Molecular Biology and Genomics Practice Questions

Q: Prenatal diagnosis of Hemophilia is best done by?

PCR. **Explanation:** **Hemophilia A and B** are X-linked recessive disorders caused by mutations in the Factor VIII and Factor IX genes, respectively. **Why PCR is the Correct Answer:** Polymerase Chain Reaction (PCR) is the gold standard for prenatal diagnosis because it allows for the **rapid amplification of fetal DNA** obtained via Chorionic Villus Sampling (CVS) or amniocentesis. Once amplified, specific techniques like **Reverse Transcriptase-PCR (RT-PCR)** or **Direct Mutation Analysis** can identify the exact genetic defect (e.g., the common Intron 22 inversion in Hemophilia A). It is preferred due to its high sensitivity, specificity, and the small amount of fetal sample required. **Analysis of Other Options:** * **Linkage Analysis:** This was historically used when the specific mutation was unknown. It tracks the disease gene using polymorphic markers (RFLPs). However, it requires DNA from multiple family members and is prone to errors due to genetic recombination. * **Cytometry (Flow Cytometry):** This is used for analyzing physical and chemical characteristics of cells (e.g., immunophenotyping in leukemia). It cannot detect single-gene mutations required for hemophilia diagnosis. * **Microarray:** While useful for detecting chromosomal imbalances (Copy Number Variations), it is not the primary tool for the point mutations or inversions typically seen in Hemophilia. **Clinical Pearls for NEET-PG:** * **Most common mutation in severe Hemophilia A:** Intron 22 inversion. * **Gold standard for carrier detection:** DNA analysis (PCR-based). * **Prenatal sampling timing:** CVS is usually done at 10–12 weeks; Amniocentesis at 15–18 weeks. * **Factor levels:** Prenatal diagnosis via fetal blood sampling (cordocentesis) to check factor levels is rarely done now due to the high risk of fetal loss compared to PCR.

Q: Where does miRNA typically bind to facilitate gene knockdown?

3' untranslated region. ### Explanation **1. Why Option A is Correct:** MicroRNAs (miRNAs) are small, non-coding RNA molecules (typically 21–25 nucleotides) that play a crucial role in post-transcriptional gene regulation. To facilitate gene knockdown, the miRNA incorporates into the **RNA-induced silencing complex (RISC)**. The "seed sequence" (nucleotides 2–7) of the miRNA then undergoes complementary base pairing primarily with the **3' Untranslated Region (3' UTR)** of the target messenger RNA (mRNA). This binding leads to either **translational repression** (if binding is partially complementary) or **mRNA degradation** (if binding is perfectly complementary), effectively silencing the gene. **2. Why Other Options are Incorrect:** * **Option B (5' UTR):** While some rare interactions occur here, the 5' UTR is primarily involved in the initiation of translation (ribosome loading). miRNA binding here is not the standard mechanism for gene knockdown. * **Option C (Introns):** Introns are non-coding sequences removed during splicing in the nucleus. miRNA-mediated silencing occurs in the **cytoplasm** on mature, processed mRNA. * **Option D:** Although experimental evidence shows occasional binding at the 5' UTR, the **canonical and most efficient site** for miRNA-mediated regulation in humans is the 3' UTR. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Origin:** miRNAs are transcribed by **RNA Polymerase II** as primary-miRNA (pri-miRNA). * **Processing Enzymes:** **Drosha** (in the nucleus) and **Dicer** (in the cytoplasm) are the key endonucleases that process miRNA. * **OncomiRs:** miRNAs that regulate oncogenes or tumor suppressor genes. For example, a decrease in **miRNA-15 and miRNA-16** is associated with Chronic Lymphocytic Leukemia (CLL). * **Therapeutic Potential:** miRNA mimics and antagomirs (inhibitors) are being researched as targeted therapies for cancer and viral infections (e.g., Hepatitis C).

Q: What cellular component carries the genetic information for hereditary diseases?

DNA. **Explanation:** The correct answer is **DNA (Deoxyribonucleic Acid)**. DNA serves as the primary repository of genetic information in almost all living organisms. It is organized into genes, which are the functional units of heredity. Mutations or alterations in the nucleotide sequence of DNA lead to the synthesis of defective proteins or regulatory errors, which manifest as **hereditary diseases** (e.g., Sickle Cell Anemia, Cystic Fibrosis). **Analysis of Options:** * **DNA (Correct):** It contains the "blueprint" of life. In eukaryotes, it is primarily located in the nucleus (genomic DNA) and mitochondria (mtDNA). * **Ribosome:** These are the cellular "protein factories." While they are essential for translating genetic code into proteins, they do not store or transmit hereditary information. * **RNA:** While RNA carries genetic information in some viruses (e.g., Retroviruses like HIV), in human cells, it primarily acts as an intermediary (mRNA, tRNA, rRNA) to convert DNA instructions into proteins. It is not the primary carrier of human hereditary traits. * **Membrane:** Cell membranes provide structural integrity and regulate transport but play no role in storing genetic information. **NEET-PG High-Yield Pearls:** * **Mitochondrial DNA (mtDNA):** Inherited exclusively from the **mother** (Maternal Inheritance). Mutations here cause diseases like Leber’s Hereditary Optic Neuropathy (LHON). * **Central Dogma:** The flow of genetic information is DNA → RNA → Protein. * **Nucleosome:** The basic unit of DNA packaging, consisting of DNA wrapped around **Histone octamers** (H2A, H2B, H3, H4). H1 is the linker histone.

Q: What is the term for the transfer of genetic material between two non-homologous chromosomes during one meiotic division?

Translocation. ### Explanation **1. Why Translocation is Correct:** Translocation is a chromosomal abnormality where a segment of DNA is transferred between **non-homologous chromosomes**. This occurs when chromosomes break and the fragments are rejoined to the wrong partners. It can be **reciprocal** (exchange of segments between two chromosomes) or **Robertsonian** (fusion of two acrocentric chromosomes). Since the question specifies the transfer between non-homologous pairs, translocation is the only fitting mechanism. **2. Why the Other Options are Incorrect:** * **Non-disjunction:** This refers to the failure of homologous chromosomes or sister chromatids to separate properly during meiosis or mitosis. It leads to **aneuploidy** (e.g., Trisomy 21), not the transfer of segments between different chromosomes. * **Inversion:** This occurs when a single chromosome undergoes two breaks, and the segment between them is reinserted in reverse order. It involves only **one chromosome**, not an exchange between non-homologous ones. * **Isochromosome:** This is a structural abnormality where a chromosome loses one of its arms and replaces it with an exact copy of the other arm (e.g., two long arms). This results from **horizontal** rather than vertical division of the centromere. **3. NEET-PG High-Yield Pearls:** * **Philadelphia Chromosome:** A classic reciprocal translocation **t(9;22)** involving the *BCR-ABL* fusion gene, diagnostic for **Chronic Myeloid Leukemia (CML)**. * **Burkitt Lymphoma:** Associated with **t(8;14)**, involving the *c-myc* oncogene. * **Robertsonian Translocation:** Most commonly involves acrocentric chromosomes (**13, 14, 15, 21, and 22**). A carrier of a t(14;21) translocation has a high risk of having a child with Down Syndrome. * **Balanced vs. Unbalanced:** Balanced translocations often show no phenotype in the individual but lead to high rates of spontaneous abortion or birth defects in offspring.

Q: Which DNA polymerase is/are involved in the repair of mammalian DNA?

beta. In mammalian cells, DNA polymerases are categorized by their specific roles in replication and repair. **Correct Answer: B. beta (β)** DNA Polymerase beta is the primary enzyme involved in **Base Excision Repair (BER)**. It is a low-fidelity polymerase that lacks 3' to 5' exonuclease (proofreading) activity. Its main function is to fill short gaps (usually a single nucleotide) created during the repair of damaged bases caused by alkylation or oxidation. Because it handles "gap-filling" rather than long-strand synthesis, it is the classic "repair polymerase." **Explanation of Incorrect Options:** * **A. alpha (α):** This enzyme is responsible for **initiating** DNA replication. It possesses primase activity and synthesizes short RNA-DNA primers (i.e., the "iDNA"). It does not have proofreading activity and is not primarily a repair enzyme. * **C. gamma (γ):** This is the exclusive polymerase for **mitochondrial DNA replication** and repair. While it does repair, it is specific to the mitochondria, whereas the question refers to general mammalian (nuclear) DNA repair. * **D. epsilon (ε):** This is a high-fidelity enzyme responsible for the synthesis of the **leading strand** during nuclear DNA replication. It has 3' to 5' exonuclease activity for proofreading. **High-Yield Clinical Pearls for NEET-PG:** * **Polymerase Delta (δ):** Primarily synthesizes the **lagging strand** (Okazaki fragments). * **Mnemonic for Eukaryotic Pols:** * **α (Alpha):** Starts (Primer). * **β (Beta):** Bad (Low fidelity/Repair). * **γ (Gamma):** Global (Mitochondria). * **δ (Delta):** Delay (Lagging strand). * **ε (Epsilon):** Early (Leading strand). * **PCNA (Proliferating Cell Nuclear Antigen):** Acts as a "sliding clamp" for Pol δ and ε to increase processivity; it is a common marker for cell proliferation in pathology.

Q: Base substitution of GAC (Asp) to GAG (Glu) is an example of which type of mutation?

Point mutation. ### Explanation **1. Why Point Mutation is Correct:** A **point mutation** is a type of mutation where a single nucleotide base is changed, inserted, or deleted from a DNA or RNA sequence. In this case, the codon **GAC** (Aspartate) changes to **GAG** (Glutamate). Since only the third base (C → G) is substituted, it is a classic example of a point mutation. Specifically, this is a **missense mutation**, a sub-type of point mutation where the base substitution results in a different amino acid being incorporated into the protein. **2. Why Other Options are Incorrect:** * **B. Silent Mutation:** This occurs when a base substitution changes the codon but, due to the degeneracy of the genetic code, the **same amino acid** is produced. Since Aspartate changed to Glutamate, it is not silent. * **C. Non-sense Mutation:** This occurs when a base substitution results in a **stop codon** (UAA, UAG, or UGA), leading to premature termination of the polypeptide chain. GAG codes for Glutamate, not a stop signal. **3. NEET-PG High-Yield Clinical Pearls:** * **Transitions vs. Transversions:** A change from Purine to Purine (A↔G) or Pyrimidine to Pyrimidine (C↔T) is a **Transition**. A change from Purine to Pyrimidine (or vice versa) is a **Transversion**. (In this question, C → G is a Transversion). * **Sickle Cell Anemia:** The most high-yield clinical example of a point (missense) mutation. A single base change (GAG → GTG) leads to the substitution of **Glutamate by Valine** at the 6th position of the β-globin chain. * **Degeneracy/Wobble Hypothesis:** Most silent mutations occur at the 3rd position of the codon (the "wobble" position).

Q: What are the proteins found in chromosomes called?

Histones. **Explanation:** **1. Why Histones are the Correct Answer:** Chromosomes are composed of **chromatin**, which is a complex of DNA and specialized proteins. The primary proteins involved are **Histones**. These are small, highly basic proteins (rich in **Arginine and Lysine**) that carry a positive charge. This allows them to bind tightly to the negatively charged phosphate backbone of DNA. The fundamental unit of chromatin is the **nucleosome**, which consists of approximately 147 base pairs of DNA wrapped around a histone octamer (two copies each of H2A, H2B, H3, and H4). Histone H1 acts as the "linker histone" to stabilize the structure. **2. Why Other Options are Incorrect:** * **A. Nucleotides:** These are the structural building blocks of nucleic acids (DNA/RNA), consisting of a nitrogenous base, a pentose sugar, and a phosphate group—not proteins. * **C. Apoproteins:** These are the protein components of lipoproteins (e.g., Apo B-100 in LDL) or enzymes that require a cofactor but lack one. They are involved in lipid transport, not DNA packaging. * **D. Glycoproteins:** These are proteins with conjugated carbohydrate chains, typically found in cell membranes or secreted (e.g., TSH, EPO). **3. NEET-PG High-Yield Clinical Pearls:** * **Post-translational modifications:** Histones undergo acetylation, methylation, and phosphorylation. **Histone Acetylation** (by HATs) neutralizes the positive charge, relaxing chromatin (Euchromatin) and increasing transcription. * **Linker Histone:** H1 is the only histone not part of the nucleosome core; it facilitates the folding of nucleosomes into the 30-nm fiber. * **Protamines:** In spermatozoa, histones are replaced by protamines to allow for even denser DNA packaging.

Q: Which snRNA is not a part of the spliceosome?

U3. ### Explanation The correct answer is **U3**. **1. Why U3 is the correct answer:** The **spliceosome** is a complex of small nuclear ribonucleoproteins (snRNPs) responsible for removing introns from pre-mRNA. The major spliceosome consists of five snRNAs: **U1, U2, U4, U5, and U6**. **U3** is a small nucleolar RNA (**snoRNA**), not a spliceosomal snRNA. It is located in the **nucleolus** and is primarily involved in the processing and methylation of **pre-ribosomal RNA (rRNA)**, rather than mRNA splicing. **2. Analysis of incorrect options:** * **U1:** This snRNA initiates splicing by binding to the **5' splice site** (GU sequence) of the intron. * **U2:** This snRNA binds to the **branch point sequence** (adenine residue) within the intron, a crucial step in forming the catalytic center. * **U4:** It acts as a chaperone or "masking" agent for U6. It must dissociate from the complex to allow U6 to become catalytically active. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Autoimmune Correlation:** Antibodies against snRNPs (specifically **Anti-Smith/Anti-Sm antibodies**) are highly specific for **Systemic Lupus Erythematosus (SLE)**. * **The Splicing Mechanism:** Splicing involves two transesterification reactions, forming a characteristic **"Lariat" structure**. * **Alternative Splicing:** This process allows a single gene to code for multiple proteins (e.g., membrane-bound vs. secreted immunoglobulins), increasing proteomic diversity. * **Rule of Thumb:** All spliceosomal snRNAs are transcribed by RNA Polymerase II, except for **U6**, which is transcribed by **RNA Polymerase III**.

Q: Which of the following is involved in the post-transcriptional modification of mRNA?

All of the above. **Explanation:** Post-transcriptional modification is the process where a primary RNA transcript (hnRNA) is converted into mature mRNA in the nucleus before being exported to the cytoplasm for translation. 1. **Splicing (Option B):** This is the removal of non-coding sequences (introns) and the joining of coding sequences (exons). It is catalyzed by the spliceosome (snRNPs). 2. **Lariat Formation (Option A):** This is a specific structural intermediate formed during the splicing process. The 2'-OH group of an adenine residue at the "branch point" attacks the 5' splice site, creating a loop-like structure called a **lariat**. Thus, lariat formation is an integral part of splicing. 3. **Methylation (Option C):** This occurs during **5' Capping**, where a 7-methylguanosine cap is added to the 5' end of the mRNA. Additionally, internal methylation (like N6-methyladenosine) can occur to regulate mRNA stability and translation. Since all three processes are essential steps in the maturation of mRNA, **Option D (All of the above)** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Capping, Tailing, Splicing:** The three pillars of mRNA processing. Tailing involves adding a Poly-A tail to the 3' end via Poly-A polymerase (does not require a template). * **Systemic Lupus Erythematosus (SLE):** Patients often produce **Anti-Smith (Anti-Sm) antibodies**, which are directed against snRNPs (involved in splicing). * **Alternative Splicing:** Allows a single gene to code for multiple proteins (e.g., membrane-bound vs. secreted antibodies). * **Beta-Thalassemia:** Often caused by mutations at splice sites, leading to defective hemoglobin synthesis.

Question 1

Arrange the cyclins and CDKs in the cell cycle from G1 to S phase progression.

Accepted Answer

CDK6/Cyclin E

Answer

CDK4/Cyclin D

Answer

CDK1/Cyclin B

Answer

CDK2/Cyclin A

Question 2

Prenatal diagnosis of Hemophilia is best done by?

Accepted Answer

PCR

Answer

Linkage analysis

Answer

Cytometry

Answer

Microarray

Question 3

Where does miRNA typically bind to facilitate gene knockdown?

Accepted Answer

3' untranslated region

Answer

5' untranslated region

Answer

Introns

Answer

Both 3' and 5' untranslated regions

Question 4

What cellular component carries the genetic information for hereditary diseases?

Accepted Answer

DNA

Answer

Ribosome

Answer

RNA

Answer

Membrane

Question 5

What is the term for the transfer of genetic material between two non-homologous chromosomes during one meiotic division?

Accepted Answer

Translocation

Answer

Non-disjunction

Answer

Inversion

Answer

Isochromosome

Question 6

Which DNA polymerase is/are involved in the repair of mammalian DNA?

Accepted Answer

beta

Answer

alpha

Answer

gamma

Answer

epsilon

Question 7

Base substitution of GAC (Asp) to GAG (Glu) is an example of which type of mutation?

Accepted Answer

Point mutation

Answer

Silent mutation

Answer

Non-sense mutation

Answer

None of the above

Question 8

What are the proteins found in chromosomes called?

Accepted Answer

Histones

Answer

Nucleotides

Answer

Apoproteins

Answer

Glycoproteins

Question 9

Which snRNA is not a part of the spliceosome?

Accepted Answer

U3

Answer

U1

Answer

U2

Answer

U4

Question 10

Which of the following is involved in the post-transcriptional modification of mRNA?

Accepted Answer

All of the above

Answer

Lariat formation

Answer

Splicing

Answer

Methylation

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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