Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 661: What is the term for the transfer of genetic material between two non-homologous chromosomes during one meiotic division?

A. Translocation (Correct Answer)
B. Non-disjunction
C. Inversion
D. Isochromosome

Explanation: ### 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.

Question 662: Which DNA polymerase is/are involved in the repair of mammalian DNA?

A. alpha
B. beta (Correct Answer)
C. gamma
D. epsilon

Explanation: 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.

Question 663: Base substitution of GAC (Asp) to GAG (Glu) is an example of which type of mutation?

A. Point mutation (Correct Answer)
B. Silent mutation
C. Non-sense mutation
D. None of the above

Explanation: ### 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).

Question 664: What are the proteins found in chromosomes called?

A. Nucleotides
B. Histones (Correct Answer)
C. Apoproteins
D. Glycoproteins

Explanation: **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.

Question 665: Which snRNA is not a part of the spliceosome?

A. U1
B. U2
C. U3 (Correct Answer)
D. U4

Explanation: ### 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**.

Question 666: Which of the following is involved in the post-transcriptional modification of mRNA?

A. Lariat formation
B. Splicing
C. Methylation
D. All of the above (Correct Answer)

Explanation: **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 667: All of the following are true statements about Cockayne syndrome, EXCEPT?

A. Defective transcription-coupled repair
B. Photosensitivity
C. Increased risk of cancer (Correct Answer)
D. Mental retardation

Explanation: **Explanation:** Cockayne syndrome (CS) is a rare autosomal recessive disorder characterized by a defect in the **Transcription-Coupled Repair (TCR)** pathway, a sub-pathway of Nucleotide Excision Repair (NER). **Why "Increased risk of cancer" is the correct (Except) answer:** Unlike Xeroderma Pigmentosum (XP), which is also a defect in NER, Cockayne syndrome is **not** associated with an increased risk of skin cancer or internal malignancies. In CS, cells with damaged DNA are hypersensitive to UV light and undergo **apoptosis** (programmed cell death) rather than surviving with mutations. This high rate of cell death leads to growth failure and neurodegeneration but prevents the survival of pre-cancerous cells. **Analysis of other options:** * **A. Defective transcription-coupled repair:** This is the hallmark of CS. Mutations in *ERCC6* (CSB) or *ERCC8* (CSA) genes prevent the repair of DNA lesions on actively transcribed strands. * **B. Photosensitivity:** Patients exhibit extreme sensitivity to sunlight, resulting in severe sunburns even with minimal exposure. * **C. Mental retardation:** CS is characterized by progressive neurological degeneration, microcephaly, and developmental delays. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Short stature (cachectic dwarfism), "bird-like" facies (sunken eyes), and premature aging (progeroid features). * **Differentiating CS from XP:** XP has a **high cancer risk** but usually lacks the severe developmental/progeroid features of CS. * **Key Feature:** "Mickey Mouse" appearance on imaging due to basal ganglia calcification. * **Pathophysiology:** TCR specifically repairs DNA damage that stalls RNA polymerase during transcription.

Question 668: Which nucleic acid base is NOT present in a codon?

A. Adenine
B. Guanine
C. Uracil
D. Thymine (Correct Answer)

Explanation: **Explanation:** The fundamental concept here lies in the difference between **DNA** and **mRNA** during the process of gene expression. A **codon** is defined as a sequence of three nucleotides on a **messenger RNA (mRNA)** molecule that specifies a particular amino acid during protein synthesis (translation). 1. **Why Thymine is the Correct Answer:** Thymine (T) is a pyrimidine base found exclusively in **DNA**. During the process of transcription, DNA is used as a template to synthesize mRNA. In this process, RNA polymerase replaces Thymine with **Uracil (U)**. Therefore, while Thymine pairs with Adenine in DNA, it is never present in the mRNA codons used by ribosomes for translation. 2. **Analysis of Incorrect Options:** * **Adenine (A) & Guanine (G):** These are purine bases present in both DNA and RNA. They are essential components of various codons (e.g., AUG for Methionine). * **Uracil (C):** This is the pyrimidine base unique to RNA. It replaces Thymine and pairs with Adenine during translation. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **The Start Codon:** **AUG** (codes for Methionine in eukaryotes and N-formylmethionine in prokaryotes). * **Stop Codons (Nonsense Codons):** **UAA** (Ochre), **UAG** (Amber), and **UGA** (Opal). These do not code for any amino acid. * **Base Pairing:** In RNA, A pairs with U (2 hydrogen bonds) and G pairs with C (3 hydrogen bonds). * **Degeneracy:** The genetic code is "degenerate" or "redundant," meaning multiple codons can code for the same amino acid (except for Methionine and Tryptophan).

Question 669: DNA replication occurs in how many phases?

A. Two
B. Three (Correct Answer)
C. Four
D. Five

Explanation: **Explanation:** DNA replication is a highly coordinated semi-conservative process that ensures the faithful transmission of genetic information. In both prokaryotes and eukaryotes, the process is divided into **three distinct phases**: 1. **Initiation:** This phase involves the recognition of the 'Origin of Replication' (oriC), the unwinding of the double helix by **DNA Helicase**, and the stabilization of single strands by Single-Stranded Binding (SSB) proteins. 2. **Elongation:** This is the core synthesis phase where **DNA Polymerase III** (in prokaryotes) or Polymerases $\alpha, \delta, \epsilon$ (in eukaryotes) add nucleotides to the growing strand. It involves the formation of the leading strand and lagging strand (Okazaki fragments). 3. **Termination:** This occurs when the replication forks meet or reach a specific termination sequence (e.g., *Ter* sites in bacteria), leading to the disassembly of the replication machinery and the ligation of nicks by **DNA Ligase**. **Analysis of Incorrect Options:** * **Option A (Two):** While some describe replication as "synthesis and proofreading," these are biochemical activities, not the structural phases of the entire process. * **Option C & D (Four/Five):** These are incorrect as they likely confuse the phases of the **Cell Cycle** (G1, S, G2, M) or the stages of **Mitosis** (Prophase, Metaphase, Anaphase, Telophase) with the specific molecular process of DNA replication. **High-Yield Clinical Pearls for NEET-PG:** * **S-Phase:** DNA replication occurs exclusively during the S-phase of the eukaryotic cell cycle. * **Directionality:** DNA synthesis always proceeds in the **5' to 3' direction**. * **Topoisomerase/Gyrase:** These enzymes relieve torsional strain (supercoiling). **Fluoroquinolones** (e.g., Ciprofloxacin) act by inhibiting DNA Gyrase in bacteria. * **Telomerase:** A specialized reverse transcriptase that maintains the ends of linear chromosomes; its activity is high in cancer cells and stem cells.

Question 670: A child develops blisters on exposure to sunlight. Irregular dark spots on the skin are also found. He is very likely to have a defect in which of the following mechanisms?

A. Nucleotide excision repair (Correct Answer)
B. Mismatch repair
C. Recombination repair
D. Thymine dimers

Explanation: ### Explanation The clinical presentation described—extreme photosensitivity (blisters on sun exposure) and irregular hyperpigmented spots (poikiloderma)—is classic for **Xeroderma Pigmentosum (XP)**. **1. Why Nucleotide Excision Repair (NER) is correct:** XP is an autosomal recessive disorder caused by a deficiency in the **Nucleotide Excision Repair** pathway. Normally, UV radiation causes the formation of **pyrimidine dimers** (specifically thymine dimers) in DNA. The NER mechanism identifies these bulky lesions, uses endonucleases to excise the damaged oligonucleotide strand, and fills the gap using DNA polymerase and ligase. In XP, this repair fails, leading to accumulated mutations, skin malignancies (Basal Cell Carcinoma, Squamous Cell Carcinoma, Melanoma), and the characteristic dermatological findings. **2. Why the other options are incorrect:** * **Mismatch Repair (MMR):** Defects here lead to **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). It corrects errors missed during DNA replication, not UV-induced damage. * **Recombination Repair:** Defects in homologous recombination repair are associated with **Ataxia-Telangiectasia** (ATM gene) or **Breast/Ovarian cancer** (BRCA1/2). * **Thymine Dimers:** This is the *result* of the damage (the lesion itself), not the *mechanism* of repair that is defective. **Clinical Pearls for NEET-PG:** * **Key Enzyme:** The most common defect in XP is a deficiency in **UV-specific endonuclease**. * **"Dry Skin":** The name *Xeroderma Pigmentosum* literally translates to "parched, pigmented skin." * **Neurological symptoms:** About 20-30% of patients may also exhibit progressive neurological degeneration. * **High-Yield Association:** NER is responsible for repairing "bulky adducts," while Base Excision Repair (BER) handles "spontaneous/deamination" insults.

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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