Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 471: The primary defect in Xeroderma pigmentosa is related to which cellular process?

A. Formation of thymidine dimers (Correct Answer)
B. Defective poly ADP ribose polymerase activity
C. Defective exonuclease activity
D. Formation of adenine dimers

Explanation: **Explanation:** **Xeroderma Pigmentosum (XP)** is an autosomal recessive genetic disorder characterized by extreme sensitivity to ultraviolet (UV) radiation. **Why Option A is Correct:** The primary defect in XP is a deficiency in the **Nucleotide Excision Repair (NER)** pathway. When skin is exposed to UV light (specifically UV-B), it causes the covalent cross-linking of adjacent pyrimidine bases, most commonly leading to the **formation of thymidine dimers** (cyclobutane pyrimidine dimers). In healthy individuals, the NER pathway identifies these bulky lesions, and specific endonucleases (XP proteins A through G) excise the damaged DNA segment. In XP patients, this repair mechanism fails, leading to the accumulation of mutations and a high risk of skin malignancies. **Why Other Options are Incorrect:** * **Option B:** Defective Poly ADP Ribose Polymerase (PARP) activity is associated with impaired Base Excision Repair (BER) and is a target in cancer therapies (PARP inhibitors), but it is not the primary defect in XP. * **Option C:** Defective 3'→5' exonuclease activity (proofreading) is typically associated with **Hereditary Non-Polyposis Colorectal Cancer (HNPCC/Lynch Syndrome)**, which involves Mismatch Repair (MMR) defects. * **Option D:** While other dimers can form, **thymidine dimers** are the hallmark lesion caused by UV radiation and the specific substrate for the NER pathway relevant to XP. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Severe sunburn with minimal sun exposure, "parchment-like" skin (xeroderma), hyperpigmented macules, and telangiectasias. * **Malignancy Risk:** 1000-fold increased risk of Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma. * **Neurological Involvement:** Seen in the **De Sanctis-Cacchione syndrome** variant of XP. * **Key Enzyme:** The specific enzyme often cited as deficient is **UV-specific endonuclease**.

Question 472: A promoter sequence is an example of which type of factor?

A. Cis acting factor (Correct Answer)
B. Trans acting factor
C. Coactivator
D. Mediator

Explanation: **Explanation:** In molecular biology, gene expression is regulated by the interaction between DNA sequences and regulatory proteins. **1. Why Cis-acting factor is correct:** A **cis-acting factor** (or element) is a specific sequence of DNA that regulates the expression of genes located on the **same chromosome** (the word "cis" means "on the same side"). A **promoter** is a classic example; it is a DNA sequence located upstream of the structural gene where RNA polymerase binds to initiate transcription. Because the promoter is a physical part of the DNA molecule it regulates, it is classified as a cis-acting element. Other examples include enhancers and silencers. **2. Why the other options are incorrect:** * **Trans-acting factor:** These are typically **proteins** (like transcription factors) encoded by genes located elsewhere in the genome. They are produced in the cytosol, enter the nucleus, and can diffuse to bind to any relevant DNA sequence regardless of its chromosomal location. * **Coactivator:** These are proteins that increase the rate of transcription by binding to transcription factors rather than binding directly to the DNA promoter sequence itself. * **Mediator:** This is a large multi-protein complex that acts as a bridge between gene-specific transcription factors (bound to enhancers) and the RNA polymerase II machinery. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **C**is is **C**ontiguous (DNA sequence); **T**rans is **T**ransferred (Protein/Factor). * **Mutations:** A mutation in a *cis*-acting element (like a TATA box mutation) usually affects only the gene on that specific allele, whereas a mutation in a *trans*-acting factor can affect the expression of multiple genes throughout the genome. * **TATA Box:** The most common promoter element in eukaryotes, located approximately 25-35 base pairs upstream of the transcription start site.

Question 473: Which of the following is true about DNA polymerase in eukaryotes?

A. Components include alpha, beta, TS, delta, and epsilon subunits. (Correct Answer)
B. Beta subunit is associated with DNA repair.
C. TS subunit is associated with DNA repair.
D. Delta subunit is associated with the synthesis of mitochondrial DNA.

Explanation: In eukaryotic DNA replication, the process is orchestrated by several specialized DNA polymerases, each designated by Greek letters. ### **Explanation of the Correct Answer** **Option A** is correct because the primary eukaryotic DNA polymerases are **Alpha (α), Beta (β), Gamma (γ), Delta (δ), and Epsilon (ε)**. While the question uses "TS" (likely a typographical variant for Gamma/$\gamma$ in some older question banks or a distractor), the core classification of eukaryotic polymerases involves these five distinct types. Unlike prokaryotes (Pol I, II, III), eukaryotes utilize these specific subunits to divide labor between leading strand synthesis, lagging strand synthesis, and repair. ### **Analysis of Incorrect Options** * **Option B & C:** While **Beta (β)** is indeed involved in DNA repair (specifically Base Excision Repair), the question asks for the "true" statement regarding the general components. However, in the context of this specific question's structure, Option A serves as the definitive structural overview. (Note: **Gamma ($\gamma$)** is the one often confused with "TS" in poorly formatted papers; Gamma is for mitochondria). * **Option D:** The **Delta (δ)** subunit is responsible for **lagging strand synthesis** and has 3’→5’ exonuclease activity. The synthesis of **mitochondrial DNA** is exclusively performed by **DNA Polymerase Gamma ($\gamma$)**. ### **High-Yield NEET-PG Pearls** * **Pol Alpha (α):** Contains **primase** activity; initiates DNA synthesis by laying down an RNA primer. * **Pol Delta (δ):** Synthesizes the **lagging strand** (Okazaki fragments); possesses proofreading ability. * **Pol Epsilon (ε):** Synthesizes the **leading strand**; highly processive. * **Pol Gamma ($\gamma$):** The only polymerase found in the **mitochondria**. * **PCNA (Proliferating Cell Nuclear Antigen):** A "sliding clamp" protein that helps Pol Delta and Epsilon stay attached to the DNA; it is a clinical marker for cell proliferation in pathology.

Question 474: Senescent cells are deficient in what?

A. RNA polymerase
B. DNA polymerase
C. Telomerase (Correct Answer)
D. Helicase

Explanation: **Explanation:** **1. Why Telomerase is the Correct Answer:** Cellular senescence, often described by the **Hayflick Limit**, refers to the phenomenon where normal cells cease to divide after a certain number of cell doublings. The primary molecular driver of this process is the progressive shortening of **telomeres** (repetitive TTAGGG sequences at chromosome ends). * **Telomerase** is a ribonucleoprotein enzyme (a reverse transcriptase) that adds telomeric repeats to the 3' end of DNA, maintaining chromosomal stability. * While germ cells, stem cells, and cancer cells express high levels of telomerase to achieve "immortality," **senescent somatic cells are deficient in telomerase**. Once telomeres reach a critical minimum length, a DNA damage response is triggered, leading to permanent cell cycle arrest (senescence). **2. Why Other Options are Incorrect:** * **A & B (RNA and DNA Polymerase):** Senescent cells are metabolically active; they continue to transcribe genes (requiring RNA polymerase) and maintain basic cellular functions. While they do not replicate DNA for division, the enzymes themselves are not fundamentally "deficient" in the way telomerase is absent or silenced. * **D (Helicase):** Helicases are essential for various processes, including DNA repair and transcription, which persist in senescent cells. Deficiency in specific helicases (e.g., *WRN* gene in Werner Syndrome) actually *causes* premature senescence rather than being a defining characteristic of a standard senescent cell. **3. Clinical Pearls for NEET-PG:** * **Hayflick Limit:** The finite number of times a normal human cell population will divide (approx. 40–60 times). * **SASP:** Senescent cells secrete a "Senescence-Associated Secretory Phenotype" (pro-inflammatory cytokines), which contributes to aging and tumorigenesis. * **Marker:** **Senescence-associated beta-galactosidase (SA-β-gal)** is the most common histochemical marker used to identify senescent cells. * **Shelterin Complex:** A protein complex that protects telomeres; mutations here also lead to genomic instability.

Question 475: Aminoacyl t-RNA is required for all except?

A. Hydroxyproline (Correct Answer)
B. Methionine
C. Cystine
D. Lysine

Explanation: **Explanation:** The process of translation (protein synthesis) requires specific amino acids to be attached to their corresponding tRNA molecules (forming **aminoacyl-tRNA**) to be incorporated into a growing polypeptide chain. This occurs only for the **20 standard (primary) amino acids** encoded by the universal genetic code. **Why Hydroxyproline is the correct answer:** Hydroxyproline is a **non-standard amino acid**. It is not incorporated into proteins during translation because there is no genetic codon or specific tRNA for it. Instead, it is formed via **post-translational modification**. In collagen synthesis, specific proline residues already incorporated into the polypeptide chain are hydroxylated by the enzyme **prolyl hydroxylase** (requiring Vitamin C and Iron). Since it does not enter the ribosome via a tRNA carrier, aminoacyl-tRNA is not required. **Analysis of incorrect options:** * **B. Methionine:** A standard amino acid (encoded by AUG) that serves as the initiating amino acid in eukaryotes. It requires methionyl-tRNA for translation. * **C. Cystine:** While cysteine is the standard amino acid, **Cystine** is formed by the oxidation of two cysteine residues. However, in the context of this classic biochemistry question, it refers to the protein-constituent sulfur amino acids which are incorporated via the tRNA pathway (as Cysteine). * **D. Lysine:** A basic standard amino acid encoded by AAA and AAG. it requires lysyl-tRNA for protein synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Post-translational modifications:** Hydroxyproline and Hydroxylysine are classic examples. They are crucial for the cross-linking and thermal stability of **Collagen**. * **Scurvy:** Deficiency of Vitamin C leads to defective hydroxylation of proline, resulting in weak collagen and symptoms like bleeding gums and poor wound healing. * **Exceptions:** **Selenocysteine** is often called the "21st amino acid." Unlike hydroxyproline, it *does* have a specialized tRNA (tRNA^Sec) that recognizes the UGA stop codon in a specific context.

Question 476: The DNA segment from which the primary mRNA transcript is copied or transcribed is called:

A. Coding strand
B. Initiator methionine domain
C. Template strand (Correct Answer)
D. Translation unit

Explanation: ### Explanation **1. Why the Correct Answer is Right:** Transcription is the process by which genetic information is transferred from DNA to RNA. DNA is double-stranded, but only one strand serves as the physical substrate for RNA polymerase. This strand is the **Template strand** (also known as the **Antisense** or **Non-coding strand**). * **Mechanism:** RNA polymerase reads the template strand in the **3' to 5' direction** to synthesize a complementary mRNA molecule in the **5' to 3' direction**. * **Complementarity:** Because of base-pairing rules, the resulting mRNA is a replica of the *other* DNA strand (the coding strand), with Uracil replacing Thymine. **2. Why the Other Options are Wrong:** * **A. Coding strand:** Also called the **Sense strand**. It has the same sequence as the mRNA (except T instead of U). While it "codes" for the protein sequence, it is **not** the strand used by RNA polymerase for copying. * **B. Initiator methionine domain:** This is a distractor. Methionine is the first amino acid in eukaryotic translation (coded by the start codon AUG), but it refers to a protein/translation concept, not a DNA segment used for transcription. * **C. Translation unit:** This is a misnomer. The correct term is a **Transcription Unit**, which includes the promoter, the structural gene, and a terminator. Translation occurs at the ribosome, not on the DNA segment. **3. High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** Remember the "3-5-5-3" rule: Template is read **3'→5'**; mRNA is synthesized **5'→3'**. * **RNA Polymerase II:** In eukaryotes, this is the specific enzyme responsible for synthesizing mRNA. * **Alpha-Amanitin:** A toxin from the *Amanita phalloides* mushroom that inhibits RNA Polymerase II, leading to severe liver failure (a common clinical correlate in biochemistry). * **Promoter Region:** The TATA box (Hogness box) is the key eukaryotic promoter sequence located upstream of the transcription start site.

Question 477: How many nonsense or genetic codons are there?

A. 2
B. 3 (Correct Answer)
C. 4
D. 5

Explanation: ### Explanation **1. Why Option B is Correct:** In the standard genetic code, there are **64 possible codons** (4³ combinations of the four nitrogenous bases). Out of these, **61 are sense codons** that code for specific amino acids. The remaining **3 codons** do not code for any amino acid and are known as **Nonsense codons** or **Stop codons**. Their primary function is to signal the termination of polypeptide chain synthesis during translation. The three nonsense codons are: * **UAA** (Ochre) * **UAG** (Amber) * **UGA** (Opal) **2. Why Other Options are Incorrect:** * **Option A (2):** While there are two types of codons (Sense and Nonsense), the numerical count of nonsense codons is three. * **Option C (4):** There are 4 distinct nitrogenous bases (A, U, G, C), but this does not correlate to the number of stop codons. * **Option D (5):** There is no biological basis for 5 nonsense codons in the standard genetic code. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Initiation Codon:** **AUG** (Methionine) is the universal start codon. In prokaryotes, it codes for N-formylmethionine. * **Degeneracy/Redundancy:** Multiple codons can code for a single amino acid (e.g., Leucine has 6 codons), but one codon never codes for more than one amino acid (**Unambiguous**). * **Nonsense Mutation:** A point mutation that changes a sense codon into a nonsense codon, leading to premature termination of the protein and usually resulting in a non-functional product (e.g., certain types of β-thalassemia). * **Exceptions:** In human mitochondria, **UGA** codes for Tryptophan rather than acting as a stop codon.

Question 478: The disorder shown in the illustration is related to which DNA repair mechanism?

A. Mismatch repair
B. Base excision repair
C. Nucleotide excision repair (Correct Answer)
D. SOS repair

Explanation: ***Nucleotide excision repair*** - The illustration shows **Xeroderma Pigmentosum (XP)**, a disorder caused by defective **nucleotide excision repair (NER)** mechanism that removes **UV-induced DNA lesions** like pyrimidine dimers. - NER defects result in **extreme photosensitivity**, **premature aging**, and **high risk of skin cancers** due to inability to repair **bulky DNA adducts** caused by UV radiation. *Mismatch repair* - Defects in **mismatch repair** cause **Lynch syndrome** (hereditary nonpolyposis colorectal cancer), not the photosensitive skin disorder shown. - This mechanism corrects **base-base mismatches** and **insertion-deletion loops** during DNA replication, not UV-induced damage. *Base excision repair* - **Base excision repair (BER)** removes **small base modifications** like **8-oxoguanine** and **uracil** through glycosylases. - BER defects are not associated with **photosensitivity** or the characteristic skin manifestations seen in this illustration. *SOS repair* - **SOS repair** is an **error-prone DNA repair** system in bacteria activated under extreme DNA damage conditions. - This mechanism does not exist in **human cells** and is not related to any human genetic disorders or photosensitivity syndromes.

Question 479: Mutations are due to changes in:

A. DNA nucleotide sequence (Correct Answer)
B. RNA nucleotide sequence
C. Amino acid sequence of ribonuclease
D. Cell walls

Explanation: **Explanation:** **1. Why Option A is Correct:** A mutation is defined as a permanent, heritable change in the **DNA nucleotide sequence**. DNA serves as the primary genetic blueprint of the cell. Any alteration in its sequence—whether through substitutions (point mutations), insertions, or deletions—can lead to changes in the genetic code. These changes are then propagated through replication and, if they occur in germ cells, are passed to offspring. **2. Why Other Options are Incorrect:** * **Option B (RNA nucleotide sequence):** While errors can occur during transcription (DNA to RNA), these are transient and not considered mutations because they are not inherited or permanent. If the underlying DNA is normal, subsequent RNA transcripts will be correct. * **Option C (Amino acid sequence):** Changes in amino acids are the *result* of a mutation (at the protein level), not the mutation itself. Ribonuclease is simply a specific enzyme; mutations affect the genome, not just one specific protein's structure directly. * **Option D (Cell walls):** Cell walls are structural components (primarily in bacteria and plants). Changes here are phenotypic consequences of genetic or environmental factors, not the source of genetic mutation. **Clinical Pearls for NEET-PG:** * **Transition vs. Transversion:** Transition is a point mutation where a purine is replaced by a purine (A↔G) or pyrimidine by pyrimidine (C↔T). Transversion is purine ↔ pyrimidine. * **Silent Mutation:** A change in DNA that does not change the amino acid (due to degeneracy of the genetic code). * **Missense Mutation:** Results in a different amino acid (e.g., Sickle Cell Anemia: Glutamic acid replaced by Valine at the 6th position of the β-globin chain). * **Nonsense Mutation:** Creates a premature stop codon (UAG, UAA, UGA), leading to a truncated protein.

Question 480: A mutation in a codon that causes a change in the coded amino acid is known as what?

A. Mitogenesis
B. Somatic mutation
C. Missense mutation (Correct Answer)
D. Recombination

Explanation: **Explanation:** **1. Why Missense Mutation is Correct:** A **missense mutation** is a type of point mutation where a single nucleotide change results in a codon that codes for a **different amino acid**. This can alter the tertiary structure and function of the resulting protein. A classic clinical example is **Sickle Cell Anemia**, where a point mutation (GAG → GTG) causes Glutamate to be replaced by Valine at the 6th position of the β-globin chain. **2. Why the Other Options are Incorrect:** * **Mitogenesis (A):** This refers to the induction of mitosis (cell division), typically by a mitogen. It is a physiological process, not a type of genetic mutation. * **Somatic Mutation (B):** This describes *where* a mutation occurs (in non-germline body cells) rather than the *effect* on the genetic code. Somatic mutations are not passed to offspring but can lead to cancer. * **Recombination (D):** This is the process by which DNA strands are broken and repaired to produce new combinations of alleles (e.g., crossing over during meiosis). It is a mechanism for genetic diversity, not a specific point mutation. **3. High-Yield Clinical Pearls for NEET-PG:** * **Silent Mutation:** A nucleotide change that codes for the *same* amino acid (due to the degeneracy of the genetic code). * **Nonsense Mutation:** A change that results in a **premature stop codon** (UAA, UAG, UGA), leading to a truncated, usually non-functional protein (e.g., β-thalassemia). * **Frameshift Mutation:** Insertion or deletion of nucleotides (not in multiples of three), altering the entire reading frame downstream (e.g., Duchenne Muscular Dystrophy). * **Transition vs. Transversion:** Transition is Purine to Purine (A↔G) or Pyrimidine to Pyrimidine (C↔T); Transversion is Purine to Pyrimidine or vice versa.

Practice by Chapter

DNA Replication and Repair Mechanisms

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Transcription Factors and Gene Regulation

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Epigenetics and DNA Methylation

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RNA Processing and Splicing

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miRNA and RNA Interference

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Protein Synthesis and Post-Translational Modifications

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Genomics and Human Genome Project

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Single Nucleotide Polymorphisms

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Gene Therapy Approaches

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CRISPR-Cas9 and Genome Editing

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DNA Fingerprinting and Forensics

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Molecular Basis of Genetic Diseases

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