Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 31: The CAP protein in the lac operon serves as an example of which of the following?

A. Positive regulator (Correct Answer)
B. Negative regulator
C. Attenuation
D. Constitutive expression

Explanation: ### Explanation **1. Why Option A is Correct:** The **Catabolite Activator Protein (CAP)**, also known as cAMP Receptor Protein (CRP), is a classic example of a **positive regulator**. In the absence of glucose, levels of cyclic AMP (cAMP) rise. cAMP binds to CAP, forming a cAMP-CAP complex. This complex binds to a specific site near the *lac* promoter, enhancing the affinity of RNA polymerase for the promoter. This "recruitment" significantly increases the rate of transcription. Therefore, CAP acts as an activator that "turns on" or upregulates gene expression. **2. Why Other Options are Incorrect:** * **B. Negative Regulator:** This refers to the **Lac Repressor protein** (encoded by the *lacI* gene). The repressor binds to the operator to block transcription; CAP does the opposite. * **C. Attenuation:** This is a regulatory mechanism seen in the **Tryptophan (*trp*) operon**, involving premature termination of transcription based on the speed of translation. It is not a feature of the *lac* operon. * **D. Constitutive Expression:** This refers to genes that are expressed at a constant rate regardless of environmental conditions (e.g., "housekeeping genes"). The *lac* operon is **inducible**, not constitutive. **3. High-Yield Clinical Pearls for NEET-PG:** * **Glucose Effect:** The *lac* operon is subject to "Catabolite Repression." Even if lactose is present, the operon is not fully active if glucose is available, because glucose inhibits Adenylate Cyclase, lowering cAMP levels and preventing CAP binding. * **Dual Control:** Remember that for maximal *lac* operon expression, two conditions must be met: **High Lactose** (to remove the repressor) and **Low Glucose** (to allow CAP binding). * **DNA Binding Motif:** CAP utilizes a **Helix-turn-helix** motif to bind to DNA, a common feature in prokaryotic regulatory proteins.

Question 32: What is the common feature of Nuclear Localization Signals?

A. Rich in acidic amino acid residues and located at the C-terminus of the protein
B. Rich in basic amino acid residues and located at the C-terminus of the protein
C. Rich in acidic amino acid residues and may be located anywhere in the protein sequence
D. Rich in basic amino acid residues and may be located anywhere in the protein sequence (Correct Answer)

Explanation: **Explanation:** **1. Why Option D is Correct:** Nuclear Localization Signals (NLS) are specific amino acid sequences that act as "zip codes," tagging proteins synthesized in the cytoplasm for transport into the nucleus through the Nuclear Pore Complex (NPC). * **Composition:** They are characteristically rich in **basic amino acids**, specifically **Lysine (K) and Arginine (R)**. These positively charged residues are essential for recognition by cytosolic receptors called **Importins**. * **Location:** Unlike signal sequences for the ER or mitochondria (which are usually N-terminal), the NLS **can be located anywhere** in the protein’s primary sequence (N-terminal, C-terminal, or internal). Crucially, the NLS is **not cleaved** after transport, allowing the protein to re-enter the nucleus if the nuclear envelope breaks down during mitosis. **2. Why Other Options are Incorrect:** * **Options A & C:** These are incorrect because NLS are rich in **basic** (positive) residues, not acidic (negative) residues like Glutamate or Aspartate. * **Option B:** While it correctly identifies the basic nature, it incorrectly restricts the NLS to the C-terminus. NLS position is flexible. **3. High-Yield Facts for NEET-PG:** * **Nuclear Export Signal (NES):** These tags direct proteins out of the nucleus and are typically rich in **Leucine** (hydrophobic). They are recognized by **Exportins**. * **Ran-GTPase:** The directionality of nuclear transport is governed by a gradient of Ran-GTP (high in the nucleus) and Ran-GDP (high in the cytosol). * **Zellweger Syndrome:** Often confused in exams, this involves defects in **Peroxisomal** targeting signals (PTS), not nuclear signals. * **Prototypical NLS:** The SV40 T-antigen sequence (PKKKRKV) is the classic example of a "monopartite" basic NLS.

Question 33: Which of the following is NOT a disease of defective DNA repair?

A. Severe combined immunodeficiency disease
B. Adenosis polyposis coli (Correct Answer)
C. Bloom syndrome
D. Breast cancer susceptibility 1 (BRCA 1)

Explanation: **Explanation:** The correct answer is **Adenosis polyposis coli (APC)**. **1. Why APC is the correct answer:** Familial Adenomatous Polyposis (FAP) is caused by a mutation in the **APC gene**, which is a **tumor suppressor gene** involved in the **Wnt signaling pathway**. The APC protein normally promotes the degradation of β-catenin. When mutated, β-catenin accumulates, translocates to the nucleus, and triggers the transcription of genes (like *c-myc*) that drive cell proliferation. It is a defect in **cell signaling and growth regulation**, not a direct defect in DNA repair mechanisms. **2. Analysis of incorrect options (DNA Repair Defects):** * **Severe Combined Immunodeficiency (SCID):** Specifically, the **RS-SCID** variant is caused by mutations in genes like *DCLRE1C* (Artemis), which are essential for **Non-Homologous End Joining (NHEJ)** during V(D)J recombination. * **Bloom Syndrome:** Caused by a mutation in the *BLM* gene, which encodes a **DNA Helicase**. This leads to genomic instability due to defective repair during DNA replication (homologous recombination). * **BRCA 1:** This gene is critical for the repair of double-strand breaks via **Homologous Recombination**. Mutations lead to an inability to repair DNA damage, significantly increasing the risk of breast and ovarian cancers. **Clinical Pearls for NEET-PG:** * **Hereditary Non-Polyposis Colorectal Cancer (HNPCC/Lynch Syndrome):** Often confused with APC, this *is* a DNA repair defect (specifically **Mismatch Repair/MMR**). * **Xeroderma Pigmentosum:** Defect in **Nucleotide Excision Repair (NER)**; inability to repair UV-induced thymine dimers. * **Ataxia Telangiectasia:** Defect in the *ATM* gene, which senses double-strand breaks. * **Fanconi Anemia:** Defect in the repair of DNA inter-strand cross-links.

Question 34: What is the function of single-strand binding proteins (SSBs) during DNA replication?

A. Relieves torsional strain
B. Deoxynucleotide polymerization
C. Prevents premature reannealing of strands (Correct Answer)
D. Seals the nick between Okazaki fragments

Explanation: **Explanation:** **1. Why Option C is Correct:** During DNA replication, the enzyme **Helicase** unwinds the double helix, creating two single strands. These strands are naturally complementary and have a high affinity for one another. **Single-strand binding proteins (SSBs)** bind to these exposed strands to stabilize them and **prevent premature reannealing** (re-forming the double helix) or the formation of secondary structures like hairpins. This ensures the strands remain accessible as templates for DNA Polymerase. **2. Analysis of Incorrect Options:** * **Option A (Relieves torsional strain):** This is the function of **DNA Topoisomerases** (e.g., DNA Gyrase in prokaryotes). They create nicks in the DNA to resolve supercoiling caused by unwinding. * **Option B (Deoxynucleotide polymerization):** This is the primary catalytic function of **DNA Polymerase III** (in prokaryotes) or **DNA Polymerase $\delta$ and $\epsilon$** (in eukaryotes). * **Option D (Seals the nick):** This is the function of **DNA Ligase**, which catalyzes the formation of phosphodiester bonds between Okazaki fragments on the lagging strand. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cooperativity:** SSB binding is "cooperative"—once one SSB molecule binds, it makes it easier for subsequent SSB molecules to bind to the adjacent DNA. * **Eukaryotic Equivalent:** In eukaryotes, the functional equivalent of the prokaryotic SSB is **Replication Protein A (RPA)**. * **Protection:** SSBs also protect the single-stranded DNA from being degraded by **nucleases** present in the cell. * **Drug Target:** Topoisomerase inhibitors (Option A) are clinically significant; for example, **Fluoroquinolones** inhibit bacterial DNA Gyrase, while **Etoposide/Teniposide** inhibit human Topoisomerase II.

Question 35: What is considered the most powerful cloning vector?

A. Plasmid
B. Virus
C. Cosmid (Correct Answer)
D. Phage

Explanation: ### Explanation **Correct Option: C (Cosmid)** In the context of standard cloning vectors, "power" is often defined by the **insert capacity**—the size of the foreign DNA fragment a vector can carry. A **Cosmid** is a hybrid vector containing the *cos* sequences (cohesive ends) of the Lambda phage and the essential elements of a plasmid. * **Mechanism:** Because it utilizes the packaging machinery of a bacteriophage but replicates like a plasmid, it can accommodate large DNA fragments ranging from **30 to 45 kb**. This makes it significantly more "powerful" for genomic library construction compared to standard plasmids or phages. **Analysis of Incorrect Options:** * **A. Plasmid:** These are small, circular extrachromosomal DNA. While easy to manipulate, they have a limited insert capacity, typically **<10 kb**. * **B. Virus:** While viral vectors (like Retrovirus or Adenovirus) are essential for gene therapy, as cloning tools, their capacity is generally restricted by the viral capsid size, usually smaller than cosmids. * **D. Phage:** Bacteriophages (like Lambda phage) typically carry inserts between **8 to 25 kb**. While more efficient than plasmids, they fall short of the cosmid’s capacity. **High-Yield NEET-PG Pearls:** * **Hierarchy of Insert Capacity (Smallest to Largest):** Plasmid (<10 kb) < Phage (up to 25 kb) < Cosmid (up to 45 kb) < BAC (Bacterial Artificial Chromosome: 100–300 kb) < YAC (Yeast Artificial Chromosome: 200–2000 kb). * **YAC** is the most powerful vector overall, but among the standard options provided in traditional exams, **Cosmid** is the preferred answer for high-capacity cloning. * **Cosmid components:** Plasmid (Origin of replication + Antibiotic resistance) + Phage (*cos* sites for packaging).

Question 36: Which of the following statements is true about protein translation?

A. Protein translation involves three main steps: initiation, elongation, and termination. (Correct Answer)
B. IF-2 (Initiation Factor 2) prevents the reassociation of ribosomal subunits.
C. IF-3 and 1A are involved in the binding of the initiating codon to the ribosome.
D. None of the above statements are true.

Explanation: **Explanation** **1. Why Option A is Correct:** Protein translation is the process by which mRNA is decoded to synthesize a specific polypeptide chain. In both prokaryotes and eukaryotes, this complex process is universally divided into three distinct, highly regulated stages: * **Initiation:** Assembly of the ribosome, mRNA, and the initiator tRNA. * **Elongation:** The sequential addition of amino acids via peptide bond formation. * **Termination:** Recognition of a stop codon and release of the completed polypeptide. **2. Why Other Options are Incorrect:** * **Option B:** This statement swaps the functions of initiation factors. **IF-3** (not IF-2) is responsible for binding to the 30S subunit to prevent the premature reassociation of the 30S and 50S subunits. **IF-2** is a GTPase that facilitates the binding of the initiator fMet-tRNA to the 30S subunit. * **Option C:** **IF-1 and IF-1A** primarily act to block the A-site, ensuring the initiator tRNA binds correctly to the P-site. They do not directly bind the codon; rather, they stabilize the initiation complex. **3. High-Yield NEET-PG Clinical Pearls:** * **Energy Requirements:** Translation is energetically expensive. 4 high-energy phosphate bonds are consumed per peptide bond: 2 from ATP (tRNA charging) and 2 from GTP (1 for binding tRNA to A-site, 1 for translocation). * **Antibiotic Targets:** Many antibiotics inhibit specific steps of translation (e.g., **Aminoglycosides** bind the 30S subunit causing mRNA misreading; **Macrolides** inhibit translocation by binding the 50S subunit). * **Diphtheria Toxin:** It inhibits eukaryotic translation by ADP-ribosylating **EF-2** (Elongation Factor 2), halting protein synthesis.

Question 37: Which of the following methods cannot be used to detect the point mutation in the beta (β)-globin gene that causes sickle cell anemia?

A. Polymerase chain reaction with allele-specific oligonucleotide hybridization
B. Southern blot analysis
C. DNA sequencing
D. Northern blot analysis (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Northern blot analysis**. **1. Why Northern Blot is the correct answer:** Sickle cell anemia is caused by a **point mutation** (missense mutation) in the DNA of the $\beta$-globin gene, where Adenine is replaced by Thymine (GAG $\rightarrow$ GTG). This results in the substitution of Glutamic acid by Valine at the 6th position of the $\beta$-globin chain. * **Northern blotting** is used to detect and quantify **RNA** (gene expression). * In sickle cell anemia, the mutation changes the *sequence* of the gene, but it does not necessarily prevent the gene from being transcribed into mRNA. Therefore, Northern blotting cannot identify a single base-pair change; it only tells you if the mRNA is present and what its size is. **2. Why the other options are incorrect:** * **A. PCR with Allele-Specific Oligonucleotide (ASO) hybridization:** This is a gold-standard technique for point mutations. Short DNA probes (ASOs) are designed to bind specifically to either the normal or the mutant DNA sequence. * **B. Southern blot analysis:** While typically used for large fragments, it can detect sickle cell anemia if combined with **Restriction Fragment Length Polymorphism (RFLP)**. The mutation abolishes a specific recognition site for the restriction enzyme *MstII*, leading to different fragment lengths on the blot. * **C. DNA sequencing:** This is the definitive method to identify any change in the genetic code, as it reads the exact nucleotide sequence (A, T, G, C) of the $\beta$-globin gene. **3. Clinical Pearls for NEET-PG:** * **SNOW DROP Mnemonic:** **S**outhern-**D**NA, **N**orthern-**R**NA, **O**o-**O**o, **W**estern-**P**rotein. * **Sickle Cell Mutation:** Glutamate (Polar) $\rightarrow$ Valine (Non-polar) at position 6 of the $\beta$-chain. * **RFLP & MstII:** The sickle mutation destroys the *MstII* site (CCTNAGG). Normal DNA yields a 1.1 kb fragment; mutant DNA yields a 1.3 kb fragment.

Question 38: What is microRNA?

A. Splicing RNA
B. Snurps
C. Gene Silencing RNA (Correct Answer)
D. Ribonuclease

Explanation: ### Explanation **Correct Answer: C. Gene Silencing RNA** **MicroRNAs (miRNAs)** are small (approximately 21–25 nucleotides), non-coding, single-stranded RNA molecules that play a crucial role in **post-transcriptional gene regulation**. They function primarily through **gene silencing**. The mechanism involves the miRNA binding to the **RNA-induced silencing complex (RISC)**. This complex then binds to a complementary sequence on a target messenger RNA (mRNA). Depending on the degree of complementarity, it leads to either: 1. **Translational repression** (partial match). 2. **mRNA degradation** (perfect match). In both scenarios, the expression of the target gene is "silenced" because the protein product is not produced. --- ### Why the other options are incorrect: * **A. Splicing RNA:** This refers to **snRNAs** (small nuclear RNAs) which, along with proteins, form the spliceosome responsible for removing introns from pre-mRNA. * **B. Snurps:** These are **Small Nuclear Ribonucleoproteins (snRNPs)**. They are the structural components of the spliceosome (e.g., U1, U2, U4, U5, U6) and are involved in splicing, not gene silencing. * **D. Ribonuclease:** These are **enzymes** (like RNase H or Dicer) that catalyze the degradation of RNA into smaller components. While miRNAs *recruit* ribonucleases within the RISC complex, the miRNA itself is a regulatory molecule, not an enzyme. --- ### High-Yield Clinical Pearls for NEET-PG: * **Dicer:** The ribonuclease III enzyme that processes pre-miRNA into mature miRNA. * **OncomiRs:** miRNAs that are dysregulated in cancer; they can act as oncogenes (by silencing tumor suppressors) or tumor suppressors. * **RNA Interference (RNAi):** A broader term including both miRNA (endogenous) and siRNA (often exogenous/synthetic). * **Clinical Application:** Patisiran is an FDA-approved siRNA drug used for hereditary transthyretin-mediated amyloidosis, demonstrating the therapeutic potential of gene silencing.

Question 39: What does palindromic DNA imply?

A. Short stretches of DNA
B. Recognised by specific restriction endonuclease
C. Codes for bacterial resistance
D. All of the above (Correct Answer)

Explanation: **Explanation:** **Palindromic DNA** refers to sequences of double-stranded DNA where the reading direction (5' to 3') is the same on both complementary strands. For example: 5'-GAATTC-3' 3'-CTTAAG-5' **Why Option D is Correct:** 1. **Short stretches of DNA:** Palindromic sequences are typically short, usually ranging from 4 to 8 base pairs in length. 2. **Recognized by specific restriction endonucleases:** This is the most significant functional feature. Restriction enzymes (molecular scissors) specifically bind to and cut DNA at these symmetrical palindromic sites. 3. **Codes for bacterial resistance:** In a clinical context, many antibiotic resistance genes (found on plasmids) are flanked by or contain palindromic sequences (transposons or "jumping genes"). These sequences allow the resistance genes to be excised and integrated into different parts of the bacterial genome or shared via horizontal gene transfer. **Analysis of Options:** * **Option A & B:** These are structural and functional definitions of palindromes in molecular biology. * **Option C:** While not all palindromes code for resistance, the mechanism of moving resistance genes (via transposons and integrons) relies heavily on these inverted repeat/palindromic sequences. Since A and B are definitively true, and C is true in the context of microbial genetics, "All of the above" is the most accurate choice. **High-Yield Clinical Pearls for NEET-PG:** * **Type II Restriction Endonucleases:** These are the most commonly used in recombinant DNA technology because they cut within the palindromic recognition site. * **Sticky vs. Blunt Ends:** Palindromic cuts can result in "sticky ends" (staggered cuts, e.g., *EcoRI*) or "blunt ends" (straight cuts, e.g., *SmaI*). * **Zinc Finger Motifs:** These are common DNA-binding proteins that often recognize palindromic sequences in the major groove of DNA.

Question 40: Heritable changes in gene expression not caused by alterations in the DNA sequence refers to which of the following?

A. Genetics
B. Epigenetics (Correct Answer)
C. Mutations
D. Transposons

Explanation: **Explanation:** **1. Why Epigenetics is Correct:** Epigenetics refers to the study of heritable changes in gene function that occur **without changing the underlying DNA sequence** (the "A-T-C-G" code). Instead, these changes involve chemical modifications to the DNA or associated proteins that dictate whether a gene is "turned on" or "off." The primary mechanisms include: * **DNA Methylation:** Usually occurs at CpG islands; typically leads to gene silencing. * **Histone Modification:** Acetylation (increases transcription) or Methylation (can increase or decrease transcription). * **Non-coding RNAs:** Such as miRNA and siRNA that regulate translation. **2. Why the Other Options are Incorrect:** * **Genetics:** This is the study of heredity and variation involving the actual DNA sequence and its transmission from parents to offspring. * **Mutations:** These are permanent, structural alterations in the DNA sequence (e.g., point mutations, deletions, insertions). Unlike epigenetic changes, mutations change the "blueprint" itself. * **Transposons:** Also known as "jumping genes," these are mobile DNA sequences that can move from one location to another within the genome, potentially causing mutations or altering genome size. **3. NEET-PG High-Yield Clinical Pearls:** * **Genomic Imprinting:** A classic epigenetic phenomenon where only one allele (either 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 *p53* or *RB*) can lead to silencing and subsequent oncogenesis. * **Drug Target:** **5-Azacytidine** is a DNA methyltransferase inhibitor used in treating myelodysplastic syndromes by reversing gene silencing.

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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