Molecular Biology and Genomics Practice Questions

Q: Which of the following statement is false regarding mitochondria?

Single stranded straight DNA. **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).

Q: Watson and Crick are associated with what?

Discovery of the helical structure of DNA. **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).

Q: Which enzyme is responsible for the unwinding of DNA?

Helicase. **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.

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

Transcription. ### 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.

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

146. ### 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).

Q: 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?

Faulty splicing. ### 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.

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

Fragile X Syndrome. **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).

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

Cystic fibrosis. **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).

Question 1

Which of the following statement is false regarding mitochondria?

Accepted Answer

Single stranded straight DNA

Answer

Guanine rich strand is referred to as the heavy strand

Answer

Each DNA molecule consists of 15,000-17,000 base pairs

Answer

Transmitted by maternal nonmendelian inheritance

Question 2

Watson and Crick are associated with what?

Accepted Answer

Discovery of the helical structure of DNA

Answer

Association of Helicobacter pylori with chronic gastritis

Answer

Discovery of the HIV virus

Answer

None of the above

Question 3

Which enzyme is responsible for the unwinding of DNA?

Accepted Answer

Helicase

Answer

Ligase

Answer

DNA primase

Answer

DNA polymerase

Question 4

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

Accepted Answer

Adenoviral vectors

Answer

Microinjection

Answer

Lipofection

Answer

Electroporation

Question 5

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

Accepted Answer

Transcription

Answer

Transduction

Answer

Translation

Answer

Translocation

Question 6

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

Accepted Answer

146

Answer

292

Answer

73

Answer

1460

Question 7

Karyotyping is useful in the diagnosis of which of the following?

Accepted Answer

Chromosomal abnormalities

Answer

Autosomal recessive disorders

Answer

X-linked recessive disorders

Answer

Biochemical abnormalities

Question 8

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?

Accepted Answer

Faulty splicing

Answer

Terminal base addition

Answer

Terminal base deletion

Answer

None of the above

Question 9

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

Accepted Answer

Fragile X Syndrome

Answer

Hereditary Non-polyposis Colorectal Cancer (HNPCC)

Answer

Fanconi Anemia

Answer

Ataxia-Telangiectasia (AT)

Question 10

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

Accepted Answer

Cystic fibrosis

Answer

Fragile-x syndrome

Answer

Huntington's disease

Answer

Friedreich's ataxia

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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