Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 351: All of the following cell types contain the enzyme telomerase which protects the length of telomeres at the end of chromosomes, except?

A. Germinal cells
B. Somatic cells (Correct Answer)
C. Hemopoietic cells
D. Tumor cells

Explanation: **Explanation:** The core concept tested here is the distribution of **Telomerase**, a specialized ribonucleoprotein (RNA-dependent DNA polymerase) that adds TTAGGG repeats to the 3' ends of chromosomes to prevent the "end-replication problem." **Why Somatic Cells are the correct answer:** Most mature **somatic cells** (differentiated body cells) lack significant telomerase activity. Consequently, with every cell division, their telomeres shorten. Once telomeres reach a critical minimum length, the cell enters **replicative senescence** (the Hayflick limit). This acts as a biological clock and a protective mechanism against uncontrolled proliferation. **Analysis of Incorrect Options:** * **Germinal cells (A):** These cells (e.g., in testes and ovaries) must pass a complete genome to the next generation. They express high levels of telomerase to maintain telomere length indefinitely. * **Hemopoietic cells (C):** Stem cells, including hematopoietic stem cells and basal cells of the epidermis, require high proliferative capacity to replenish blood and tissues. They maintain telomerase activity to support lifelong self-renewal. * **Tumor cells (D):** Approximately 85-90% of cancer cells upregulate or reactivate telomerase. This allows them to bypass senescence and achieve **immortality**, a hallmark of malignancy. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** Telomerase consists of two main components: **TERC** (RNA template) and **TERT** (Reverse Transcriptase enzyme). * **Progeria:** Accelerated telomere shortening is linked to premature aging syndromes like Hutchinson-Gilford Progeria. * **Dyskeratosis Congenita:** A genetic disorder caused by mutations in telomerase components, leading to bone marrow failure. * **Alternative Lengthening of Telomeres (ALT):** A telomerase-independent mechanism used by some cancer cells to maintain telomere length via homologous recombination.

Question 352: Mitochondrial DNA is:

A. Circular double stranded (Correct Answer)
B. Circular single stranded
C. Linear double helix
D. None of these

Explanation: **Explanation:** Human mitochondrial DNA (mtDNA) is a unique genetic material distinct from nuclear DNA. It consists of a **circular, double-stranded** molecule (Option A). This structure is reminiscent of bacterial DNA, supporting the **Endosymbiotic Theory**, which suggests mitochondria evolved from ancient aerobic prokaryotes. Each mitochondrion contains multiple copies of this circular genome. It is approximately 16.5 kb in size and encodes 37 genes: 13 for oxidative phosphorylation proteins, 22 for tRNA, and 2 for rRNA. Unlike nuclear DNA, mtDNA lacks introns and is not packaged with histones. **Analysis of Incorrect Options:** * **Option B (Circular single stranded):** While some viruses possess single-stranded circular DNA, human mtDNA is always double-stranded, consisting of a "Heavy" (H) strand and a "Light" (L) strand. * **Option C (Linear double helix):** This describes **nuclear DNA**. Nuclear DNA is organized into linear chromosomes associated with histone proteins, whereas mtDNA remains circular and "naked." **High-Yield Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother because the sperm's mitochondria are degraded upon fertilization. * **Heteroplasmy:** This refers to the presence of a mixture of more than one type of organellar genome (mutated vs. wild-type) within a cell. It explains the variable clinical severity of mitochondrial diseases. * **High Mutation Rate:** mtDNA has a mutation rate 10 times higher than nuclear DNA due to the lack of robust repair mechanisms and proximity to free radicals generated by the electron transport chain. * **Leber’s Hereditary Optic Neuropathy (LHON)** and **MELAS** are classic examples of mitochondrial inheritance disorders.

Question 353: In a DNA, the coding region reads 5'-CGT-3'. This would code in the RNA as

A. 5'-CGU-3' (Correct Answer)
B. 5'-GCA-3'
C. 5'-ACG-3'
D. 5'-UGC-3'

Explanation: ### Explanation **1. Why Option A is Correct:** In molecular biology, DNA consists of two strands: the **Coding strand** (Sense strand) and the **Template strand** (Antisense strand). * The **Template strand** (3' to 5') is used by RNA polymerase to synthesize mRNA. * The **Coding strand** (5' to 3') has the same sequence and polarity as the resulting mRNA, with the sole exception that **Thymine (T)** in DNA is replaced by **Uracil (U)** in RNA. Since the question provides the coding region sequence as **5'-CGT-3'**, the mRNA sequence will be identical but with Uracil: **5'-CGU-3'**. **2. Why Other Options are Incorrect:** * **Option B (5'-GCA-3'):** This is the complementary sequence to the coding strand. It would be the sequence found on the template DNA strand, not the mRNA. * **Option C (5'-ACG-3'):** This is the sequence read in reverse (3' to 5') or a scrambled version; it does not follow the rules of transcription polarity. * **Option D (5'-UGC-3'):** This represents the "Anticodon" sequence found on tRNA, which would pair with the mRNA codon 5'-GCA-3', but it does not match the transcription product of the given DNA. **3. NEET-PG High-Yield Pearls:** * **The "Golden Rule":** mRNA sequence = Coding strand sequence (replace T with U). * **Directionality:** Transcription always proceeds in the **5' → 3' direction**. * **Template vs. Coding:** RNA Polymerase reads the template strand in the 3' → 5' direction to synthesize mRNA 5' → 3'. * **Clinical Correlation:** Mutations in the coding region (Exons) can lead to altered protein function, whereas mutations in the promoter region (upstream of the coding sequence) usually affect the *quantity* of mRNA produced.

Question 354: What was the contribution of Arthur Kornberg to molecular genetics?

A. Chemical synthesis of ribonucleotides (Correct Answer)
B. Sequencing of amino acids
C. Base pairing rules
D. Structure of DNA

Explanation: **Explanation:** **Arthur Kornberg** was a Nobel Prize-winning biochemist primarily recognized for his work on the enzymatic synthesis of DNA. He discovered **DNA Polymerase I** (often called the Kornberg enzyme) in *E. coli*. In the context of this question, his contribution involves the **chemical/enzymatic synthesis of polynucleotides** (specifically DNA/ribonucleotides), demonstrating how genetic material is replicated in a cell-free system. This laid the foundation for understanding DNA replication and recombinant DNA technology. **Analysis of Options:** * **Option A (Correct):** Kornberg successfully synthesized DNA in a test tube using DNA polymerase, primers, and nucleotide triphosphates. While he is most famous for DNA, his work fundamentally addressed the synthesis of polynucleotide chains from nucleotide precursors. * **Option B (Incorrect):** The sequencing of amino acids (specifically insulin) was the landmark contribution of **Frederick Sanger**, who won his first Nobel Prize for this achievement. * **Option C (Incorrect):** Base pairing rules (A=T and G=C) were established by **Erwin Chargaff** (Chargaff’s Rules), which provided the crucial hint for the double helix model. * **Option D (Incorrect):** The double-helical structure of DNA was proposed by **Watson and Crick**, based on X-ray diffraction data provided by **Rosalind Franklin** and Maurice Wilkins. **High-Yield Clinical Pearls for NEET-PG:** * **Kornberg Enzyme:** Refers to **DNA Polymerase I**, which has 5'→3' polymerase, 3'→5' exonuclease (proofreading), and 5'→3' exonuclease (primer removal) activities. * **Har Gobind Khorana:** Often confused with Kornberg; Khorana is credited with the chemical synthesis of oligonucleotides and deciphering the genetic code. * **DNA Poly III:** The primary enzyme for *E. coli* DNA replication; DNA Poly I (Kornberg's) is mainly for repair and primer removal.

Question 355: The process which occurs over cytidine residues and often results in gene inactivation is?

A. Gene rearrangement
B. Pseudogene formation
C. Histone acetylation
D. DNA Methylation (Correct Answer)

Explanation: **Explanation:** **DNA Methylation** is a key epigenetic mechanism involved in the regulation of gene expression. It involves the addition of a methyl group (–CH₃) to the 5th carbon of the **cytidine residue**, typically occurring at **CpG islands** (regions with a high frequency of cytosine-guanine dinucleotide pairs) located in gene promoters. This process is catalyzed by the enzyme **DNA Methyltransferase (DNMT)**. * **Why it is correct:** Methylation of promoter regions physically impedes the binding of transcription factors and recruits methyl-CpG-binding domain proteins (MBDs), which further recruit histone deacetylases. This leads to chromatin condensation (heterochromatin), effectively **silencing or inactivating the gene.** **Analysis of Incorrect Options:** * **A. Gene rearrangement:** This involves the physical shuffling of DNA segments (e.g., VDJ recombination in B-cells). While it changes gene structure, it is not a specific process targeting cytidine for inactivation. * **B. Pseudogene formation:** These are non-functional segments of DNA that resemble functional genes but have lost their expression ability due to accumulated mutations over evolutionary time, not via cytidine modification. * **C. Histone acetylation:** This process occurs on lysine residues of histone tails. It neutralizes positive charges, relaxing the chromatin (euchromatin) and typically **activates** gene transcription—the opposite of the question's premise. **NEET-PG High-Yield Pearls:** * **Genomic Imprinting:** DNA methylation is the basis for imprinting (e.g., **Prader-Willi and Angelman syndromes**), where one parental allele is silenced. * **Fragile X Syndrome:** Characterized by hypermethylation of the FMR1 gene due to CGG triplet repeats. * **Cancer:** Hypermethylation of tumor suppressor genes (like *p16* or *BRCA1*) is a common mechanism in oncogenesis. * **5-Azacytidine:** A drug used in myelodysplastic syndrome that inhibits DNA methyltransferase to "reactivate" silenced genes.

Question 356: All of the following disorders are due to defective nucleotide excision repair, EXCEPT?

A. Xeroderma pigmentosum
B. Cockayne syndrome
C. Trichothiodystrophy
D. Hereditary nonpolyposis colon cancer (Correct Answer)

Explanation: **Explanation:** The correct answer is **Hereditary Nonpolyposis Colon Cancer (HNPCC)**, also known as Lynch Syndrome. **1. Why HNPCC is the correct answer:** HNPCC is caused by a defect in **Mismatch Repair (MMR)**, not Nucleotide Excision Repair (NER). Mutations typically occur in the *MSH2* or *MLH1* genes. This defect leads to **microsatellite instability (MSI)**, characterized by the accumulation of errors in short, repetitive DNA sequences, significantly increasing the risk of colorectal, endometrial, and ovarian cancers. **2. Analysis of incorrect options (NER defects):** Nucleotide Excision Repair (NER) is responsible for removing "bulky" DNA lesions, such as pyrimidine dimers caused by UV radiation. * **Xeroderma Pigmentosum (XP):** The classic example of an NER defect. Patients have extreme photosensitivity and a 2000-fold increased risk of skin cancer due to the inability to repair UV-induced damage. * **Cockayne Syndrome:** A rare autosomal recessive disorder caused by a defect in transcription-coupled NER. It presents with growth failure, microcephaly, and "progeroid" (premature aging) features, but notably *no* increased risk of skin cancer. * **Trichothiodystrophy (TTD):** Another NER-related disorder characterized by sulfur-deficient brittle hair, developmental delay, and ichthyosis. **3. High-Yield Clinical Pearls for NEET-PG:** * **NER Defect:** Xeroderma Pigmentosum, Cockayne Syndrome, Trichothiodystrophy. * **Mismatch Repair (MMR) Defect:** HNPCC (Lynch Syndrome). * **Base Excision Repair (BER) Defect:** MUTYH-associated polyposis. * **Homologous Recombination Defect:** BRCA 1 & 2 (Breast/Ovarian cancer), Fanconi Anemia, Bloom Syndrome. * **Non-homologous End Joining (NHEJ) Defect:** SCID (Severe Combined Immunodeficiency), Ataxia-telangiectasia.

Question 357: Supercoiling occurs in:

A. Eukaryotes only
B. Prokaryotes only
C. Both Eukaryotes and Prokaryotes (Correct Answer)
D. Viruses only

Explanation: **Explanation:** **1. Why the Correct Answer is Right:** DNA supercoiling is a fundamental biological process required to package long strands of DNA into the tiny dimensions of a cell or nucleus. It involves the twisting of the DNA helix upon itself, much like a telephone cord. * **In Prokaryotes:** Since they lack a nucleus, supercoiling (primarily negative supercoiling) is essential to condense the circular genomic DNA into a compact **nucleoid**. This is mediated by the enzyme **DNA Gyrase** (Topoisomerase II). * **In Eukaryotes:** Linear DNA is wrapped around histone octamers to form nucleosomes. This wrapping inherently induces supercoiling, which is necessary for chromatin folding and regulating access to the genome during transcription and replication. **2. Why Incorrect Options are Wrong:** * **A & B (Eukaryotes/Prokaryotes only):** These are incorrect because supercoiling is a universal phenomenon. Both circular (prokaryotic) and linear (eukaryotic) DNA molecules require topological tension management to fit within cellular boundaries and to facilitate the "unzipping" of strands during replication. * **D (Viruses only):** While some viruses exhibit supercoiling in their DNA genomes, it is not exclusive to them. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Topoisomerases:** These enzymes regulate supercoiling. **Type I** cuts one strand (no ATP), while **Type II** cuts both strands (requires ATP). * **Pharmacological Relevance:** * **Quinolones/Fluoroquinolones** (e.g., Ciprofloxacin) inhibit bacterial **DNA Gyrase**, preventing the relief of supercoiling and halting replication. * **Etoposide/Teniposide** inhibit eukaryotic Topoisomerase II (used in cancer chemotherapy). * **Irinotecan/Topotecan** inhibit eukaryotic Topoisomerase I. * **Linking Fact:** Most biological DNA is **negatively supercoiled**, which makes it easier to separate the strands for replication compared to positively supercoiled DNA.

Question 358: Chromosome 21 belongs to which of the following subtypes?

A. Acrocentric chromosome (Correct Answer)
B. Metacentric chromosome
C. Submetacentric chromosome
D. Telocentric chromosome

Explanation: **Explanation:** Chromosomes are classified based on the position of the **centromere**, which determines the relative lengths of the short arm (p) and the long arm (q). **1. Why Acrocentric is Correct:** In **acrocentric chromosomes**, the centromere is located very close to one end. This results in one extremely short arm (p-arm) that often contains repetitive DNA sequences and forms **satellites** involved in organizing the nucleolus. In humans, the acrocentric chromosomes are **13, 14, 15, 21, and 22**. Therefore, Chromosome 21 is a classic example of an acrocentric chromosome. **2. Analysis of Incorrect Options:** * **Metacentric (B):** The centromere is located exactly in the middle, resulting in arms of equal length (e.g., Chromosomes 1 and 3). * **Submetacentric (C):** The centromere is slightly off-center, creating a distinct short arm (p) and a long arm (q) (e.g., Chromosomes 2, 4 through 12, and X). * **Telocentric (D):** The centromere is located at the very tip of the chromosome. **Telocentric chromosomes do not occur in humans**; they are found in other species like mice. **Clinical Pearls & High-Yield Facts:** * **Robertsonian Translocation:** This specific type of translocation occurs only between **acrocentric chromosomes** (most commonly 14 and 21). This is a high-yield cause of familial Down Syndrome. * **Nucleolar Organizer Regions (NORs):** The p-arms of acrocentric chromosomes contain NORs, which house the genes for 45S ribosomal RNA (rRNA). * **Denver Classification:** Chromosome 21 belongs to **Group G** (small acrocentric chromosomes, 21-22 and Y). Note: While the Y chromosome is acrocentric, it does not have satellites.

Question 359: Which type of RNA molecule carries the anticodon?

A. mRNA
B. tRNA (Correct Answer)
C. rRNA
D. hnRNA

Explanation: **Explanation:** **1. Why tRNA is the correct answer:** Transfer RNA (tRNA) acts as the "adapter molecule" during protein synthesis (translation). It possesses a specific cloverleaf secondary structure. The **anticodon loop** contains a triplet of nucleotides called the **anticodon**, which is complementary to the codon found on the mRNA. This base-pairing ensures that the correct amino acid (attached to the 3' end of the tRNA) is incorporated into the growing polypeptide chain according to the genetic code. **2. Why the other options are incorrect:** * **mRNA (Messenger RNA):** Carries the genetic information from DNA in the form of **codons**. It serves as the template for translation but does not contain the anticodon. * **rRNA (Ribosomal RNA):** The structural and catalytic component of ribosomes. It facilitates the binding of mRNA and tRNA and catalyzes peptide bond formation (peptidyl transferase activity) but does not carry anticodons. * **hnRNA (Heterogeneous nuclear RNA):** The primary transcript (pre-mRNA) found in the nucleus of eukaryotes. It contains both introns and exons and must undergo processing (splicing, capping, tailing) to become mature mRNA. **3. High-Yield NEET-PG Pearls:** * **Wobble Hypothesis:** Proposed by Francis Crick; it explains why the 3rd base of the tRNA anticodon can undergo non-standard base pairing with the 3rd base of the mRNA codon, allowing one tRNA to recognize multiple codons. * **Aminoacyl-tRNA Synthetase:** The enzyme responsible for "charging" tRNA by attaching the correct amino acid. This is the actual "translator" of the genetic code. * **CCA Tail:** All tRNAs have a CCA sequence at the 3' end (added post-transcriptionally), which serves as the amino acid attachment site. * **Rare Bases:** tRNA contains unusual bases like pseudouridine, dihydrouridine (D-arm), and ribothymidine (T-arm).

Question 360: Which enzyme is responsible for DNA proofreading and repair?

A. DNA polymerase (Correct Answer)
B. DNA ligase
C. DNA gyrase
D. DNA primase

Explanation: **Explanation:** **1. Why DNA Polymerase is Correct:** DNA polymerases (specifically **DNA Pol III** in prokaryotes and **Pol $\delta/\epsilon$** in eukaryotes) are the primary enzymes for DNA synthesis. Their "proofreading" ability is attributed to their **3' $\rightarrow$ 5' exonuclease activity**. If an incorrect nucleotide is added, the enzyme pauses, removes the mismatched base in the 3' to 5' direction, and replaces it with the correct one. This ensures high fidelity during replication. Additionally, specific DNA polymerases (like **DNA Pol I** in prokaryotes or **Pol $\beta$** in eukaryotes) play a vital role in DNA repair pathways like Base Excision Repair (BER). **2. Why Other Options are Incorrect:** * **DNA Ligase:** Known as the "molecular glue," its role is to catalyze the formation of phosphodiester bonds to seal nicks between DNA fragments (e.g., joining Okazaki fragments). It does not have catalytic proofreading activity. * **DNA Gyrase (Topoisomerase II):** This enzyme relieves torsional strain (supercoiling) ahead of the replication fork by creating double-stranded breaks. It is the target of Fluoroquinolones. * **DNA Primase:** This is an RNA polymerase that synthesizes short RNA primers required to initiate DNA synthesis, as DNA polymerase cannot start a chain *de novo*. **3. NEET-PG High-Yield Pearls:** * **Directionality:** Synthesis occurs 5' $\rightarrow$ 3'; Proofreading occurs **3' $\rightarrow$ 5'**. * **DNA Pol I:** Unique for having **5' $\rightarrow$ 3' exonuclease activity**, used to remove RNA primers. * **Clinical Correlation:** Defects in mismatch repair (MMR) genes lead to **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer), while defects in nucleotide excision repair (NER) lead to **Xeroderma Pigmentosum**.

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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