Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 501: Which of the following methods is used in gene therapy?

A. Electroporation (Correct Answer)
B. Electrofocusing
C. Selectively targeted recombination
D. Intracytoplasmic injection

Explanation: ### Explanation **Correct Option: A. Electroporation** Gene therapy requires the delivery of therapeutic genetic material into target cells. **Electroporation** is a physical method used for this purpose. It involves applying a high-voltage electrical pulse to the cell membrane, which creates temporary nanometer-sized pores (aqueous pathways). These pores allow large, charged molecules like DNA or RNA—which are normally membrane-impermeable—to enter the cytoplasm or nucleus. Once the pulse stops, the membrane reseals, trapping the genetic material inside. **Analysis of Incorrect Options:** * **B. Electrofocusing (Isoelectric Focusing):** This is a technique used in protein chemistry to separate proteins based on their isoelectric point (pI) in a pH gradient. It is not a delivery method for genetic material. * **C. Selectively targeted recombination:** While "homologous recombination" is a mechanism used in gene editing (like CRISPR), the term provided is not a standard delivery method for gene therapy. * **D. Intracytoplasmic injection:** While "Intracytoplasmic Sperm Injection" (ICSI) is a common IVF technique, it refers to injecting a whole sperm into an oocyte, not the standard clinical delivery of therapeutic genes into somatic cells. **High-Yield Clinical Pearls for NEET-PG:** * **Vectors in Gene Therapy:** * **Viral:** Retrovirus (integrates into host genome), Adenovirus (non-integrating, high immunogenicity), and Adeno-associated virus (AAV - preferred for long-term expression). * **Non-Viral:** Liposomes (lipofection), Electroporation, and Gene Gun (biolistics). * **First Gene Therapy:** Successfully used for **ADA-SCID** (Adenosine Deaminase deficiency) in 1990. * **Ex-vivo vs. In-vivo:** Electroporation is most commonly used in *ex-vivo* gene therapy, where cells (like T-cells for CAR-T therapy) are removed from the patient, modified, and then re-infused.

Question 502: Which is the only prokaryotic DNA polymerase with 5'-3' exonuclease activity?

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

Explanation: ### Explanation **DNA Polymerase I (Pol I)** is the only prokaryotic DNA polymerase that possesses **5'-3' exonuclease activity**. While all three major prokaryotic polymerases (I, II, and III) have 3'-5' exonuclease activity for proofreading, the unique 5'-3' exonuclease function allows Pol I to remove RNA primers and damaged DNA strands ahead of it while simultaneously synthesizing new DNA (a process known as **Nick Translation**). #### Analysis of Options: * **DNA Polymerase I (Correct):** It is a multifunctional enzyme. Its 5'-3' exonuclease activity is essential for **removing RNA primers** during lagging strand synthesis (Okazaki fragment processing) and for **DNA repair** (Base Excision Repair). * **DNA Polymerase II:** Primarily involved in **DNA repair** (SOS response). It lacks 5'-3' exonuclease activity. * **DNA Polymerase III:** The **primary enzyme for replicative DNA synthesis** in *E. coli*. It has high processivity and 3'-5' proofreading activity but lacks the 5'-3' exonuclease activity required to remove primers. * **DNA Polymerase IV & V:** These are "Y-family" polymerases involved in **translesion synthesis** (error-prone repair). They lack both 3'-5' and 5'-3' exonuclease activities. #### NEET-PG High-Yield Pearls: 1. **Klenow Fragment:** If DNA Pol I is treated with the protease subtilisin, it cleaves into a large fragment (Klenow) which retains polymerization and 3'-5' exonuclease activity but **loses the 5'-3' exonuclease activity**. 2. **Function Summary:** * **Pol I:** Primer removal and gap filling. * **Pol II:** DNA Repair. * **Pol III:** Main replicative enzyme (highest processivity). 3. **Eukaryotic Counterpart:** DNA Polymerase **Delta ($\delta$)** and **Epsilon ($\epsilon$)** are the main replicative enzymes in humans, while **RNase H** and **FEN1** perform the primer removal role that Pol I handles in prokaryotes.

Question 503: What is the smallest fundamental unit that codes for DNA synthesis?

A. Cistron (Correct Answer)
B. Operon
C. Replicon
D. Anticodon

Explanation: **Explanation:** The correct answer is **Cistron**. **1. Why Cistron is correct:** In molecular biology, a **cistron** is defined as the smallest functional unit of genetic material that determines the synthesis of a single polypeptide chain or a functional RNA molecule. It is often used synonymously with the term "gene." Since DNA synthesis (replication) and subsequent protein synthesis are governed by these functional segments, the cistron is considered the fundamental coding unit. In eukaryotes, genes are typically **monocistronic** (one gene codes for one protein), whereas in prokaryotes, they are often **polycistronic**. **2. Why other options are incorrect:** * **Operon:** This is a coordinated unit of gene expression found in prokaryotes (e.g., Lac Operon). It consists of a cluster of genes under the control of a single promoter and operator, rather than being the smallest fundamental unit. * **Replicon:** This is a unit of DNA that contains an origin of replication and is capable of replicating as a single entity. While it relates to DNA synthesis, it refers to the *process* of replication rather than the *coding* unit for a product. * **Anticodon:** This is a sequence of three nucleotides in a tRNA molecule that corresponds to a complementary codon in mRNA. It is involved in translation (protein synthesis) but does not code for DNA synthesis. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Monocistronic vs. Polycistronic:** Human mRNA is almost exclusively monocistronic. * **Introns vs. Exons:** In eukaryotes, cistrons are interrupted by non-coding sequences called **introns**, which are removed during splicing. * **Muton and Recon:** Historically, the *Muton* is the smallest unit of mutation, and the *Recon* is the smallest unit of recombination. * **TATA Box:** The promoter region (rich in A and T) where transcription factors bind to initiate the expression of a cistron.

Question 504: The gene responsible for coding plasma cholinesterase is located at which chromosome and position?

A. Chromosome 3 at q26 (Correct Answer)
B. Chromosome 3 at q27
C. Chromosome 4 at q26
D. Chromosome 4 at q27

Explanation: **Explanation:** The gene responsible for coding **Plasma Cholinesterase** (also known as Butyrylcholinesterase or BCHE) is located on the **long arm (q) of Chromosome 3 at position 26 (3q26.1–q26.2)**. 1. **Why Option A is Correct:** The *BCHE* gene provides instructions for making the plasma cholinesterase enzyme, which is synthesized in the liver and secreted into the blood. It is distinct from Acetylcholinesterase (AChE), which is found at neuromuscular junctions and is coded on Chromosome 7. 2. **Why Options B, C, and D are Incorrect:** These options represent incorrect chromosomal loci. While Chromosome 4 is often associated with other plasma proteins (like Albumin at 4q13), it does not harbor the *BCHE* gene. Position q27 on Chromosome 3 is adjacent but does not contain the primary locus for the *BCHE* gene. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Succinylcholine Apnea:** This is the most critical clinical correlation. Patients with genetic variants (mutations) in the *BCHE* gene at 3q26 have "Atypical Cholinesterase." They cannot efficiently hydrolyze Succinylcholine (a depolarizing muscle relaxant), leading to prolonged neuromuscular blockade and respiratory paralysis. * **Dibucaine Number:** This is a qualitative test used to detect atypical cholinesterase. Normal individuals have a Dibucaine number of ~80 (Dibucaine inhibits 80% of the enzyme), while homozygotes for the atypical gene have a number of ~20. * **Organophosphate Poisoning:** Plasma cholinesterase levels are used as a sensitive marker for exposure to organophosphates, as the enzyme is irreversibly inhibited by these compounds.

Question 505: Which of the following are DNA sequencing techniques?

A. Sanger's technique
B. Maxam and Gilbert technique
C. Next generation sequencing
D. All of the above (Correct Answer)

Explanation: **Explanation:** DNA sequencing is the process of determining the precise order of nucleotides (Adenine, Guanine, Cytosine, and Thymine) within a DNA molecule. All the options listed represent different generations of sequencing technology. 1. **Sanger’s Technique (Chain Termination Method):** Known as the "First Generation" sequencing method. It utilizes **dideoxynucleotides (ddNTPs)** which lack a 3'-OH group, causing the termination of DNA strand synthesis. It remains the "Gold Standard" for validating clinical mutations. 2. **Maxam and Gilbert Technique (Chemical Degradation Method):** Also a first-generation method, it involves the radioactive labeling of DNA and its subsequent partial breakdown using base-specific chemicals (e.g., dimethyl sulfate, hydrazine). It is rarely used today due to the use of hazardous chemicals. 3. **Next-Generation Sequencing (NGS):** This represents "Second Generation" technology (e.g., Illumina, Ion Torrent). It allows for **massively parallel sequencing**, enabling the sequencing of millions of fragments simultaneously. It is significantly faster and cheaper for whole-genome analysis compared to traditional methods. **Why "All of the above" is correct:** Since Sanger, Maxam-Gilbert, and NGS are all established methodologies used to decode the genetic sequence, Option D is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Automated Sanger Sequencing** uses fluorescently labeled ddNTPs and capillary electrophoresis. * **Pyrosequencing** (an NGS method) detects the release of **pyrophosphate** during nucleotide incorporation. * **Third-Generation Sequencing** (e.g., Oxford Nanopore, PacBio) allows for real-time, single-molecule sequencing of long DNA fragments without PCR amplification.

Question 506: What is the function of a promoter site on DNA?

A. Transcribes a repressor protein
B. Initiates transcription (Correct Answer)
C. Codes for RNA polymerase
D. Regulates transcription termination

Explanation: ### Explanation **1. Why Option B is Correct:** The **promoter** is a specific regulatory sequence of DNA located upstream (5') of a gene. Its primary function is to serve as the **recognition and binding site for RNA polymerase** and associated transcription factors. By positioning the RNA polymerase correctly at the start site, the promoter ensures that transcription begins at the right location and in the correct orientation. In eukaryotes, key promoter elements include the **TATA box** (Hogness box) and the **CAAT box**, which dictate the efficiency and initiation of transcription. **2. Why Other Options are Incorrect:** * **Option A:** A **regulatory gene** (not the promoter) transcribes the mRNA that codes for a repressor protein. The repressor then binds to the *operator* site to inhibit transcription. * **Option C:** RNA polymerase is an enzyme (protein). It is coded by specific **structural genes**, not by a promoter site. The promoter is a non-coding regulatory element. * **Option D:** Transcription termination is regulated by specific **termination sequences** (like the rho-dependent or rho-independent signals in prokaryotes) located at the 3' end of the gene, opposite to the promoter's location. **3. NEET-PG High-Yield Pearls:** * **Pribnow Box (TATAAT):** The prokaryotic equivalent of the TATA box, located at the -10 position. * **Enhancers vs. Promoters:** While promoters *initiate* transcription and are position-dependent, **enhancers** increase the *rate* of transcription and can be located far away from the gene (position-independent). * **Clinical Correlation:** Mutations in promoter regions can lead to decreased protein synthesis. A classic example is **β-thalassemia**, where mutations in the β-globin gene promoter reduce transcription, leading to a deficiency of hemoglobin chains.

Question 507: What is the smallest unit of genetic expression?

A. Gene
B. Cistron (Correct Answer)
C. Codon
D. Genetic code

Explanation: ### Explanation **Why Cistron is the Correct Answer:** In molecular biology, a **cistron** is defined as the smallest functional unit of genetic expression that codes for a single polypeptide chain. The term was coined by Seymour Benzer through the "cis-trans" test. While a gene is a broader functional unit, the cistron specifically refers to the DNA sequence required to produce a functional protein product. In eukaryotes, most mRNA is **monocistronic** (codes for one protein), whereas in prokaryotes, it is often **polycistronic** (one mRNA codes for multiple proteins, as seen in the Lac Operon). **Analysis of Incorrect Options:** * **Gene:** This is a broader term representing the basic unit of heredity. It includes not only the coding region (cistron) but also regulatory sequences like promoters and enhancers. It is not the "smallest" unit of expression. * **Codon:** A codon is a sequence of three nucleotides that corresponds to a specific amino acid. While it is a unit of the genetic code, it cannot express a functional product on its own; it is merely a "word" within the cistron. * **Genetic Code:** This refers to the universal set of rules by which information encoded in genetic material is translated into proteins. It is a system/language, not a physical unit of expression. **High-Yield NEET-PG Pearls:** * **Muton:** The smallest unit of DNA capable of undergoing **mutation** (a single nucleotide). * **Recon:** The smallest unit of DNA capable of undergoing **recombination**. * **Introns vs. Exons:** Remember that cistrons in eukaryotes contain non-coding intervening sequences (introns) that are removed via splicing. * **Polycistronic mRNA** is a hallmark of prokaryotic gene regulation, allowing coordinate control of genes with related functions.

Question 508: Guanosine triphosphate (GTP) is required by which of the following steps in protein synthesis?

A. Aminoacyl-tRNA synthetase activation of amino acids
B. Attachment of ribosomes to the endoplasmic reticulum
C. Translocation of the tRNA-nascent protein complex from A to P sites (Correct Answer)
D. Attachment of mRNA to ribosomes

Explanation: ### Explanation Protein synthesis (translation) is an energy-intensive process that utilizes both ATP and GTP at specific stages. **1. Why Option C is Correct:** The translocation step involves the movement of the ribosome along the mRNA template by one codon. This process moves the peptidyl-tRNA from the **A (Aminoacyl) site** to the **P (Peptidyl) site**. This mechanical movement is catalyzed by **Elongation Factor-2 (EF-2)** in eukaryotes (or EF-G in prokaryotes), which functions as a GTPase. The hydrolysis of **GTP to GDP** provides the conformational energy required for this translocation. **2. Analysis of Incorrect Options:** * **Option A:** The activation of amino acids (charging of tRNA) by aminoacyl-tRNA synthetase requires **ATP**, not GTP. The reaction produces an aminoacyl-adenylate intermediate and releases inorganic pyrophosphate (PPi). * **Option B:** The attachment of ribosomes to the Rough Endoplasmic Reticulum (RER) is mediated by the **Signal Recognition Particle (SRP)** and its receptor. While the SRP receptor is a GTPase, the structural "attachment" itself is primarily a docking mechanism involving translocons (Sec61 complex). * **Option D:** The initial binding of mRNA to the small ribosomal subunit (40S) involves eukaryotic Initiation Factors (eIFs). While GTP is required for the later stage of initiation (joining the 60S subunit), the initial mRNA attachment is more dependent on the 5' cap recognition and ATP-dependent scanning. **3. High-Yield Clinical Pearls for NEET-PG:** * **Diphtheria & Pseudomonas Toxins:** Both inhibit protein synthesis by ADP-ribosylation of **EF-2**, specifically blocking the GTP-dependent translocation step. * **Energy Budget:** For every amino acid added, **4 high-energy bonds** are consumed: 2 from ATP (during tRNA charging) and 2 from GTP (one for binding the aminoacyl-tRNA to the A-site via EF-1α, and one for translocation via EF-2). * **Initiation:** The first GTP used in translation is carried by **eIF-2** to bring the initiator methionyl-tRNA to the P-site.

Question 509: What is the initiator codon in prokaryotes?

A. UAA
B. UGA
C. AUG (Correct Answer)
D. UAG

Explanation: **Explanation:** In both prokaryotes and eukaryotes, the standard **initiator codon** is **AUG**. This triplet marks the start of the translation process on the mRNA strand. 1. **Why AUG is correct:** The AUG codon codes for the amino acid **Methionine**. In prokaryotes, this methionine is specifically modified to **N-formylmethionine (fMet)**. The initiation complex recognizes AUG downstream of the **Shine-Dalgarno sequence** (the ribosomal binding site in prokaryotes), ensuring the correct reading frame is established for protein synthesis. 2. **Why other options are incorrect:** * **UAA (Ochre), UGA (Opal), and UAG (Amber):** These are collectively known as **Stop Codons** or nonsense codons. They do not code for any amino acid. Instead, they signal the termination of translation by causing the ribosomal complex to dissociate from the mRNA. **High-Yield Clinical Pearls for NEET-PG:** * **Wobble Hypothesis:** Explains why multiple codons can code for the same amino acid, usually differing at the 3rd base. * **Non-Standard Start Codons:** While AUG is the primary initiator, prokaryotes occasionally use **GUG** (Valine) or **UUG** (Leucine) as alternative start codons, though they still incorporate fMet when acting as initiators. * **Antibiotic Correlation:** Many antibiotics (like Aminoglycosides and Tetracyclines) work by interfering with the prokaryotic initiation complex or the 30S ribosomal subunit, exploiting the differences between bacterial and human translation. * **Mitochondrial DNA:** Note that human mitochondria (which follow a "prokaryotic" endosymbiotic model) use AUA as an additional start codon.

Question 510: The 43S preinitiation complex includes all of the following except:

A. IF3
B. IF 1A
C. 1E2β
D. IF-4F (Correct Answer)

Explanation: In eukaryotic translation, the formation of the **43S Preinitiation Complex (PIC)** is a critical early step. This complex is formed by the assembly of several components onto the **40S ribosomal subunit** before it attaches to the mRNA. ### Why Option D is Correct: **eIF-4F** (the cap-binding complex) is **not** part of the 43S PIC. Instead, eIF-4F binds directly to the 5' cap of the **mRNA** to form the "activated mRNA." The 43S PIC and the eIF-4F-bound mRNA then combine to form the **48S initiation complex**. eIF-4F consists of three subunits: eIF-4E (cap-binding), eIF-4A (helicase), and eIF-4G (scaffold). ### Why Other Options are Incorrect: The 43S PIC is composed of: * **40S ribosomal subunit** * **eIF-2-GTP-Met-tRNAi** (The Ternary Complex) * **eIF-3:** Prevents premature reassociation of 40S and 60S subunits (**Option A**). * **eIF-1 and eIF-1A:** Bind to the A and E sites to monitor start codon selection (**Option B**). * **eIF-2:** A heterotrimeric protein ($\alpha, \beta, \gamma$). **eIF-2$\beta$** is a structural component of the ternary complex required for the 43S PIC (**Option C**). * **eIF-5:** Acts as a GTPase activating protein. ### High-Yield Clinical Pearls for NEET-PG: * **Rate-limiting step:** The binding of **eIF-4E** to the mRNA cap is the rate-limiting step of translation initiation. * **Vanishing White Matter Disease:** Caused by mutations in **eIF-2B** (the guanine nucleotide exchange factor for eIF-2). * **Heme-Regulated Inhibitor (HRI):** In reticulocytes, heme deficiency activates a kinase that phosphorylates the $\alpha$-subunit of **eIF-2**, inhibiting translation to prevent globin synthesis without heme. * **43S vs. 48S:** 43S = 40S + Ternary Complex + eIFs; 48S = 43S + mRNA.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

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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

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Single Nucleotide Polymorphisms

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Gene Therapy Approaches

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CRISPR-Cas9 and Genome Editing

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DNA Fingerprinting and Forensics

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Molecular Basis of Genetic Diseases

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