Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 551: What is the function of the poly 'A' tail attached at the 3' end of mRNA?

A. Unwinding of mRNA
B. Stabilization of mRNA (Correct Answer)
C. Polymerization of mRNA
D. Transcription of mRNA

Explanation: **Explanation:** The **poly-A tail** is a stretch of 20–250 adenine residues added post-transcriptionally to the 3' end of eukaryotic mRNA. This process, known as polyadenylation, is catalyzed by the enzyme **Poly(A) Polymerase**. **Why the correct answer is right:** The primary function of the poly-A tail is to **stabilize the mRNA molecule**. It protects the mRNA from degradation by 3' exonucleases in the cytoplasm, thereby increasing its half-life. Additionally, it facilitates the export of mRNA from the nucleus to the cytoplasm and enhances translation efficiency by interacting with Poly(A) Binding Proteins (PABP), which help form the translation initiation complex. **Why the other options are incorrect:** * **A. Unwinding of mRNA:** This is typically performed by helicases during translation or RNA-induced silencing complexes, not by the poly-A tail. * **C. Polymerization of mRNA:** This refers to the synthesis of the RNA strand itself, which is performed by **RNA Polymerase II** during transcription. * **D. Transcription of mRNA:** This is the process of copying DNA into RNA. The poly-A tail is added *after* the transcription of the main gene body is complete (post-transcriptional modification). **High-Yield Clinical Pearls for NEET-PG:** * **The Signal:** The consensus sequence for polyadenylation is **AAUAAA**, located upstream of the cleavage site. * **Exception:** **Histone mRNAs** are the only eukaryotic mRNAs that do **not** have a poly-A tail; they use a stem-loop structure instead. * **Laboratory Utility:** In molecular biology, oligo(dT) primers are used to isolate mRNA from total cellular RNA by binding to the poly-A tail.

Question 552: Which among the following is a transcription inhibitor?

A. Chloramphenicol
B. Streptomycin
C. Puromycin
D. Amanitin (Correct Answer)

Explanation: **Explanation:** The correct answer is **Amanitin** (specifically $\alpha$-amanitin), a potent toxin found in the *Amanita phalloides* (death cap) mushroom. **1. Why Amanitin is Correct:** Transcription is the process of synthesizing RNA from a DNA template. **$\alpha$-amanitin** acts as a specific inhibitor of **RNA Polymerase II** in eukaryotes. By binding to the enzyme, it prevents the synthesis of mRNA, leading to a cessation of protein synthesis and eventual cell death (primarily hepatotoxicity). At higher concentrations, it can also inhibit RNA Polymerase III. **2. Why the Other Options are Incorrect:** The other three options are inhibitors of **Translation** (protein synthesis), not transcription: * **Chloramphenicol:** Inhibits the bacterial **50S** ribosomal subunit by blocking peptidyl transferase activity. * **Streptomycin:** An aminoglycoside that binds to the bacterial **30S** ribosomal subunit, causing misreading of mRNA and inhibition of initiation. * **Puromycin:** Acts as a structural analog of aminoacyl-tRNA; it causes **premature chain termination** in both prokaryotes and eukaryotes. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **RNA Polymerase Sensitivity:** RNA Pol I (rRNA) is insensitive to $\alpha$-amanitin; RNA Pol II (mRNA) is highly sensitive; RNA Pol III (tRNA) is moderately sensitive. * **Rifampicin:** A key antitubercular drug that inhibits **prokaryotic** transcription by binding to the $\beta$-subunit of bacterial DNA-dependent RNA polymerase. * **Actinomycin D:** Inhibits transcription in both prokaryotes and eukaryotes by intercalating between DNA bases. * **Mushroom Poisoning:** Clinical presentation of *Amanita* ingestion typically involves a latent period followed by severe GI distress and fulminant liver failure.

Question 553: Codons are present on:

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

Explanation: **Explanation:** The correct answer is **mRNA**. In molecular biology, the **genetic code** is organized into **codons**, which are sequences of three nucleotides that specify a particular amino acid during protein synthesis. While the genetic information originates in DNA, the term "codon" specifically refers to the triplets found on the **messenger RNA (mRNA)**. During transcription, DNA acts as a template to create mRNA; this mRNA then carries the genetic "blueprint" from the nucleus to the cytoplasm, where it is read by ribosomes. **Analysis of Incorrect Options:** * **DNA:** While DNA contains the original genetic sequence, these triplets are technically referred to as **genetic codes** or **triplets**, not codons. * **tRNA:** Transfer RNA contains the **anticodon**, a three-nucleotide sequence that is complementary to the mRNA codon. The anticodon ensures that the correct amino acid is delivered to the ribosome. * **Ribosomal RNA (rRNA):** rRNA provides the structural and catalytic framework of the ribosome (ribozyme activity) but does not carry the specific triplet codes for amino acids. **NEET-PG High-Yield Pearls:** * **Start Codon:** **AUG** (codes for Methionine in eukaryotes and N-formylmethionine in prokaryotes). * **Stop Codons (Nonsense Codons):** **UAA** (Ochre), **UAG** (Amber), and **UGA** (Opal). These do not code for any amino acid. * **Degeneracy/Redundancy:** A single amino acid can be coded by multiple codons (e.g., Leucine has six), which protects against minor mutations. * **Wobble Hypothesis:** Proposed by Francis Crick, it explains why the third base of a codon can sometimes vary without changing the amino acid assigned.

Question 554: Which of the following is NOT a mechanism of epigenetics?

A. Non-heritable changes (Correct Answer)
B. Acetylation of Histone
C. Hereditary changes
D. Methylation of DNA

Explanation: **Explanation:** Epigenetics refers to the study of heritable changes in gene expression that occur **without** altering the underlying DNA sequence. The fundamental hallmark of an epigenetic change is that it is stable enough to be passed down through cell divisions (mitosis) and sometimes across generations (meiosis), yet it remains reversible. **Why Option A is correct:** By definition, epigenetic modifications are **heritable**. Therefore, "Non-heritable changes" is a contradiction to the core concept of epigenetics. These changes create a "cellular memory" that ensures a daughter cell maintains the same phenotype as the parent cell. **Analysis of Incorrect Options:** * **B. Acetylation of Histone:** This is a classic epigenetic mechanism. Histone Acetyltransferases (HATs) add acetyl groups to lysine residues, neutralizing their positive charge. This relaxes the chromatin (euchromatin), making DNA accessible for transcription ("Acetylation Activates"). * **C. Hereditary changes:** As stated, epigenetics involves the transmission of gene expression patterns from parent to progeny cells. Examples include genomic imprinting (e.g., Prader-Willi and Angelman syndromes). * **D. Methylation of DNA:** This typically occurs at CpG islands in promoter regions. DNA Methyltransferase (DNMT) adds a methyl group to cytosine, which usually leads to gene silencing ("Methylation Mutes"). **High-Yield Clinical Pearls for NEET-PG:** * **Genomic Imprinting:** A specialized form of epigenetics where only one allele (either maternal or paternal) is expressed. * **Drugs targeting epigenetics:** **5-Azacytidine** (DNA Methyltransferase inhibitor) and **Vorinostat** (HDAC inhibitor) are used in treating certain malignancies. * **Chromatin States:** Heterochromatin (densely packed, transcriptionally inactive) vs. Euchromatin (loosely packed, active).

Question 555: Which type of RNA contains codons?

A. Transfer RNA (t-RNA)
B. Ribosomal RNA (r-RNA)
C. Messenger RNA (m-RNA) (Correct Answer)
D. Small interfering RNA (si-RNA)

Explanation: **Explanation:** The correct answer is **Messenger RNA (m-RNA)**. In molecular biology, the "Central Dogma" dictates that genetic information flows from DNA to RNA to protein. **Why m-RNA is correct:** Messenger RNA serves as the intermediary template between DNA and protein synthesis. It carries genetic information in the form of **codons**—sequences of three nucleotides that specify a particular amino acid. During translation, these codons are read by ribosomes to assemble amino acids in the correct order to form a polypeptide chain. **Analysis of Incorrect Options:** * **Transfer RNA (t-RNA):** Does not contain codons; instead, it contains **anticodons**. The anticodon is a three-nucleotide sequence complementary to the codon on the m-RNA, allowing the t-RNA to deliver the specific amino acid to the ribosome. * **Ribosomal RNA (r-RNA):** This is a structural and catalytic component of ribosomes. It does not carry the genetic code (codons) but provides the environment and ribozyme activity (peptidyl transferase) necessary for protein synthesis. * **Small interfering RNA (si-RNA):** These are short, double-stranded RNA molecules involved in the RNA interference (RNAi) pathway. They function in gene silencing and regulation rather than protein coding. **High-Yield NEET-PG Pearls:** * **Start Codon:** AUG (codes for Methionine in eukaryotes and N-formylmethionine in prokaryotes). * **Stop Codons (Nonsense Codons):** UAA (Ochre), UAG (Amber), and UGA (Opal). These do not code for any amino acid. * **Degeneracy/Redundancy:** Most amino acids are coded by more than one codon (except Methionine and Tryptophan). * **Wobble Hypothesis:** Explains why the third base of a codon can undergo non-standard base pairing with the anticodon, allowing one t-RNA to recognize multiple codons.

Question 556: Which of the following is NOT a nucleic acid test?

A. Western blot (Correct Answer)
B. Southern blot
C. Northern blot
D. Microarray

Explanation: **Explanation:** The core concept tested here is the identification of specific macromolecules using blotting techniques. **Nucleic acid tests (NATs)** are methods used to detect specific sequences of DNA or RNA. **Why Western Blot is the correct answer:** Western blot is used for the detection of specific **proteins**, not nucleic acids. In this technique, proteins are separated by electrophoresis (usually SDS-PAGE), transferred to a membrane (nitrocellulose or PVDF), and probed using labeled **antibodies**. Since it identifies amino acid chains rather than nucleotide sequences, it is not a nucleic acid test. **Why the other options are incorrect:** * **Southern Blot:** Used for the detection of specific **DNA** sequences. It involves DNA digestion by restriction endonucleases and hybridization with a DNA probe. (Mnemonic: **S**outhern = **D**NA). * **Northern Blot:** Used for the detection of specific **RNA** sequences (usually mRNA) to study gene expression. (Mnemonic: **N**orthern = **R**NA). * **Microarray:** A high-throughput nucleic acid technique where thousands of DNA or RNA probes are fixed to a solid surface to study gene expression or genetic variations (SNPs) simultaneously. **High-Yield Clinical Pearls for NEET-PG:** * **SNOW DROP Mnemonic:** **S**outhern-**D**NA, **N**orthern-**R**NA, **O**-O, **W**estern-**P**rotein. * **Clinical Use of Western Blot:** Historically the confirmatory test for **HIV** (detecting antibodies against p24, gp120, and gp41) and Lyme disease. * **Southwestern Blot:** A hybrid technique used to identify **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Dot Blot:** A simplified version of blotting where the sample is applied directly to the membrane without prior electrophoretic separation.

Question 557: Highly repetitive DNA is characteristically found in which chromosomal region?

A. Centromere (Correct Answer)
B. Microsatellite DNA
C. Telomere
D. All of the above

Explanation: **Explanation:** The human genome is categorized based on the frequency of sequence repetition. **Highly repetitive DNA** (also known as Satellite DNA) consists of short sequences (2–10 bp) repeated millions of times. These sequences are non-coding and are characteristically concentrated in the **centromeres** and pericentromeric regions of chromosomes. They play a structural role in maintaining chromosomal integrity and facilitating spindle fiber attachment during cell division. **Analysis of Options:** * **A. Centromere (Correct):** This region contains **Alpha-satellite DNA**, a classic example of highly repetitive DNA. It is essential for the formation of the kinetochore. * **B. Microsatellite DNA:** While repetitive, these are classified as **moderately repetitive DNA** (specifically Short Tandem Repeats or STRs). They are dispersed throughout the genome rather than being localized to a specific chromosomal landmark like the centromere. * **C. Telomere:** Telomeres contain repetitive sequences (TTAGGG), but they are categorized as **telomeric repeats**. While repetitive, the term "Highly Repetitive DNA" in classical biochemistry specifically refers to the bulk satellite DNA found at the centromeres. * **D. All of the above:** Incorrect, as the classification of "highly repetitive" specifically targets satellite DNA found at centromeres in standard medical nomenclature. **NEET-PG High-Yield Pearls:** * **Satellite DNA** can be separated from bulk DNA using **density gradient centrifugation**, appearing as separate "satellite" bands due to different buoyant densities. * **Minisatellites (VNTRs)** and **Microsatellites (STRs)** are highly polymorphic and are the basis for **DNA Fingerprinting**. * **Trinucleotide repeats** (a type of microsatellite) are associated with diseases like Huntington’s chorea and Fragile X syndrome.

Question 558: Splicing activity is a function of which type of RNA?

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

Explanation: **Explanation:** The correct answer is **snRNA (Small nuclear RNA)**. **Why snRNA is correct:** Splicing is the process of removing non-coding sequences (**introns**) and joining coding sequences (**exons**) from the primary transcript (pre-mRNA). This process is mediated by the **Spliceosome**, a large complex composed of **snRNAs** (U1, U2, U4, U5, and U6) and specific proteins. These are collectively called **snRNPs** (Small Nuclear Ribonucleoproteins, pronounced "snurps"). The snRNA component is crucial because it recognizes the consensus sequences at the 5' and 3' splice sites through complementary base pairing, facilitating the transesterification reactions required for splicing. **Why other options are incorrect:** * **mRNA (Messenger RNA):** This is the template for protein synthesis. It undergoes splicing but does not perform the catalytic activity itself. * **tRNA (Transfer RNA):** Its primary role is to act as an adapter molecule that carries specific amino acids to the ribosome during translation. * **rRNA (Ribosomal RNA):** This is a structural and catalytic component of ribosomes. While it has ribozyme activity (peptidyl transferase), it is involved in translation, not splicing. **High-Yield Clinical Pearls for NEET-PG:** * **Systemic Lupus Erythematosus (SLE):** Patients often develop **anti-Smith (anti-Sm) antibodies**, which are directed against the proteins associated with snRNPs. * **Alternative Splicing:** A mechanism where different combinations of exons are joined, allowing a single gene to code for multiple proteins (increases genetic diversity). * **Splice Site Mutations:** Mutations at splice sites can lead to abnormal proteins, a classic example being certain forms of **β-thalassemia**.

Question 559: At physiological pH, what is the charge of DNA molecules?

A. Positively charged
B. Negatively charged (Correct Answer)
C. Neutral
D. Amphipathic

Explanation: **Explanation:** **1. Why DNA is Negatively Charged:** The charge of DNA is primarily determined by its **phosphate backbone**. Each nucleotide in the DNA strand contains a phosphate group ($PO_4^{3-}$). At physiological pH (approximately 7.4), the phosphoric acid groups are deprotonated, leaving oxygen atoms with a negative charge. Since these phosphate groups are located on the exterior of the double helix, the entire DNA molecule acts as a polyanion (a molecule with multiple negative charges). **2. Analysis of Incorrect Options:** * **A. Positively charged:** DNA is not positively charged. However, it interacts closely with **Histones**, which are highly basic proteins rich in Arginine and Lysine. These histones carry a positive charge, allowing them to bind tightly to the negative DNA backbone to form nucleosomes. * **C. Neutral:** DNA cannot be neutral at physiological pH because the $pK_a$ of the phosphate groups is much lower than 7.4, ensuring they remain in an ionized (negative) state. * **D. Amphipathic:** This term refers to molecules having both hydrophilic and hydrophobic parts (like phospholipids). While DNA has a hydrophobic core (bases) and a hydrophilic exterior (sugar-phosphate), it is characterized chemically by its overall negative charge rather than amphipathic behavior in this context. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Electrophoresis:** The negative charge of DNA is the fundamental principle behind **Agarose Gel Electrophoresis**, where DNA fragments migrate toward the **Anode (positive electrode)**. * **Histone Acetylation:** This process neutralizes the positive charge on histones, weakening their bond with DNA. This results in "relaxed" DNA (Euchromatin), which is transcriptionally active. * **Precipitation:** In DNA extraction, salts (like Sodium Acetate) are added to neutralize the negative charge of the phosphate backbone, allowing DNA to precipitate out of the solution in the presence of ethanol.

Question 560: Catabolite Activator Protein (CAP) in the Lac operon functions as a:

A. Positive regulator (Correct Answer)
B. Promoter
C. Repressor
D. Negative regulator

Explanation: ### Explanation **1. Why Option A is Correct:** The **Catabolite Activator Protein (CAP)**, also known as cAMP Receptor Protein (CRP), acts as a **positive regulator** (activator) of the Lac operon. In the absence of glucose, intracellular levels of **cAMP** rise. cAMP binds to CAP, forming a cAMP-CAP complex. This complex binds to a specific site upstream of the promoter, significantly increasing the affinity of RNA polymerase for the promoter. This "turns up" transcription, ensuring the cell can efficiently metabolize lactose when glucose is unavailable. **2. Why Other Options are Incorrect:** * **Option B (Promoter):** The promoter is a DNA sequence where RNA polymerase binds to initiate transcription. CAP is a protein that binds *near* the promoter, not the promoter itself. * **Option C & D (Repressor/Negative Regulator):** These terms refer to the **Lac Repressor protein** (encoded by the *lacI* gene). The repressor binds to the **operator** to inhibit transcription when lactose is absent. CAP does the opposite; it enhances transcription. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Dual Control:** The Lac operon is under both negative control (by the Lac Repressor) and positive control (by CAP). * **Glucose Effect:** This is also known as **Catabolite Repression**. High glucose levels lead to low cAMP, preventing the CAP complex from forming, which keeps the operon "off" even if lactose is present. * **Inducer:** Allolactose (an isomer of lactose) is the natural inducer that inactivates the repressor. * **IPTG:** A synthetic, non-metabolizable inducer often used in laboratory settings to study the Lac operon. * **Requirement for Expression:** For maximal expression of the Lac operon, two conditions must be met: **High Lactose** (to remove the repressor) and **Low Glucose** (to allow CAP binding).

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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