Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 211: Which of the following is the smallest autosome?

A. Chromosome 1
B. Y chromosome
C. Chromosome 21 (Correct Answer)
D. Chromosome 22

Explanation: **Explanation:** The human karyotype is traditionally arranged by size, from largest (Chromosome 1) to smallest. While **Chromosome 21** was originally thought to be larger than Chromosome 22 based on physical appearance under older microscopy, genomic sequencing has confirmed that **Chromosome 21 is the smallest autosome**, containing approximately 48 million base pairs (compared to ~51 million in Chromosome 22). **Analysis of Options:** * **Chromosome 21 (Correct):** It is the smallest autosome by DNA content. Its small size is clinically significant, as it is one of the few autosomes where a trisomy is compatible with survival (Down Syndrome), likely due to the relatively low number of genes it carries. * **Chromosome 1 (Incorrect):** This is the **largest** human autosome, containing nearly 249 million base pairs and the highest number of genes. * **Chromosome 22 (Incorrect):** Long considered the smallest, it is actually the second-smallest autosome. It was the first human chromosome to be fully sequenced. * **Y Chromosome (Incorrect):** While the Y chromosome is physically smaller than Chromosome 21 (containing only ~57 million base pairs), it is a **sex chromosome**, not an autosome. The question specifically asks for the smallest *autosome*. **High-Yield NEET-PG Pearls:** * **Largest Chromosome:** Chromosome 1. * **Smallest Chromosome overall:** Y Chromosome. * **Smallest Autosome:** Chromosome 21. * **Gene Density:** Chromosome 19 has the highest gene density; Chromosome 13 and the Y chromosome have the lowest. * **Clinical Correlation:** Trisomy 21 (Down Syndrome) is the most common viable autosomal trisomy.

Question 212: Which of the following is NOT a potential complication of gene therapy?

A. Genotoxicity
B. Genome integration (Correct Answer)
C. Gene silencing
D. Immunotoxicity

Explanation: **Explanation:** The goal of gene therapy is to introduce genetic material into a patient’s cells to treat or prevent disease. In many therapeutic strategies, particularly those using viral vectors like Lentiviruses or Retroviruses, **Genome integration** is a **desired outcome** rather than a complication. Integration allows the therapeutic gene to be permanently incorporated into the host DNA, ensuring stable expression and transmission to daughter cells during cell division. **Why the other options are complications:** * **Genotoxicity (Option A):** This occurs when the integration of the transgene happens at an inappropriate site (insertional mutagenesis), potentially activating oncogenes or disrupting tumor suppressor genes, leading to cancer (e.g., leukemia). * **Gene silencing (Option C):** The host cell may recognize the foreign DNA and "switch it off" via epigenetic mechanisms like DNA methylation or histone modification, rendering the therapy ineffective. * **Immunotoxicity (Option D):** The patient’s immune system may mount an inflammatory response against the viral vector or the newly synthesized therapeutic protein, potentially leading to severe systemic inflammation or organ failure (e.g., the Jesse Gelsinger case). **High-Yield Clinical Pearls for NEET-PG:** * **Vectors:** Adeno-associated virus (AAV) is currently preferred for many therapies as it is non-integrating (remains episomal) and has low immunogenicity. * **Ex-vivo vs. In-vivo:** Ex-vivo therapy involves modifying cells outside the body (e.g., CAR-T cell therapy), while in-vivo involves direct injection of the vector. * **Severe Combined Immunodeficiency (SCID):** One of the first diseases treated with gene therapy; however, early trials were complicated by genotoxicity leading to T-cell leukemia.

Question 213: Base stacking of DNA is characterized by?

A. Electrophoresis
B. Hyperchromicity (Correct Answer)
C. Linear dichromicity
D. Hypochromicity

Explanation: ### Explanation **Correct Answer: B. Hyperchromicity** **Underlying Concept:** DNA base stacking refers to the arrangement of nitrogenous bases in a parallel, "stacked" orientation within the double helix. This stacking, along with hydrogen bonding, stabilizes the DNA structure. In a double-stranded DNA (dsDNA) molecule, the hydrophobic bases are tightly packed, which **quenches** their ability to absorb UV light. When DNA is denatured (due to heat or chemicals), the base stacking is disrupted, and the strands separate. This exposure of the bases leads to a significant **increase in UV light absorption at 260 nm**, a phenomenon known as **Hyperchromicity**. Therefore, the degree of base stacking is directly characterized by the change in absorbance (Hyperchromic shift) during denaturation. **Analysis of Incorrect Options:** * **A. Electrophoresis:** This is a technique used to separate DNA fragments based on their size and charge, not to characterize internal molecular forces like base stacking. * **C. Linear dichromicity:** While this relates to the orientation of molecules relative to polarized light, it is not the standard clinical or biochemical parameter used to characterize the loss of base stacking in DNA. * **D. Hypochromicity:** This refers to a *decrease* in light absorption. Native dsDNA is hypochromic relative to denatured single-stranded DNA (ssDNA). Base stacking causes hypochromicity, but the process of *characterizing* that stacking is typically measured by the hyperchromic shift upon melting. **High-Yield Clinical Pearls for NEET-PG:** * **$T_m$ (Melting Temperature):** The temperature at which 50% of DNA is denatured. $T_m$ is higher in DNA with high **G-C content** because G-C pairs have 3 hydrogen bonds (compared to 2 in A-T pairs) and stronger base-stacking interactions. * **Absorbance Peak:** DNA and RNA absorb maximally at **260 nm** due to the resonance of the purine and pyrimidine rings. * **Purity Check:** The ratio of absorbance at **260/280 nm** is used to assess DNA purity (a ratio of ~1.8 is considered pure DNA; lower indicates protein contamination).

Question 214: Which term describes a centromere located near the end of a chromosome?

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

Explanation: **Explanation:** Chromosomes are classified based on the position of the **centromere**, which divides the chromosome into a short arm (**p arm**) and a long arm (**q arm**). **1. Why Acrocentric is correct:** In **acrocentric** chromosomes, the centromere is located **near the end**. This results in one arm being extremely short, often terminating in "satellites" (stalk-like structures containing ribosomal RNA genes). In humans, chromosomes **13, 14, 15, 21, and 22** are acrocentric. **2. Analysis of Incorrect Options:** * **Metacentric:** The centromere is located in the **middle**, resulting in two arms of approximately equal length (e.g., Chromosome 1). * **Submetacentric:** The centromere is slightly off-center, creating a distinct short arm (p) and a long arm (q). * **Telocentric:** The centromere is at the **very tip (telomere)**, resulting in only one visible arm. **Note:** Telocentric chromosomes do not occur naturally in humans but are found in other species like mice. **3. High-Yield Clinical Pearls for NEET-PG:** * **Robertsonian Translocation:** This unique type of translocation occurs **only between acrocentric chromosomes**. The long arms fuse at the centromere, and the short arms are lost. This is a significant cause of familial Down Syndrome (Trisomy 21). * **Nucleolar Organizer Regions (NORs):** In acrocentric chromosomes, the satellites contain NORs, which are essential for the formation of the nucleolus. * **Mnemonic for Human Acrocentrics:** "D and G group chromosomes" (13-15 and 21-22).

Question 215: Which of the following statements is FALSE regarding Ribozyme?

A. Are protein enzymes (Correct Answer)
B. Can catalyze a reaction
C. Act on RNA molecules
D. Assist new proteins to fold into their appropriate conformation

Explanation: **Explanation:** **1. Why Option A is the Correct (False) Statement:** Ribozymes are **RNA molecules** that possess catalytic activity. The fundamental concept here is that while most enzymes are proteins, ribozymes are a unique class of non-protein catalysts. Therefore, stating they are "protein enzymes" is biochemically incorrect. **2. Analysis of Other Options:** * **Option B (Can catalyze a reaction):** This is **true**. Ribozymes lower the activation energy of specific biochemical reactions, similar to protein enzymes. * **Option C (Act on RNA molecules):** This is **true**. Most ribozymes function by cleaving or ligating phosphodiester bonds in RNA. A classic example is the **Peptidyl transferase** activity of the 23S rRNA (in bacteria) or 28S rRNA (in eukaryotes), which catalyzes peptide bond formation. * **Option D (Assist new proteins to fold):** This is **true**. Specific ribozymes and ribosomal RNA components have been shown to possess "RNA chaperone" activity, assisting in the proper folding of nascent polypeptide chains. **3. High-Yield Clinical Pearls for NEET-PG:** * **Discovery:** Thomas Cech and Sidney Altman won the Nobel Prize for discovering ribozymes (RNase P and Tetrahymena group I intron). * **Key Examples:** * **RNase P:** Involved in tRNA processing. * **Spliceosome:** Small nuclear RNAs (snRNAs) catalyze the splicing of pre-mRNA. * **Peptidyl Transferase:** The most high-yield ribozyme; it is the RNA component of the large ribosomal subunit, not a protein. * **Medical Significance:** Ribozymes are being researched as "molecular scissors" for gene therapy to cleave viral RNA (e.g., HIV) or oncogenic mRNA.

Question 216: Which of the following enzymes does not require a primer for its activity?

A. DNA Polymerase I
B. RNA Polymerase (Correct Answer)
C. DNA Polymerase II
D. DNA Polymerase III

Explanation: ### Explanation The fundamental difference between DNA and RNA synthesis lies in the requirement for a **3'-OH primer**. **Why RNA Polymerase is the Correct Answer:** RNA Polymerase is unique because it can initiate the synthesis of a polynucleotide chain *de novo*. It does not require a pre-existing primer; it simply binds to a specific DNA sequence called the **promoter** and begins joining ribonucleotides together. In fact, in DNA replication, a specialized RNA polymerase called **Primase** is responsible for creating the initial RNA primer that DNA polymerases then extend. **Why the Other Options are Incorrect:** * **DNA Polymerase I, II, and III:** All DNA polymerases (prokaryotic and eukaryotic) share a common limitation: they cannot start a new DNA strand from scratch. They can only add deoxyribonucleotides to an existing **3'-OH group** of a primer (usually an RNA primer). * **DNA Pol III** is the primary enzyme for elongation. * **DNA Pol I** is involved in removing RNA primers (5'→3' exonuclease activity) and filling the gaps. * **DNA Pol II** is primarily involved in DNA repair. **High-Yield Clinical Pearls for NEET-PG:** * **Primase:** A DNA-dependent RNA polymerase that synthesizes the 10-12 nucleotide RNA primer required for DNA replication. * **Reverse Transcriptase:** An RNA-dependent DNA polymerase (found in HIV) that **does** require a primer (specifically host tRNA). * **Directionality:** All polymerases (DNA and RNA) synthesize chains in the **5' to 3' direction**. * **Rifampicin:** A high-yield antibiotic that inhibits **bacterial RNA polymerase**, preventing the initiation of transcription.

Question 217: Which of the following is a stop codon?

A. UAG (Correct Answer)
B. UCA
C. UAC
D. AUG

Explanation: ### Explanation In molecular biology, **stop codons** (also known as nonsense codons) are sequences of three nucleotides in mRNA that signal the termination of protein synthesis during translation. They do not code for any amino acid; instead, they trigger the binding of release factors, causing the newly synthesized polypeptide chain to be released from the ribosome. **Correct Option: A (UAG)** There are three universal stop codons: 1. **UAG** (Amber) 2. **UAA** (Ochre) 3. **UGA** (Opal) *Mnemonic to remember: **U** **A**re **G**one, **U** **A**re **A**way, **U** **G**o **A**way.* **Analysis of Incorrect Options:** * **B. UCA:** This codes for the amino acid **Serine**. * **C. UAC:** This codes for the amino acid **Tyrosine**. It is often confused with UAG, but it is a functional sense codon. * **D. AUG:** This is the **Start Codon** (Initiation codon). It codes for **Methionine** in eukaryotes and **N-formylmethionine (fMet)** in prokaryotes. --- ### High-Yield Clinical Pearls for NEET-PG: * **Nonsense Mutation:** A point mutation that changes a sense codon into a stop codon (UAG, UAA, or UGA), leading to a prematurely truncated, usually non-functional protein. * **Exceptions to the Rule:** In human **mitochondria**, the genetic code differs slightly; **UGA** codes for Tryptophan rather than acting as a stop codon, while **AGA** and **AGG** (normally Arginine) act as stop codons. * **Selenocysteine:** Known as the 21st amino acid, it is encoded by the stop codon **UGA** when a specific insertion sequence (SECIS element) is present in the mRNA.

Question 218: The 7-methyl guanosine cap is characteristic of which type of nucleic acid?

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

Explanation: **Explanation:** The **7-methylguanosine (m7G) cap** is a specialized structure added to the **5' end of eukaryotic mRNA** shortly after the initiation of transcription. This modification is a hallmark of post-transcriptional processing of pre-mRNA. **Why mRNA is correct:** The cap is formed by a 5' to 5' triphosphate linkage of a 7-methylguanosine residue. Its primary functions are: 1. **Protection:** It prevents degradation by 5' exonucleases. 2. **Nuclear Export:** It is recognized by the Cap Binding Complex (CBC) for transport into the cytoplasm. 3. **Translation Initiation:** It serves as a recognition signal for the eIF4F complex, allowing the ribosome to bind and initiate translation. **Why other options are incorrect:** * **tRNA:** These undergo different modifications, such as the addition of a CCA tail at the 3' end and the presence of unusual bases (e.g., pseudouridine, dihydrouridine), but they do not possess a 5' cap. * **rRNA:** These are transcribed by RNA Polymerase I (except 5S rRNA) and are heavily methylated and cleaved, but they lack the m7G cap. * **DNA:** DNA is a double-stranded molecule that does not undergo capping; its stability is maintained by its helical structure and histone packaging. **High-Yield Clinical Pearls for NEET-PG:** * **Enzymatic Steps:** Capping involves three enzymes: RNA triphosphatase, Guanylyltransferase, and Guanine-7-methyltransferase (which uses **S-adenosylmethionine/SAM** as the methyl donor). * **RNA Polymerase II:** Only RNA Pol II transcripts are capped because its C-terminal domain (CTD) recruits the capping enzymes. * **Clinical Correlation:** Defective mRNA processing is linked to various diseases; for instance, certain viruses (like Influenza) "snatch" host caps to stabilize their own viral mRNA.

Question 219: Which of the following contributes to polypeptide synthesis?

A. Leader sequence
B. Enhancer (Correct Answer)
C. tRNA
D. ncRNA

Explanation: **Explanation:** The synthesis of a polypeptide is a multi-step process regulated primarily at the level of **transcription**. **Why Enhancer is correct:** An **Enhancer** is a cis-acting DNA regulatory element that significantly increases the rate of transcription of a specific gene. It functions by binding to transcription factors (activators), which then interact with the promoter-bound RNA polymerase complex via DNA looping. By boosting mRNA production, enhancers directly contribute to the overall yield and availability of templates for polypeptide synthesis. In the context of "contributing" to the synthesis process, the enhancer acts as the primary regulatory "gas pedal." **Analysis of Incorrect Options:** * **Leader sequence:** Also known as the 5' UTR (untranslated region), it is involved in mRNA stability and ribosome binding but is **not translated** into the polypeptide itself. * **tRNA:** While essential for translation, tRNA acts as an adapter molecule that carries amino acids. It is a tool for synthesis rather than a regulatory element that "contributes" to the initiation or rate-limiting control of the gene product in the same regulatory context as an enhancer. * **ncRNA (Non-coding RNA):** These are RNA molecules (like miRNA or siRNA) that are not translated into proteins. In fact, many ncRNAs function to *silence* or inhibit protein synthesis rather than contribute to it. **High-Yield NEET-PG Pearls:** * **Enhancers vs. Promoters:** Promoters are orientation and distance-dependent (located 5' to the gene), whereas **Enhancers** are **position and orientation independent** (can be upstream, downstream, or within introns). * **Vanishing White Matter Disease:** Caused by mutations in eukaryotic initiation factors (eIF2B), highlighting the clinical importance of translation regulation. * **Alpha-fetoprotein (AFP):** The enhancer for the AFP gene is a classic example of developmental regulation, being active in the fetal liver but silenced after birth.

Question 220: During which phase of the cell cycle does DNA replication occur?

A. M phase
B. G1 phase
C. G2 phase
D. S phase (Correct Answer)

Explanation: **Explanation:** The cell cycle is a highly regulated sequence of events that leads to cell division. The correct answer is **S phase (Synthesis phase)** because this is the specific period during which the cell replicates its entire nuclear DNA content. This ensures that each daughter cell receives an identical set of chromosomes. **Breakdown of the Cell Cycle Phases:** * **S phase (Correct):** DNA polymerase and other enzymes synthesize a new DNA strand. By the end of this phase, the DNA content of the cell doubles (from 2n to 4n), although the chromosome number remains the same. * **G1 phase (Incorrect):** This is the "Gap 1" or growth phase. The cell increases in size and synthesizes RNA and proteins required for DNA replication, but no DNA synthesis occurs here. * **G2 phase (Incorrect):** The "Gap 2" phase follows DNA replication. The cell continues to grow and synthesizes proteins (like tubulin) necessary for spindle formation in preparation for mitosis. * **M phase (Incorrect):** This is the Mitotic phase where physical division occurs (Prophase to Telophase). DNA is condensed and segregated, but not replicated. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** The transition from **G1 to S phase** (regulated by Cyclin D-CDK4/6 and the Retinoblastoma (Rb) protein) is the primary checkpoint for cell cycle progression. * **Quiescent Phase (G0):** Cells that stop dividing (like mature neurons or cardiac myocytes) exit the cycle and enter G0. * **Pharmacology Link:** Many chemotherapy drugs are "S-phase specific," such as **Antimetabolites** (Methotrexate, 5-Fluorouracil, Cytarabine). * **Histone Synthesis:** Histone proteins are also synthesized primarily during the **S phase** to package the newly formed DNA.

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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