Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 421: Which of the following is necessarily present in an expression vector but not in a cloning vector?

A. Origin of replication
B. Restriction site
C. Selectable marker
D. Ribosomal entry site (Correct Answer)

Explanation: ### Explanation The fundamental difference between a **cloning vector** and an **expression vector** lies in their purpose: a cloning vector is designed to simply replicate and store a DNA fragment, whereas an expression vector is designed to produce the protein encoded by that DNA. **Why "Ribosomal Entry Site" is Correct:** For a protein to be synthesized (translation), the mRNA transcribed from the vector must bind to a ribosome. In prokaryotic expression vectors, this requires a **Shine-Dalgarno sequence**, and in eukaryotic systems, an **Internal Ribosome Entry Site (IRES)** or a Kozak sequence. Since cloning vectors only aim to replicate DNA and not translate it into protein, they do not require these translation-initiation signals. **Analysis of Incorrect Options:** * **Origin of Replication (ori):** Essential for both. It allows the plasmid to replicate independently within the host cell. * **Restriction Site:** Essential for both. These are specific sequences where restriction endonucleases cut the DNA to allow the insertion of the target gene (Multiple Cloning Site). * **Selectable Marker:** Essential for both. These are typically antibiotic resistance genes (e.g., Ampicillin resistance) that allow researchers to identify and select host cells that have successfully taken up the vector. **High-Yield NEET-PG Pearls:** * **Expression Vectors** must also contain a **strong promoter** (like *tac* or *T7*) and a **transcription termination signal**, which are absent in basic cloning vectors. * **Shine-Dalgarno Sequence:** A purine-rich sequence (AGGAGG) located 8-13 nucleotides upstream of the start codon (AUG) in prokaryotes. * **cDNA Requirement:** When using expression vectors in bacteria to produce human proteins (like Insulin), **cDNA** must be used because bacteria cannot perform post-transcriptional splicing to remove introns.

Question 422: Acetylation of histones results in which of the following modification?

A. Increased Heterochromatin formation
B. Increased mRNA production (Correct Answer)
C. Activate Deacetylases
D. Arginine Methylation

Explanation: **Explanation:** **1. Why Option B is Correct:** Histone acetylation is a key epigenetic modification that regulates gene expression. Histones are rich in basic amino acids (Lysine and Arginine), giving them a positive charge that binds tightly to the negatively charged DNA phosphate backbone [5]. * **Mechanism:** The enzyme **Histone Acetyltransferase (HAT)** adds acetyl groups to the lysine residues on histone tails [3]. * **Result:** This neutralizes the positive charge of the histones, weakening their affinity for DNA. The chromatin shifts from a tightly packed state (Heterochromatin) to a relaxed, open state (**Euchromatin**). This allows RNA polymerase and transcription factors access to the DNA, leading to **increased mRNA production (transcription)** [1], [4]. **2. Why Other Options are Incorrect:** * **Option A:** Acetylation *decreases* heterochromatin formation by promoting the formation of euchromatin [4]. * **Option C:** Acetylation is the *result* of HAT activity. Deacetylases (HDACs) perform the opposite function—they remove acetyl groups to silence genes [2]. * **Option D:** Arginine methylation is a distinct modification. While both are epigenetic markers, acetylation specifically targets lysine residues to activate transcription [3]. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **A**cetylation **A**ctivates transcription; **M**ethylation **M**utes (usually) or **M**odifies DNA. * **HDAC Inhibitors:** Drugs like **Vorinostat** and **Valproic acid** inhibit Histone Deacetylases, keeping chromatin in an acetylated (active) state [2]. These are used in treating certain cancers (e.g., Cutaneous T-cell lymphoma) to reactivate tumor suppressor genes. * **DNA Methylation:** Unlike histone acetylation, DNA methylation (usually at CpG islands) typically leads to **gene silencing**.

Question 423: A patient presents with a condition highly suggestive of a defect in which of the following cellular mechanisms?

A. Peptidyl transferase activity (Correct Answer)
B. Cutting DNA at a specific site
C. Participating in DNA synthesis
D. GTPase activity

Explanation: ***Peptidyl transferase activity*** - This **ribosomal enzyme** located in the **23S rRNA** of the large ribosomal subunit (60S in eukaryotes) catalyzes **peptide bond formation** during protein synthesis. - Clinical relevance: antibiotics like **chloramphenicol** and **linezolid** specifically inhibit peptidyl transferase, leading to impaired protein synthesis and cellular dysfunction. *Cutting DNA at a specific site* - This describes **restriction endonucleases** that cleave DNA at specific recognition sequences, primarily used in molecular biology techniques. - These enzymes are not involved in routine cellular protein synthesis or common pathological conditions affecting ribosomal function. *Participating in DNA synthesis* - This refers to **DNA polymerases** that synthesize new DNA strands during replication and repair processes. - Defects in DNA synthesis would present with different clinical features related to **DNA repair disorders** or **replication errors**, not protein synthesis defects. *GTPase activity* - This involves enzymes that hydrolyze **GTP to GDP**, including **elongation factors** (eEF1A, eEF2) and **signal transduction proteins**. - While GTPases participate in protein synthesis, the primary defect suggested by the clinical presentation points to the **peptide bond formation step** rather than GTP hydrolysis.

Question 424: Which of the following statements is true about ribosomes?

A. Ribosomes are conserved across different species.
B. Ribosomes facilitate the interaction between tRNA and mRNA.
C. RNA is transcribed from DNA, and proteins are synthesized from RNA.
D. All of the above (Correct Answer)

Explanation: **Explanation:** Ribosomes are the essential ribonucleoprotein complexes responsible for protein synthesis (translation) in all living cells. This question tests the fundamental understanding of the Central Dogma of Molecular Biology. * **Option A (Conserved across species):** Ribosomes are evolutionary ancient and highly conserved. While there are structural differences (Prokaryotes: 70S; Eukaryotes: 80S), the core catalytic mechanism—the **peptidyl transferase activity**—is conserved across all domains of life. * **Option B (Facilitating tRNA-mRNA interaction):** The ribosome acts as a physical scaffold. The small subunit (40S/30S) ensures correct base-pairing between the **mRNA codon** and the **tRNA anticodon**, while the large subunit (60S/50S) catalyzes peptide bond formation. * **Option C (Central Dogma):** This describes the flow of genetic information: DNA is transcribed into mRNA in the nucleus, which is then translated into functional proteins by ribosomes in the cytoplasm. Since all three statements accurately describe ribosomal function and biological context, **Option D** is the correct answer. **NEET-PG High-Yield Pearls:** 1. **Ribozyme Activity:** The peptidyl transferase activity in ribosomes is mediated by **28S rRNA** (in eukaryotes) and **23S rRNA** (in prokaryotes), making the ribosome a ribozyme. 2. **Antibiotic Target:** Many antibiotics exploit the differences between prokaryotic and eukaryotic ribosomes (e.g., **Aminoglycosides** and **Tetracyclines** bind the 30S subunit; **Macrolides** and **Chloramphenicol** bind the 50S subunit). 3. **Shine-Dalgarno Sequence:** In prokaryotes, the 16S rRNA of the small subunit recognizes this sequence on mRNA to initiate translation.

Question 425: Defect in snRNPs causes which of the following?

A. Defect in 5' capping
B. Defect in addition of poly-A tail
C. Defect in splicing (Correct Answer)
D. Defect in terminal addition of nucleotide

Explanation: ### Explanation **Correct Answer: C. Defect in splicing** **Concept:** Small nuclear ribonucleoproteins (**snRNPs**, pronounced "snurps") are the core components of the **Spliceosome**. In eukaryotes, primary mRNA transcripts (pre-mRNA) contain non-coding sequences called **introns** and coding sequences called **exons**. Splicing is the process of removing introns and joining exons to form mature mRNA. The spliceosome consists of five small nuclear RNAs (U1, U2, U4, U5, and U6) complexed with proteins to form snRNPs. These complexes recognize specific consensus sequences at the 5' donor site (GU) and 3' acceptor site (AG) of the intron. A defect in snRNPs prevents the formation of the spliceosome, leading to the retention of introns and the production of non-functional proteins. **Analysis of Incorrect Options:** * **A. Defect in 5' capping:** This process involves the addition of a 7-methylguanosine cap to the 5' end of the mRNA. It is catalyzed by **guanylyltransferase** and **methyltransferase**, not snRNPs. * **B. Defect in addition of poly-A tail:** Polyadenylation at the 3' end is performed by the enzyme **Poly-A polymerase** after cleavage by specific endonucleases. * **D. Defect in terminal addition of nucleotide:** This refers to processes like the addition of "CCA" to the 3' end of tRNA (by **tRNA nucleotidyltransferase**) or telomere lengthening (by **telomerase**). **Clinical Pearls for NEET-PG:** * **Systemic Lupus Erythematosus (SLE):** Patients often produce **Anti-Smith (Anti-Sm) antibodies**, which are highly specific for SLE. These antibodies are directed against the proteins associated with snRNPs. * **Spinal Muscular Atrophy (SMA):** Caused by a mutation in the *SMN1* gene, leading to defective assembly of snRNPs and subsequent motor neuron degeneration. * **Splicing Rule:** Introns almost always begin with **GU** and end with **AG** (the "GU-AG rule").

Question 426: Genomic imprinting is seen in which of the following conditions?

A. Angelman syndrome (Correct Answer)
B. Turner syndrome
C. Fragile-X syndrome
D. Noonan syndrome

Explanation: **Explanation:** **Genomic Imprinting** is an epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner. While most autosomal genes are expressed from both alleles, imprinted genes are "silenced" (usually via methylation) in either the maternal or paternal germline. **Why Angelman Syndrome is Correct:** Angelman syndrome and Prader-Willi syndrome are the classic examples of imprinting defects involving the **15q11-q13** region. * **Angelman Syndrome ("Happy Puppet"):** Occurs when the **maternal** allele is lost (deletion) or silenced, and the paternal allele is already imprinted (silenced). It is characterized by intellectual disability, seizures, ataxia, and frequent laughter. * **Prader-Willi Syndrome:** Occurs when the **paternal** allele is lost/silenced. **Analysis of Incorrect Options:** * **Turner Syndrome (45, XO):** A chromosomal numerical abnormality (monosomy) caused by nondisjunction, not imprinting. * **Fragile-X Syndrome:** Caused by a **trinucleotide repeat expansion (CGG)** in the *FMR1* gene. While it involves DNA methylation, it is classified as a triplet repeat disorder, not classic genomic imprinting. * **Noonan Syndrome:** An autosomal dominant disorder caused by mutations in the RAS-MAPK pathway (most commonly the *PTPN11* gene). **High-Yield NEET-PG Pearls:** 1. **Uniparental Disomy (UPD):** If a child inherits two copies of Chromosome 15 from the father (and none from the mother), it results in Angelman Syndrome. 2. **Other Imprinting Disorders:** Beckwith-Wiedemann Syndrome (Chromosome 11p15) and Silver-Russell Syndrome. 3. **Mnemonic:** **P**ader-Willi = **P**aternal deletion; **A**ngelman = **M**aternal deletion (**M**aple Syrup is **S**weet - **M**aternal **A**ngelman).

Question 427: What property of DNA synthesis does the Sanger method of DNA sequencing take advantage of to generate a sequencing ladder?

A. The addition of nucleotides requires a free 3'-hydroxyl group on the DNA strand. (Correct Answer)
B. DNA possesses a free 5'-phosphoryl group.
C. DNA polymerase exhibits proofreading capabilities.
D. Nucleotides are linked via phosphodiester bonds.

Explanation: ### Explanation The **Sanger method**, also known as the **dideoxy chain termination method**, is the gold standard for DNA sequencing. It relies on the fundamental biochemical requirement that DNA polymerase needs a **free 3'-hydroxyl (-OH) group** on the preceding nucleotide to catalyze the formation of a phosphodiester bond with the incoming deoxynucleotide (dNTP). **Why Option A is Correct:** In Sanger sequencing, **dideoxynucleotides (ddNTPs)** are added to the reaction mix. These ddNTPs lack the 3'-OH group (possessing a 3'-H instead). When a DNA polymerase incorporates a ddNTP into the growing chain, no further nucleotides can be added because there is no 3'-OH available for the next phosphodiester bond. This results in **chain termination**. By using four different ddNTPs, a "ladder" of DNA fragments of varying lengths is generated, which can be separated by size to determine the sequence. **Analysis of Incorrect Options:** * **Option B:** While DNA has a 5'-phosphoryl group, it is the 3'-OH end that is the site of elongation. The 5' end is the "start" of the strand, not the regulatory point for termination in this method. * **Option C:** Proofreading (3' to 5' exonuclease activity) is a mechanism to ensure fidelity, but it is not the principle used to generate the sequencing ladder; in fact, high-fidelity polymerases used in sequencing are often modified to minimize interference with ddNTP incorporation. * **Option D:** Phosphodiester bonds are the linkages in all DNA molecules, but the *interruption* of this linkage formation (due to the lack of a 3'-OH) is the specific property Sanger sequencing exploits. ### High-Yield Clinical Pearls for NEET-PG * **Components of Sanger Mix:** Template DNA, Primer, DNA Polymerase, dNTPs (dATP, dCTP, dGTP, dTTP), and **ddNTPs** (the limiting factor). * **Automated Sequencing:** Modern Sanger sequencing uses **fluorescently labeled ddNTPs**, allowing the reaction to occur in a single tube and be read by capillary electrophoresis. * **Clinical Application:** Used for validating mutations identified by Next-Generation Sequencing (NGS) and for diagnosing single-gene disorders (e.g., Cystic Fibrosis, Sickle Cell Anemia).

Question 428: DNA replication follows which of the following models?

A. Conservative
B. Semiconservative (Correct Answer)
C. Dispersive
D. All of the above

Explanation: **Explanation:** DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. The correct model is **Semiconservative**, as proven by the landmark **Meselson-Stahl experiment** (1958). **Why Semiconservative is Correct:** In this model, the two strands of the parental DNA double helix separate. Each parental strand serves as a **template** for the synthesis of a new complementary strand. Consequently, each resulting daughter DNA molecule consists of one "old" (conserved) parental strand and one "newly" synthesized strand. This ensures high fidelity in the transmission of genetic information. **Why Other Options are Incorrect:** * **Conservative:** This model suggested that the original DNA molecule remains entirely intact and a completely new double-stranded molecule is synthesized. This does not occur in biological systems. * **Dispersive:** This model proposed that the parental strands are broken into fragments and interspersed with new DNA segments, resulting in daughter strands that are mosaics of old and new DNA. This was disproven by density gradient centrifugation. **High-Yield Clinical Pearls for NEET-PG:** * **Enzymology:** DNA replication occurs during the **S-phase** of the cell cycle. The primary enzyme responsible for synthesis in eukaryotes is **DNA Polymerase δ and ε**, while **DNA Polymerase III** is the main replicative enzyme in prokaryotes. * **Directionality:** Synthesis always occurs in the **5' to 3' direction**. * **Inhibitors:** Several chemotherapeutic agents target this process (e.g., **5-Fluorouracil** inhibits thymidylate synthase; **Quinolones** inhibit DNA gyrase/Topoisomerase II in bacteria). * **Topoisomerase:** These enzymes prevent "supercoiling" or torsional strain during replication. Camptothecins (Irinotecan) inhibit Topoisomerase I, while Etoposide inhibits Topoisomerase II.

Question 429: Which of the following represents the most characteristic function of a Type II Restriction Enzyme?

A. Prevent folding of proteins
B. Remove formed DNA
C. Prevent supercoiling
D. Cut DNA at palindromic sites (Correct Answer)

Explanation: ### Explanation **Type II Restriction Enzymes** (Restriction Endonucleases) are essential tools in molecular biology and recombinant DNA technology. They function as "molecular scissors" that recognize specific, short sequences of double-stranded DNA—typically 4 to 8 base pairs long—known as **palindromic sequences** (sequences that read the same 5' to 3' on both strands). Unlike Type I or III enzymes, Type II enzymes cut the DNA **within or at a fixed distance** from this recognition site, producing predictable fragments (sticky or blunt ends). This precision is why they are the only type used extensively in gene cloning and DNA mapping. **Analysis of Incorrect Options:** * **A. Prevent folding of proteins:** This is the function of **Chaperones** (e.g., Heat Shock Proteins), which assist in the correct folding of polypeptide chains. * **B. Remove formed DNA:** This describes the action of **Nucleases** (specifically Exonucleases or DNAse) involved in DNA degradation or repair, rather than sequence-specific site cleavage. * **C. Prevent supercoiling:** This is the role of **Topoisomerases** (e.g., DNA Gyrase), which relieve torsional strain during replication and transcription. **High-Yield Facts for NEET-PG:** * **Nomenclature:** Named after the organism (e.g., *EcoRI* from *E. coli*). * **Cofactor:** Type II enzymes require **Magnesium ($Mg^{2+}$)** for their catalytic activity. * **Methylation:** Bacteria protect their own DNA from these enzymes by methylating their recognition sites using **Methyltransferases** (Restriction-Modification System). * **Clinical Application:** Used in **RFLP (Restriction Fragment Length Polymorphism)** for forensic analysis, paternity testing, and diagnosing genetic diseases like Sickle Cell Anemia.

Question 430: Which of the following statements regarding satellite DNA is incorrect?

A. It consists of repeated DNA sequences arranged in tandem.
B. It is often found clustered around centromeres.
C. It is also found clustered around telomeres.
D. It is transcriptionally active. (Correct Answer)

Explanation: ### Explanation **1. Why Option D is the Correct Answer:** Satellite DNA consists of highly repetitive, non-coding DNA sequences. In the hierarchy of chromatin structure, satellite DNA is the primary component of **constitutive heterochromatin**. Heterochromatin is characterized by being highly condensed and tightly packed, which makes it inaccessible to RNA polymerase. Therefore, satellite DNA is **transcriptionally inactive** (silent). It does not code for proteins or functional RNA, making the statement in Option D incorrect. **2. Analysis of Incorrect Options:** * **Option A:** Satellite DNA is defined by **tandem repeats** (head-to-tail orientation) of short nucleotide sequences. These repeats can range from a few base pairs (microsatellites) to hundreds of base pairs (macrosatellites). * **Option B & C:** These sequences are structural rather than informational. They are predominantly localized at the **centromeres** (providing a scaffold for the kinetochore) and **telomeres** (protecting chromosome ends). In humans, Alpha-satellite DNA is the most common type found at the centromere of all chromosomes. **3. High-Yield Clinical Pearls for NEET-PG:** * **Density Gradient:** The term "Satellite" comes from their behavior during **Cesium Chloride (CsCl) density gradient centrifugation**, where they form small "satellite" bands separate from the main bulk of genomic DNA due to differences in G-C content. * **DNA Profiling:** Variations in the number of tandem repeats (VNTRs and STRs) are the basis for **DNA fingerprinting** and forensic analysis. * **Microsatellite Instability (MSI):** Defective DNA mismatch repair (MMR) leads to MSI, a hallmark of **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). * **Telomeres:** Human telomeric repeat sequence is **TTAGGG**.

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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