cytoplasm; sperm mitochondria are typically degraded after fertilization. This follows a **non-Mendelian (matrilineal)** pattern. **Clinical Pearls for NEET-PG:** 1. **Heteroplasmy:** The presence of a mixture of more than one type of organellar genome (mutated vs. wild-type) within a cell. This explains the variable expressivity in mitochondrial diseases. 2. **Threshold Effect:** A certain percentage of mutated mtDNA must be present before clinical symptoms appear. 3. **High Mutation Rate:** mtDNA lacks histones and has less efficient repair mechanisms compared to nuclear DNA, making it 10 times more prone to mutations. 4. **Key Diseases:** MELAS, MERRF, and Leber’s Hereditary Optic Neuropathy (LHON).

Question 112: Watson and Crick are associated with what?

A. Discovery of the helical structure of DNA (Correct Answer)
B. Association of Helicobacter pylori with chronic gastritis
C. Discovery of the HIV virus
D. None of the above

Explanation: **Explanation:** **James Watson and Francis Crick** are credited with the discovery of the **double-helical structure of DNA** in 1953. Using X-ray diffraction data produced by Rosalind Franklin and Maurice Wilkins, they proposed the "B-form" DNA model. This model established that DNA consists of two antiparallel strands held together by hydrogen bonds between complementary nitrogenous bases (Adenine-Thymine and Cytosine-Guanine). This discovery laid the foundation for modern molecular biology, explaining how genetic information is stored and replicated. They were awarded the Nobel Prize in Physiology or Medicine in 1962. **Analysis of Incorrect Options:** * **Option B:** The association of *Helicobacter pylori* with chronic gastritis and peptic ulcer disease was discovered by **Barry Marshall and Robin Warren** (Nobel Prize, 2005). * **Option C:** The Human Immunodeficiency Virus (HIV) was discovered by **Luc Montagnier and Françoise Barré-Sinoussi** (Nobel Prize, 2008). **High-Yield Clinical Pearls for NEET-PG:** * **Chargaff’s Rule:** Watson and Crick’s model relied on Erwin Chargaff’s finding that in DNA, the amount of A = T and G = C. * **DNA Forms:** While Watson and Crick described **B-DNA** (right-handed, 10 bp/turn), remember that **Z-DNA** is left-handed and occurs in regions with alternating purine-pyrimidine sequences. * **Central Dogma:** Francis Crick also proposed the "Central Dogma of Molecular Biology" (DNA → RNA → Protein). * **Bonding:** Remember that A-T pairs have **2 hydrogen bonds**, while G-C pairs have **3**, making G-C rich regions more thermally stable (higher melting temperature, Tm).

Question 113: Which enzyme is responsible for the unwinding of DNA?

A. Ligase
B. DNA primase
C. Helicase (Correct Answer)
D. DNA polymerase

Explanation: **Explanation:** The correct answer is **Helicase**. **1. Why Helicase is correct:** DNA replication requires the double-stranded DNA (dsDNA) template to be separated into single strands. **Helicase** is the enzyme responsible for this "unwinding" process. It functions by breaking the hydrogen bonds between complementary nitrogenous bases (A=T and G≡C). This process is energy-dependent and requires **ATP hydrolysis**. As helicase moves along the DNA, it creates the **replication fork**. **2. Why other options are incorrect:** * **Ligase:** Known as the "molecular glue," it joins DNA fragments (like Okazaki fragments) by catalyzing the formation of phosphodiester bonds. It does not unwind DNA. * **DNA Primase:** This is an RNA polymerase that synthesizes a short **RNA primer**. This primer provides the free 3'-OH group necessary for DNA polymerase to begin synthesis. * **DNA Polymerase:** Its primary role is the synthesis of new DNA strands by adding nucleotides complementary to the template. It cannot initiate synthesis or unwind the helix on its own. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** Helicase **H**elps **H**ydrogen bonds break (Unzips the genes). * **Topoisomerases:** While Helicase unwinds the DNA, Topoisomerases (e.g., DNA Gyrase in prokaryotes) relieve the **supercoiling** (torsional strain) created ahead of the replication fork. * **Clinical Correlation:** Deficiencies in specific helicases lead to genomic instability syndromes, such as **Bloom Syndrome** (BLM gene) and **Werner Syndrome** (WRN gene), characterized by premature aging and cancer predisposition. * **Single-Stranded Binding Proteins (SSBs):** These stabilize the unwound DNA to prevent it from re-annealing before replication is complete.

Question 114: Which of the following is the most common gene delivery system for 'in vivo' gene therapy?

A. Microinjection
B. Lipofection
C. Adenoviral vectors (Correct Answer)
D. Electroporation

Explanation: **Explanation:** Gene therapy involves the introduction of genetic material into cells to treat or prevent disease. The delivery systems are broadly classified into **viral vectors** and **non-viral methods**. **Why Adenoviral Vectors are correct:** For *in vivo* gene therapy (where the vector is injected directly into the patient), **Adenoviral vectors** are the most commonly used system. They are highly efficient at transducing a wide range of both dividing and non-dividing cells. Unlike retroviruses, they do not integrate into the host genome (remaining episomal), which reduces the risk of insertional mutagenesis, though it makes the expression transient. Their high "tropism" and ability to carry large genetic payloads make them the clinical gold standard for many *in vivo* applications. **Analysis of Incorrect Options:** * **Microinjection (A):** This is a physical method used primarily for *in vitro* fertilization or creating transgenic animals by injecting DNA directly into a single cell (oocyte). It is technically impossible to perform *in vivo* on a systemic level. * **Lipofection (B):** A non-viral method using cationic liposomes. While safer (low immunogenicity), its efficiency *in vivo* is significantly lower than viral vectors. * **Electroporation (D):** Uses high-voltage pulses to create pores in cell membranes. It is primarily used for *ex vivo* gene therapy (modifying cells outside the body) or localized skin/muscle treatments, but it is not the "most common" systemic delivery system. **High-Yield Clinical Pearls for NEET-PG:** * **Retroviral vectors:** Require cell division for integration; carry a risk of oncogenesis. * **Adeno-associated virus (AAV):** Currently gaining popularity for long-term expression with low immunogenicity. * **SCID-X1:** The first disease successfully treated with gene therapy (using retroviral vectors). * **Episomal vs. Integrating:** Adenoviruses stay **episomal** (outside the chromosome), whereas Retroviruses **integrate** into the host DNA.

Question 115: The process by which a base sequence of messenger RNA is synthesized on a template of complementary DNA is called?

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

Explanation: ### Explanation **Correct Answer: A. Transcription** **Why it is correct:** Transcription is the first step of gene expression where a specific segment of DNA is copied into **messenger RNA (mRNA)** by the enzyme **RNA Polymerase**. This process occurs in the nucleus (in eukaryotes). The DNA strand acts as a template, and the resulting mRNA carries the genetic "message" from the nucleus to the cytosol for protein synthesis. **Analysis of Incorrect Options:** * **B. Transduction:** This is a process of horizontal gene transfer in bacteria where genetic material is moved from one bacterium to another by a **bacteriophage** (virus). It is not part of the standard central dogma of protein synthesis. * **C. Translation:** This is the subsequent step where the mRNA sequence is decoded by ribosomes to synthesize a specific **polypeptide/protein**. It involves "translating" the nucleotide language into an amino acid language. * **D. Translocation:** This term has two meanings in genetics: (1) In translation, it is the movement of the ribosome along the mRNA strand; (2) In cytogenetics, it refers to a chromosomal abnormality where a segment of one chromosome breaks off and attaches to another. **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** RNA synthesis always occurs in the **5' to 3' direction**, while the DNA template is read in the 3' to 5' direction. * **Inhibitors:** **Rifampicin** inhibits bacterial RNA polymerase (used in TB), while **Actinomycin D** inhibits transcription in both prokaryotes and eukaryotes (used in chemotherapy). * **Post-transcriptional modifications:** In eukaryotes, the primary transcript (hnRNA) undergoes 5' capping, 3' polyadenylation, and splicing (removal of introns) before becoming mature mRNA.

Question 116: What is the approximate number of base pairs associated with a single nucleosome?

A. 146 (Correct Answer)
B. 292
C. 73
D. 1460

Explanation: ### Explanation **1. Why Option A is Correct:** The nucleosome is the fundamental structural unit of chromatin. It consists of a **nucleosome core particle** and linker DNA. The core particle is composed of an octamer of histone proteins (two each of H2A, H2B, H3, and H4) around which DNA is wrapped. Specifically, **146 base pairs (bp)** of DNA wrap approximately **1.75 times** around this histone octamer in a left-handed superhelix. This "beads-on-a-string" structure is essential for compacting the large eukaryotic genome into the cell nucleus. **2. Why Other Options are Incorrect:** * **Option B (292):** This is exactly double the core DNA length. While a "chromatosome" (core + H1 histone + linker DNA) involves about 160–200 bp, 292 bp does not represent a standard structural unit. * **Option C (73):** This is half of 146. While 146 bp completes nearly two turns, 73 bp would represent roughly a single turn around the octamer, which is not the complete functional unit. * **Option D (1460):** This is a ten-fold overestimation and does not correspond to any specific repeating unit in chromatin architecture. **3. NEET-PG High-Yield Clinical Pearls:** * **Histone Composition:** Histones are rich in **Lysine and Arginine**, giving them a positive charge to bind the negatively charged phosphate backbone of DNA. * **Linker Histone:** **Histone H1** is not part of the core octamer; it sits outside the core to stabilize the entry and exit of DNA (forming the chromatosome). * **Epigenetics:** Acetylation of histones (by HATs) neutralizes the positive charge, leading to relaxed chromatin (**Euchromatin**) and increased transcription. Deacetylation (by HDACs) leads to condensed chromatin (**Heterochromatin**). * **Clinical Link:** In **Systemic Lupus Erythematosus (SLE)**, patients often develop antibodies against histones (Anti-histone antibodies), especially in drug-induced SLE (e.g., Hydralazine, Procainamide).

Question 117: Karyotyping is useful in the diagnosis of which of the following?

A. Autosomal recessive disorders
B. X-linked recessive disorders
C. Chromosomal abnormalities (Correct Answer)
D. Biochemical abnormalities

Explanation: **Explanation:** **Karyotyping** is a cytogenetic technique used to examine the complete set of chromosomes in a cell. It involves arresting cells in **metaphase** (using colchicine), staining them (usually G-banding), and arranging them in a systematic order based on size, centromere position, and banding patterns. **1. Why the Correct Answer is Right:** Karyotyping is specifically designed to detect **Chromosomal Abnormalities**. These include: * **Numerical abnormalities:** Aneuploidies such as Trisomy 21 (Down Syndrome), 18 (Edwards), 13 (Patau), and Monosomy X (Turner Syndrome). * **Structural abnormalities:** Large-scale deletions, duplications, inversions, and translocations (e.g., Philadelphia chromosome $t(9;22)$). Because karyotyping looks at the "macro" structure of the genome, it can only detect changes involving approximately **5–10 Megabases (Mb)** of DNA or more. **2. Why the Other Options are Wrong:** * **Autosomal Recessive (A) and X-linked Recessive (B) Disorders:** These are typically **single-gene (point) mutations** (e.g., Sickle Cell Anemia, Hemophilia). They involve changes at the nucleotide level which are too small to be seen under a light microscope. These require molecular techniques like PCR or DNA sequencing. * **Biochemical Abnormalities (D):** These refer to metabolic derangements (e.g., Phenylketonuria). These are diagnosed via enzyme assays or metabolite analysis (e.g., Tandem Mass Spectrometry), not by looking at chromosomes. **High-Yield Clinical Pearls for NEET-PG:** * **Sample Source:** Lymphocytes (most common), amniotic fluid, or chorionic villi. * **Best Phase:** Cells are arrested in **Metaphase** because chromosomes are most condensed. * **Resolution:** Standard karyotyping cannot detect **microdeletions** (e.g., DiGeorge Syndrome); for those, **FISH** (Fluorescence In Situ Hybridization) is the investigation of choice. * **Barr Body:** In females, one X chromosome is inactivated (Lyonization), appearing as a Barr body in interphase nuclei. The number of Barr bodies = (Total X chromosomes - 1).

Question 118: A 10-year-old female patient with a known history of beta-thalassemia presents with complaints of tiredness and shortness of breath. Which of the following genetic mechanisms can cause beta-thalassemia?

A. Terminal base addition
B. Faulty splicing (Correct Answer)
C. Terminal base deletion
D. None of the above

Explanation: ### Explanation **Correct Answer: B. Faulty splicing** **Mechanism:** Beta-thalassemia is a heterogeneous group of genetic disorders characterized by reduced ($\beta^+$) or absent ($\beta^0$) synthesis of the beta-globin chains of hemoglobin. While various mutations can occur, **faulty splicing** is one of the most common molecular mechanisms. Most often, point mutations occur within **introns** or at the **exon-intron junctions** (splice sites). These mutations can: 1. Destroy existing splice sites. 2. Create "cryptic" splice sites. This leads to the production of abnormal mRNA transcripts that are either degraded or translated into non-functional proteins, resulting in a deficiency of beta-globin chains. **Analysis of Incorrect Options:** * **A & C (Terminal base addition/deletion):** These are not standard mechanisms for beta-thalassemia. While frame-shift mutations (insertions or deletions within the coding sequence) can cause the disease, they are typically internal, not "terminal." Terminal modifications usually refer to post-transcriptional polyadenylation or capping, which are rarely the primary cause of this pathology. * **D (None of the above):** Incorrect, as splicing defects are a hallmark of the molecular pathology of beta-thalassemia. **NEET-PG High-Yield Pearls:** * **Most common mutation in India:** The most common mutation causing beta-thalassemia in the Indian population is **IVS 1-5 (G→C)**, which is a splice-site mutation. * **Alpha vs. Beta Thalassemia:** Remember that **Alpha-thalassemia** is most commonly caused by **large gene deletions**, whereas **Beta-thalassemia** is most commonly caused by **point mutations** (including splicing, nonsense, and promoter mutations). * **Clinical Presentation:** Look for microcytic hypochromic anemia, increased HbA2 (>3.5%), and "crew-cut" appearance on skull X-ray due to extramedullary hematopoiesis.

Question 119: All of the following are true regarding DNA repair defect syndromes EXCEPT?

A. Hereditary Non-polyposis Colorectal Cancer (HNPCC)
B. Fanconi Anemia
C. Ataxia-Telangiectasia (AT)
D. Fragile X Syndrome (Correct Answer)

Explanation: **Explanation:** The correct answer is **Fragile X Syndrome** because it is a **Trinucleotide Repeat Expansion disorder**, not a primary defect in DNA repair mechanisms. It is caused by the expansion of CGG repeats in the *FMR1* gene, leading to hypermethylation and gene silencing. **Analysis of Options:** * **Hereditary Non-polyposis Colorectal Cancer (HNPCC/Lynch Syndrome):** This is caused by a defect in the **Mismatch Repair (MMR)** genes (primarily *MSH2* and *MLH1*). This leads to microsatellite instability (MSI). * **Fanconi Anemia:** This is a defect in **Interstrand Cross-link (ICL) repair**. Patients exhibit bone marrow failure, physical anomalies, and a high risk of AML. * **Ataxia-Telangiectasia (AT):** This is caused by a mutation in the *ATM* gene, which is responsible for detecting **Double-Strand Breaks (DSB)** and initiating the repair process via Non-Homologous End Joining (NHEJ) or Homologous Recombination. **High-Yield Clinical Pearls for NEET-PG:** 1. **Xeroderma Pigmentosum:** Defect in **Nucleotide Excision Repair (NER)**; inability to repair pyrimidine dimers caused by UV light. 2. **Cockayne Syndrome:** Defect in transcription-coupled DNA repair (a subtype of NER); characterized by "bird-like" facies and dwarfism but *no* increased risk of skin cancer. 3. **Bloom Syndrome:** Defect in **DNA Helicase** (*BLM* gene); presents with sister chromatid exchanges and "butterfly" rash. 4. **BRCA 1 & 2:** Defects in **Homologous Recombination** (Double-strand break repair).

Question 120: Triplet repeat expansion is seen in all of the following conditions EXCEPT:

A. Fragile-x syndrome
B. Huntington's disease
C. Friedreich's ataxia
D. Cystic fibrosis (Correct Answer)

Explanation: **Explanation:** The correct answer is **Cystic Fibrosis (Option D)**. **1. Why Cystic Fibrosis is the correct answer:** Cystic Fibrosis is an **autosomal recessive** disorder caused by mutations in the **CFTR gene** on chromosome 7. The most common mutation is a **deletion of three nucleotides** (CTT) resulting in the loss of phenylalanine at position 508 (**ΔF508**). It is a classic example of a single-gene mutation involving a deletion, not a triplet repeat expansion. **2. Analysis of Incorrect Options (Triplet Repeat Disorders):** * **Fragile-X Syndrome (Option A):** Caused by the expansion of **CGG** repeats in the *FMR1* gene. It is the most common cause of inherited intellectual disability. * **Huntington’s Disease (Option B):** Caused by the expansion of **CAG** repeats in the *HTT* gene. It is an autosomal dominant neurodegenerative disorder characterized by chorea and dementia. * **Friedreich’s Ataxia (Option C):** Caused by the expansion of **GAA** repeats in the *FXN* gene (frataxin). It is unique as it is an autosomal recessive triplet repeat disorder. **3. High-Yield Clinical Pearls for NEET-PG:** * **Anticipation:** This phenomenon, where the disease becomes more severe or has an earlier onset in successive generations, is a hallmark of triplet repeat expansions. * **Location of Repeats:** * **Exonic (Coding):** Huntington’s (CAG). * **Untranslated Regions (Non-coding):** Fragile-X (5' UTR), Myotonic Dystrophy (3' UTR), Friedreich’s Ataxia (Intron 1). * **Parental Transmission:** Huntington’s expansion usually occurs during **paternal** meiosis (spermatogenesis), whereas Fragile-X expansion occurs during **maternal** meiosis (oogenesis).

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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