Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 251: The mapping of the human genome was completed in which year?

A. 2000
B. 2001
C. 2002
D. 2003 (Correct Answer)

Explanation: **Explanation:** The **Human Genome Project (HGP)** was an international scientific research project coordinated by the National Institutes of Health (NIH) and the U.S. Department of Energy. Its primary goal was to determine the base pairs that make up human DNA and to identify and map all the genes of the human genome. 1. **Why 2003 is correct:** The HGP was launched in 1990 and was originally planned to take 15 years. However, due to rapid technological advances in DNA sequencing, the project was completed ahead of schedule. The **final completion** of the human genome sequence was announced in **April 2003**, coinciding with the 50th anniversary of the description of the DNA double helix by Watson and Crick. 2. **Why other options are incorrect:** * **2000:** In June 2000, a "working draft" of the human genome was announced by President Bill Clinton and Tony Blair. * **2001:** The first detailed analysis of the **initial draft** was published in the journals *Nature* and *Science* in February 2001. * **2002:** While significant mapping of individual chromosomes (like Chromosome 20) occurred this year, it was not the year of project completion. **High-Yield Facts for NEET-PG:** * **Methodology:** The HGP primarily used **Sanger Sequencing** (Chain Termination Method). * **Key Findings:** The human genome contains approximately **3 billion base pairs** and roughly **20,000–25,000 protein-coding genes**. * **The "Telomere-to-Telomere" (T2T) Consortium:** While the HGP was "completed" in 2003, about 8% of the genome (highly repetitive heterochromatin) remained unsequenced. The truly "gapless" sequence was finally published in **2022**. * **Largest Gene:** Dystrophin (2.4 million bases). * **Smallest Gene:** TDF (Testis Determining Factor) on the Y chromosome.

Question 252: Choose the most appropriate statement about Epigenetics?

A. Irreversible modification of DNA
B. Change in genotype without change in phenotype
C. Change in nucleotide sequence
D. Heritable changes in gene expression (Correct Answer)

Explanation: ### Explanation **1. Why Option D is Correct:** Epigenetics refers to the study of changes in organisms caused by modification of gene expression rather than alteration of the genetic code itself. The core concept is that these changes are **heritable** (can be passed from mother to daughter cells during mitosis or through generations) and result in a functional change in gene activity (expression) without changing the underlying DNA sequence. The primary mechanisms include **DNA methylation** (usually silencing genes) and **Histone modification** (acetylation/deacetylation). **2. Why the Other Options are Incorrect:** * **Option A (Irreversible modification):** Epigenetic changes are generally **reversible**. For example, DNA demethylation or histone acetylation can "turn back on" a silenced gene. This reversibility is the basis for many new epigenetic cancer therapies. * **Option B (Change in genotype without change in phenotype):** This is the opposite of epigenetics. Epigenetics involves a **change in phenotype** (how the cell functions or looks) **without a change in genotype** (the DNA sequence remains the same). * **Option C (Change in nucleotide sequence):** Any change in the nucleotide sequence is defined as a **mutation**. Epigenetics specifically excludes changes to the base-pair sequence. **3. High-Yield Clinical Pearls for NEET-PG:** * **DNA Methylation:** Occurs at **CpG islands** via the enzyme DNA Methyltransferase (DNMT). It typically leads to transcriptional silencing. * **Histone Acetylation:** Mediated by Histone Acetyltransferases (HATs); it relaxes chromatin (euchromatin) and **increases** transcription. Histone Deacetylases (HDACs) decrease transcription. * **Genomic Imprinting:** A classic epigenetic phenomenon where only one allele (maternal or paternal) is expressed. Examples: **Prader-Willi Syndrome** (paternal deletion/maternal imprinting) and **Angelman Syndrome** (maternal deletion/paternal imprinting) on Chromosome 15. * **Cancer:** Hypermethylation of tumor suppressor genes (like *p16* or *BRCA1*) is a common epigenetic event in oncogenesis.

Question 253: Which statement is true about the transgenic process?

A. Removing DNA from a cell
B. Inserting DNA into a cell (Correct Answer)
C. Tumor carcinogenesis can be studied
D. In "Knock out" animals, a specific gene is targeted for inducing mutation or inactivation

Explanation: **Explanation:** **1. Why Option B is Correct:** The core definition of a **transgenic process** is the deliberate introduction of a foreign gene (transgene) into the genome of a living organism. This process involves **inserting DNA** into a cell (usually an embryo or stem cell) so that the organism expresses a new trait or produces a specific protein. This is the fundamental step in creating Genetically Modified Organisms (GMOs). **2. Analysis of Other Options:** * **Option A:** Removing DNA is generally referred to as DNA extraction or purification, not transgenics. * **Option C:** While transgenic models *can* be used to study cancer, this is an **application** of the technology, not the definition of the process itself. The question asks what the process *is*. * **Option D:** This describes **Gene Knockout** technology. While related to transgenics, "Knockout" specifically refers to the **inactivation** or "silencing" of an endogenous gene, whereas "Transgenic" typically refers to the **addition** of a new genetic element. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Transgenic Animals:** Most commonly mice. Used to produce human proteins (e.g., α1-antitrypsin in sheep milk). * **Methods of Gene Transfer:** * **Microinjection:** Direct injection into the male pronucleus of a fertilized ovum. * **Viral Vectors:** Using retroviruses to carry DNA into cells. * **Liposomes:** Using lipid bilayers to facilitate DNA entry. * **Knock-in vs. Knock-out:** "Knock-in" involves replacing a gene with a mutated version to study gain-of-function, while "Knock-out" studies loss-of-function. * **CRISPR-Cas9:** The most modern and precise tool for genome editing (Nobel Prize 2020), often tested in recent NEET-PG cycles.

Question 254: Which of the following cis-acting elements typically resides adjacent to or overlaps with many prokaryotic promoters?

A. Regulatory gene
B. Structural gene(s)
C. Repressor
D. Operator (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right:** In prokaryotic gene regulation (the **Operon model**), the **Operator** is a specific DNA sequence (a **cis-acting element**) that serves as the binding site for regulatory proteins. It is strategically positioned between the promoter and the structural genes, often overlapping with the promoter itself. When a repressor protein binds to the operator, it physically obstructs RNA polymerase from binding to the promoter or moving forward, thereby inhibiting transcription. This spatial proximity is essential for the "on/off" switch mechanism of operons like *lac* and *trp*. **2. Why the Incorrect Options are Wrong:** * **A. Regulatory gene:** This is a DNA segment that codes for a regulatory protein (like a repressor). It is often located at a distance from the operon it regulates and is not a cis-acting element that overlaps the promoter. * **B. Structural gene(s):** These are the genes that code for the actual enzymes or proteins (e.g., *lacZ, lacY*). They are located downstream of the promoter and operator, not overlapping them. * **C. Repressor:** This is a **trans-acting factor** (a protein), not a cis-acting element (DNA sequence). It binds to the operator but is not part of the DNA architecture itself. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cis-acting elements:** DNA sequences on the same molecule (e.g., Promoter, Operator, Enhancer). * **Trans-acting factors:** Diffusible molecules, usually proteins, that bind to DNA (e.g., Repressors, Activators). * **Polycistronic mRNA:** A hallmark of prokaryotes where one promoter/operator complex regulates multiple structural genes. * **Inducible vs. Repressible:** The *lac* operon is the classic example of an **inducible** system (turned on by the presence of lactose/allolactose), whereas the *trp* operon is **repressible**.

Question 255: What is the macromolecular complex that associates with introns during mRNA splicing?

A. Splicer
B. Dicer
C. Nuclear body
D. Spliceosome (Correct Answer)

Explanation: **Explanation:** The correct answer is **Spliceosome**. mRNA splicing is a critical post-transcriptional modification in eukaryotes where non-coding sequences (**introns**) are removed and coding sequences (**exons**) are joined together to form mature mRNA. 1. **Why Spliceosome is correct:** The spliceosome is a large macromolecular complex composed of **snRNPs** (small nuclear ribonucleoproteins, pronounced "snurps")—specifically U1, U2, U4, U5, and U6—and additional protein factors. These snRNPs recognize specific consensus sequences at the 5' splice site (GU) and 3' splice site (AG) to facilitate the transesterification reactions required for splicing. 2. **Why other options are incorrect:** * **Dicer:** An enzyme involved in the **RNA interference (RNAi)** pathway. It cleaves double-stranded RNA or pre-miRNA into short fragments (siRNA/miRNA) to silence gene expression. * **Nuclear body:** These are non-membrane-bound structures within the nucleus (e.g., Cajal bodies, Nucleoli) that serve various organizational functions but are not the specific machinery for intron removal. * **Splicer:** This is a distractor term and not a recognized biological complex in molecular biology. **Clinical Pearls for NEET-PG:** * **Systemic Lupus Erythematosus (SLE):** Patients often develop **anti-Smith (anti-Sm) antibodies**, which are directed against the proteins in snRNPs. This is a highly specific diagnostic marker for SLE. * **Spinal Muscular Atrophy (SMA):** Caused by a defect in the SMN1 gene, leading to impaired assembly of snRNPs. * **Beta-Thalassemia:** Some forms are caused by mutations at splice sites, leading to improper intron removal and non-functional hemoglobin chains.

Question 256: What is the attachment site of miRNA to inhibit gene expression?

A. 3' end of mRNA (Correct Answer)
B. 5' end of mRNA
C. Binding to sigma factor
D. Cap binding protein

Explanation: **Explanation:** MicroRNAs (miRNAs) are small, non-coding RNA molecules (typically 21–25 nucleotides long) that play a crucial role in post-transcriptional gene regulation. **Why Option A is Correct:** The primary mechanism of miRNA action involves binding to the **3' Untranslated Region (3' UTR)** of the target mRNA. This binding occurs via base-pairing between the "seed sequence" (nucleotides 2–7) of the miRNA and the complementary sequence on the 3' end of the mRNA. Once attached, the miRNA-induced silencing complex (miRISC) inhibits translation or promotes the degradation of the mRNA, thereby "silencing" the gene. **Why Other Options are Incorrect:** * **Option B (5' end of mRNA):** While the 5' end is involved in translation initiation, it is not the standard physiological target site for miRNA-mediated inhibition. * **Option C (Sigma factor):** Sigma factors are prokaryotic transcription factors required for RNA polymerase binding to promoters. miRNAs are eukaryotic regulatory elements and do not interact with sigma factors. * **Option D (Cap binding protein):** Proteins like eIF4E bind to the 5' cap to initiate translation. While miRNAs may interfere with the function of these proteins indirectly, they do not physically attach to them to inhibit gene expression. **High-Yield NEET-PG Pearls:** * **Biogenesis:** miRNAs are processed by enzymes **Drosha** (in the nucleus) and **Dicer** (in the cytosol). * **Mechanism:** If the complementarity is perfect, mRNA is cleaved; if it is partial (common in humans), translation is repressed. * **Clinical Link:** Dysregulation of miRNAs is linked to various cancers (acting as "oncomiRs") and cardiovascular diseases. * **RNA Interference (RNAi):** Unlike siRNA (which is exogenous and double-stranded), miRNA is endogenous and single-stranded.

Question 257: Which part of the gene is retained in the mature mRNA?

A. Exon (Correct Answer)
B. Intron
C. hnRNA
D. miRNA

Explanation: ### Explanation **1. Why Exons are Correct:** In eukaryotes, the initial transcript of a gene is a long precursor molecule called **pre-mRNA** (or hnRNA). This precursor contains both coding and non-coding sequences. **Exons** are the "Expressed" sequences that carry the genetic code for proteins. During the process of **splicing**, exons are joined together while non-coding regions are removed. Therefore, exons are the only segments retained in the mature mRNA that exits the nucleus for translation. **2. Analysis of Incorrect Options:** * **B. Intron:** These are "Intervening" non-coding sequences. They are removed (spliced out) during post-transcriptional modification and are **not** present in mature mRNA. * **C. hnRNA (Heterogeneous nuclear RNA):** This is the primary transcript (pre-mRNA) found in the nucleus. It is the precursor to mRNA and contains both introns and exons; it is not the final "mature" product. * **D. miRNA (microRNA):** These are small, non-coding RNA molecules (approx. 22 nucleotides) involved in RNA silencing and post-transcriptional regulation of gene expression. They are not a structural part of a gene's mRNA. **3. NEET-PG High-Yield Clinical Pearls:** * **Splicing Machinery:** Splicing is carried out by **snRNPs** (small nuclear ribonucleoproteins, pronounced "snurps"). * **Clinical Correlation:** Antibodies against snRNPs (specifically **Anti-Smith antibodies**) are highly specific for **Systemic Lupus Erythematosus (SLE)**. * **Alternative Splicing:** This process allows a single gene to code for multiple proteins by including different combinations of exons (e.g., membrane-bound vs. secreted antibodies). * **Mutation Site:** Mutations at the **splice donor site** (GU) or **splice acceptor site** (AG) can lead to improper splicing, a common cause of diseases like **β-Thalassemia**.

Question 258: What has been the primary goal of the Human Genome Project?

A. Introduction of a gene sequence into a cell to modify its behavior
B. Identify genes and sequence of base pairs in the DNA of the human genome (Correct Answer)
C. Development of new diagnostic techniques such as restriction enzymes
D. Confirmation of Hardy Weinberg law

Explanation: ### Explanation The **Human Genome Project (HGP)** was an international scientific research project (1990–2003) aimed at determining the base pairs that make up human DNA and identifying, mapping, and sequencing all the genes of the human genome from both a physical and a functional standpoint. **Why Option B is Correct:** The primary objective of the HGP was to provide a complete and accurate sequence of the **3 billion DNA base pairs** that make up the human genome and to identify all estimated **20,000–25,000 human genes**. This "blueprint" serves as the foundation for modern personalized medicine, pharmacogenomics, and the understanding of genetic diseases. **Analysis of Incorrect Options:** * **Option A:** This describes **Gene Therapy**, which is a therapeutic application resulting from genomic knowledge, but not the primary goal of the HGP itself. * **Option C:** **Restriction enzymes** (molecular scissors) were a *tool* used to achieve the sequencing goals of the HGP, but their development predates the project (discovered in the 1960s/70s). * **Option D:** The **Hardy-Weinberg Law** is a principle of population genetics regarding allele frequencies. While genomic data can be used to test it, the law was established in 1908, long before the HGP. **High-Yield Clinical Pearls for NEET-PG:** * **Key Findings:** Only about **1.5%** of the human genome codes for proteins (exons). * **Reference Genome:** The HGP used a "mosaic" of DNA from several anonymous donors. * **Technique:** The HGP primarily used **Sanger Sequencing** (Chain Termination Method), whereas modern "Next-Generation Sequencing" (NGS) allows for much faster, high-throughput analysis. * **ELSI:** 3-5% of the HGP budget was dedicated to studying the **Ethical, Legal, and Social Implications** of genomic research.

Question 259: Which step in prokaryotic translation is inhibited by Tetracycline?

A. Initiation
B. Binding of aminoacyl-tRNA to the A site on the ribosome (Correct Answer)
C. Peptide-bond formation
D. Translocation

Explanation: ### Explanation **Mechanism of Action (The Correct Answer):** Tetracyclines (e.g., Doxycycline, Minocycline) are bacteriostatic antibiotics that inhibit protein synthesis by binding reversibly to the **30S subunit** of the bacterial ribosome. Specifically, they block the **A site (aminoacyl site)**, preventing the attachment of the incoming **aminoacyl-tRNA** to the mRNA-ribosome complex. Since the tRNA cannot bind, no new amino acids can be added to the growing polypeptide chain, effectively halting translation. **Analysis of Incorrect Options:** * **A. Initiation:** This step involves the assembly of the 30S and 50S subunits with mRNA and fMet-tRNA. While Aminoglycosides (like Streptomycin) interfere with initiation by causing mRNA misreading, Tetracyclines act after the initiation complex is formed. * **C. Peptide-bond formation:** This step is catalyzed by peptidyl transferase (part of the 50S subunit). It is specifically inhibited by **Chloramphenicol**. * **D. Translocation:** This involves the movement of the ribosome along the mRNA. This step is inhibited by **Macrolides** (e.g., Erythromycin) and **Clindamycin**, which bind to the 50S subunit. **High-Yield Clinical Pearls for NEET-PG:** * **Resistance:** Primarily occurs via **efflux pumps** (encoded by *tet* genes) that push the drug out of the bacterial cell. * **Selectivity:** They are selective for bacteria because eukaryotic cells lack the specific energy-dependent transport system required for tetracycline uptake. * **Side Effects:** Look for "teeth discoloration" and "bone growth retardation" (due to calcium chelation) and "photosensitivity." * **Mnemonic:** **"Buy AT 30, CELL at 50"** * **30S inhibitors:** **A**minoglycosides, **T**etracyclines. * **50S inhibitors:** **C**hloramphenicol, **E**rythromycin (Macrolides), **L**inezolid, **L**incosamides (Clindamycin).

Question 260: What is the name of the enzyme that synthesizes a double-stranded DNA copy from a single-stranded RNA template molecule?

A. DNA polymerase
B. RNA polymerase
C. Reverse transcriptase (Correct Answer)
D. Phosphokinase

Explanation: **Explanation:** The correct answer is **Reverse transcriptase**. This enzyme is an **RNA-dependent DNA polymerase**. In the standard "Central Dogma" of molecular biology, genetic information flows from DNA to RNA. However, reverse transcriptase reverses this process by using a single-stranded RNA template to synthesize a complementary DNA (cDNA) strand, which then becomes double-stranded DNA. **Analysis of Options:** * **DNA polymerase:** These are DNA-dependent DNA polymerases. They synthesize a new DNA strand using an existing **DNA** template during replication and repair. * **RNA polymerase:** These are DNA-dependent RNA polymerases. They synthesize RNA from a **DNA** template during the process of transcription. * **Phosphokinase (Kinases):** These enzymes are involved in signal transduction and metabolism; they catalyze the transfer of a phosphate group from high-energy donor molecules (like ATP) to specific substrates. **Clinical Pearls for NEET-PG:** * **Retroviruses:** Reverse transcriptase is a hallmark of retroviruses like **HIV**. It allows the viral RNA genome to integrate into the host's DNA as a "provirus." * **Pharmacology Link:** Nucleoside Reverse Transcriptase Inhibitors (**NRTIs**) like Zidovudine and Abacavir, and **NNRTIs** like Efavirenz, target this specific enzyme to treat HIV. * **Biotechnology:** In the lab, reverse transcriptase is essential for **RT-PCR** (Reverse Transcription Polymerase Chain Reaction), used to detect RNA viruses (e.g., SARS-CoV-2) or measure gene expression. * **Telomerase:** This enzyme, which maintains chromosomal ends, is a specialized type of reverse transcriptase that carries its own RNA template.

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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