Molecular Biology and Genomics Practice Questions

Q: Which of the following statements about Polymerase Chain Reaction (PCR) is false?

PCR utilizes a heat-labile DNA polymerase.. ### Explanation **1. Why Option A is the Correct (False) Statement:** The Polymerase Chain Reaction (PCR) process involves a **denaturation step** where the reaction mixture is heated to approximately **94°C–96°C** to separate double-stranded DNA. A heat-labile (heat-sensitive) enzyme would be permanently denatured and inactivated at this temperature. Therefore, PCR requires a **heat-stable (thermostable) DNA polymerase**, most commonly **Taq polymerase** (derived from the bacterium *Thermus aquaticus*), which can withstand repeated heating cycles. **2. Analysis of Other Options:** * **Option B:** This is a true statement. Heat-stable polymerases are essential for the automation of PCR, as they remain functional throughout multiple cycles of heating and cooling. * **Options C & D:** These are true statements defining the core purpose of PCR. It is an *in vitro* enzymatic technique used for the exponential **amplification** of a specific DNA segment, yielding **millions of copies** from a single template. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Steps of PCR:** 1. Denaturation (~95°C), 2. Annealing (~55°C), 3. Extension (~72°C). * **Components:** Template DNA, Primers (forward and reverse), dNTPs (nucleotides), and Taq Polymerase. * **RT-PCR:** Reverse Transcriptase PCR is used to amplify **RNA** (e.g., for diagnosing COVID-19 or HIV viral load) by first converting RNA into complementary DNA (cDNA). * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes. * **Application:** PCR is the "gold standard" for diagnosing slow-growing or non-culturable organisms (e.g., *Mycobacterium tuberculosis*, *Chlamydia*).

Q: Which of the following is true about telomerase?

Ribonucleoprotein. **Explanation:** **Telomerase** is a specialized enzyme responsible for maintaining the length of telomeres (the repetitive TTAGGG sequences at the ends of eukaryotic chromosomes). It functions as a **Ribonucleoprotein (RNP)** complex, meaning it consists of both protein and RNA components. 1. **Why Option A is Correct:** Telomerase contains two essential components: * **TERT (Telomerase Reverse Transcriptase):** The catalytic protein component. * **TERC (Telomerase RNA Component):** An integral RNA molecule that acts as a **template** for synthesizing telomeric DNA. Because it uses its own RNA template to synthesize DNA, telomerase is classified as an **RNA-dependent DNA polymerase**. 2. **Why Other Options are Incorrect:** * **Option B:** Telomerase is an enzyme involved in DNA replication, not a transcription factor. Transcription factors regulate the synthesis of mRNA from DNA. * **Option C:** It is a nuclear enzyme, not a membrane-bound receptor. * **Option D:** While it interacts with DNA, "DNA binding domain" is a structural motif characteristic of transcription factors or histones; telomerase is defined primarily by its catalytic reverse transcriptase activity. **High-Yield Clinical Pearls for NEET-PG:** * **The End-Replication Problem:** DNA polymerase cannot replicate the 3' end of linear chromosomes, leading to progressive shortening. Telomerase solves this. * **Cancer & Aging:** Telomerase activity is high in **germ cells, stem cells, and cancer cells** (conferring "immortality"), but is low or absent in most somatic cells (leading to cellular senescence). * **Prokaryotes:** Do not have telomerase because their DNA is **circular** and lacks ends.

Q: Which of the following toxins inhibits the peptidyl transferase?

Ricin. **Explanation:** The correct answer is **Ricin**. Ricin is a potent toxin derived from the seeds of the castor oil plant (*Ricinus communis*). It acts as a **Type II Ribosome-Inactivating Protein (RIP)**. **1. Why Ricin is correct:** Ricin functions as an **N-glycosidase** that specifically removes a single adenine residue from the 28S ribosomal RNA of the 60S eukaryotic ribosomal subunit. This site is part of the "sarcin/ricin loop," which is critical for the binding of elongation factors. By depurinating this specific adenine, Ricin irreversibly damages the ribosome, preventing it from facilitating **peptidyl transferase activity** and binding elongation factors, thereby halting protein synthesis. **2. Why other options are incorrect:** * **Diphtheria toxin:** Inhibits protein synthesis by catalyzing the ADP-ribosylation of **Elongation Factor-2 (eEF-2)**, not by targeting peptidyl transferase directly. * **Pertussis toxin:** Acts by ADP-ribosylating the **Gi (inhibitory) protein**, leading to increased levels of cAMP; it does not directly inhibit translation. * **Amanitin (α-amanitin):** Found in *Amanita phalloides* (death cap mushroom), it specifically inhibits **RNA Polymerase II**, thereby blocking mRNA synthesis (transcription) rather than translation. **Clinical Pearls for NEET-PG:** * **Shiga Toxin:** Produced by *S. dysenteriae*, it shares the same mechanism as Ricin (cleaving the 28S rRNA). * **Cycloheximide:** A laboratory tool that specifically inhibits eukaryotic peptidyl transferase. * **Chloramphenicol:** Inhibits **prokaryotic** (70S) peptidyl transferase; its side effect is bone marrow suppression due to its effect on mitochondrial ribosomes. * **Mnemonic for Ricin:** **R**icin **R**emoves **R**ibosomal adenine.

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

Electroporation. ### 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.

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

DNA Polymerase I. ### 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.

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

Cistron. **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.

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

IF-4F. 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.

Q: Which of the following is an example of post-translational modification?

Phosphorylation. **Explanation:** **Post-translational modification (PTM)** refers to the covalent and generally enzymatic modification of proteins following protein biosynthesis. These modifications occur after the polypeptide chain has been synthesized on the ribosome and are crucial for protein folding, stability, localization, and biological activity. **Why Phosphorylation is Correct:** **Phosphorylation** is the most common PTM. It involves the addition of a phosphate group (usually to Serine, Threonine, or Tyrosine residues) by enzymes called **kinases** and its removal by **phosphatases**. This process acts as a molecular "on/off" switch, regulating enzyme activity and signal transduction pathways (e.g., the Insulin signaling pathway). **Analysis of Incorrect Options:** * **Methylation:** While it can occur on proteins (like histones), in the context of standard NEET-PG biochemistry, it is more frequently discussed as a **pre-translational/epigenetic** modification of DNA (Cytosine residues) to regulate gene expression. * **Oxidation:** This is generally a form of **protein damage** caused by reactive oxygen species (ROS) rather than a regulated, functional post-translational modification. * **Splicing:** This is a **post-transcriptional** modification. It involves the removal of introns and joining of exons from the primary RNA transcript (hnRNA) to form mature mRNA. **High-Yield Clinical Pearls for NEET-PG:** * **Zymogen Activation:** Proteolysis (e.g., Trypsinogen to Trypsin) is an irreversible PTM. * **Gamma-carboxylation:** Occurs on Glutamate residues of Clotting Factors II, VII, IX, and X; it requires **Vitamin K** as a cofactor. * **Hydroxylation:** Proline and Lysine residues in collagen require **Vitamin C** for hydroxylation; deficiency leads to Scurvy. * **Glycosylation:** Occurs in the ER and Golgi apparatus; it is essential for membrane protein function.

Question 1

Which enzymatic mutation is responsible for immortality of cancer cells?

Accepted Answer

Telomerase

Answer

DNA reverse transcriptase

Answer

RNA polymerase

Answer

DNA polymerase

Question 2

Which of the following statements about Polymerase Chain Reaction (PCR) is false?

Accepted Answer

PCR utilizes a heat-labile DNA polymerase.

Answer

PCR utilizes a heat-stable DNA polymerase.

Answer

PCR is a technique for DNA amplification.

Answer

PCR is used to yield multiple copies of DNA.

Question 3

Which of the following is true about telomerase?

Accepted Answer

Ribonucleoprotein

Answer

Transcriptional factor activation

Answer

Membrane receptor

Answer

DNA binding domain

Question 4

Which of the following toxins inhibits the peptidyl transferase?

Accepted Answer

Ricin

Answer

Diphtheria toxin

Answer

Pertussis toxin

Answer

Amanitin

Question 5

Which of the following methods is used in gene therapy?

Accepted Answer

Electroporation

Answer

Electrofocusing

Answer

Selectively targeted recombination

Answer

Intracytoplasmic injection

Question 6

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

Accepted Answer

DNA Polymerase I

Answer

DNA Polymerase II

Answer

DNA Polymerase III

Answer

DNA Polymerase IV

Question 7

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

Accepted Answer

Cistron

Answer

Operon

Answer

Replicon

Answer

Anticodon

Question 8

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

Accepted Answer

Translocation of the tRNA-nascent protein complex from A to P sites

Answer

Aminoacyl-tRNA synthetase activation of amino acids

Answer

Attachment of ribosomes to the endoplasmic reticulum

Answer

Attachment of mRNA to ribosomes

Question 9

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

Accepted Answer

IF-4F

Answer

IF3

Answer

IF 1A

Answer

1E2β

Question 10

Which of the following is an example of post-translational modification?

Accepted Answer

Phosphorylation

Answer

Methylation

Answer

Oxidation

Answer

Splicing

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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