Molecular Biology and Genomics Practice Questions

Q: In which of the following techniques are oligonucleotide primers used?

Polymerase Chain Reaction (PCR). **Explanation:** **Correct Option: B. Polymerase Chain Reaction (PCR)** PCR is an *in vitro* enzymatic method used to amplify specific DNA sequences. The process requires **oligonucleotide primers**, which are short, single-stranded DNA sequences (usually 15–30 nucleotides long) complementary to the target DNA's flanking regions. These primers provide a free 3'-OH group, allowing **Taq Polymerase** to initiate DNA synthesis. Without these specific primers, the polymerase cannot bind and extend the DNA chain. **Analysis of Incorrect Options:** * **A. RFLP:** This technique relies on **Restriction Endonucleases** to cut DNA at specific recognition sites. The fragments are then separated by electrophoresis and visualized using labeled probes, not primers. * **C. FISH:** This technique uses fluorescently labeled **DNA/RNA probes** to detect and locate specific gene sequences directly on intact chromosomes. It is a hybridization technique, not an amplification technique requiring primers. * **D. Chromosomal Walking:** While this method is used to sequence long stretches of DNA by using the end of one cloned fragment as a probe to find the next, the fundamental tool used to identify overlapping clones is a **probe**, not a primer-driven amplification (though modern variations may use PCR, the classical definition relies on genomic library screening). **NEET-PG High-Yield Pearls:** * **PCR Steps:** Denaturation (94°C) → Annealing (55°C; where primers bind) → Extension (72°C). * **Components of PCR:** Template DNA, Primers, dNTPs, and Heat-stable DNA polymerase (Taq). * **Clinical Use:** PCR is the gold standard for diagnosing viral loads (e.g., HIV, Hepatitis C) and detecting genetic mutations.

Q: What is the typical size of a microsatellite repeat sequence?

2-6 base pairs (bp). **Explanation:** The correct answer is **B (2-6 base pairs)**. This question tests the classification of repetitive DNA sequences, specifically **Short Tandem Repeats (STRs)**, also known as **Microsatellites**. Microsatellites are tracts of repetitive DNA where a short motif, typically **2 to 6 base pairs** in length, is repeated 5 to 50 times. They are highly polymorphic and distributed throughout the human genome. Their high degree of variability between individuals makes them the "gold standard" marker for DNA profiling and linkage analysis. **Analysis of Options:** * **Option A ( 3 kb):** These sizes are characteristic of **Minisatellites** (Variable Number Tandem Repeats or VNTRs), where the repeat unit is typically 10-60 bp, and the total array can span several kilobases. **High-Yield Clinical Pearls for NEET-PG:** 1. **Microsatellite Instability (MSI):** This is a critical diagnostic marker for **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). It occurs due to mutations in **Mismatch Repair (MMR) genes** (e.g., *MLH1, MSH2*), leading to uncontrolled expansion or contraction of these repeats. 2. **Trinucleotide Repeat Disorders:** Many neurological diseases are caused by microsatellite expansions (e.g., **Huntington’s disease** [CAG], **Fragile X syndrome** [CGG]). 3. **Forensics:** STRs are the primary tools used in **DNA Fingerprinting** because the number of repeats at specific loci varies significantly between individuals.

Q: Which of the following is not required for PCR, a cell-free test tube method for amplifying a target sequence of DNA?

Radiolabeled DNA probe. **Explanation** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic method used to amplify specific DNA sequences. To understand why a **radiolabeled DNA probe** is not required, one must look at the fundamental components of the PCR "cocktail." 1. **Why the Correct Answer is Right:** A **radiolabeled DNA probe** is used in hybridization techniques like Southern Blotting to *detect* a specific DNA sequence after it has been separated by electrophoresis. In PCR, the goal is *amplification* (synthesis). While probes are used in Real-Time PCR (qPCR) for quantification, they are not a fundamental requirement for the basic PCR process itself. Furthermore, modern labs use fluorescent dyes rather than hazardous radioactive labels. 2. **Why the Other Options are Wrong:** * **Taq Polymerase (A):** A heat-stable DNA polymerase (derived from *Thermus aquaticus*) is essential to synthesize new DNA strands at high temperatures without denaturing. * **dNTPs (B):** Deoxynucleotide triphosphates (dATP, dCTP, dGTP, dTTP) are the "building blocks" required to construct the new DNA polymer. * **Primers (C):** PCR requires two synthetic oligonucleotide primers that are complementary to the sequences flanking the target DNA. They provide the 3'-OH group necessary for DNA polymerase to initiate synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **Steps of PCR:** Denaturation (94-96°C) → Annealing (50-65°C) → Extension (72°C). * **RT-PCR vs. qPCR:** Reverse Transcriptase PCR (RT-PCR) is used to amplify RNA (e.g., for COVID-19 testing), while Quantitative PCR (qPCR) measures DNA concentration in real-time. * **Magnesium ions (MgCl₂):** A critical cofactor for Taq polymerase activity; its concentration can affect the specificity of the reaction.

Q: All are true about the Northern blotting technique EXCEPT:

It is used to detect RNA molecules.. The question asks for the **EXCEPT** statement regarding Northern blotting. Note that the provided key lists Option C as the "correct" answer (the false statement), but in standard molecular biology, Northern blotting **is** used to detect RNA. However, in the context of competitive exams like NEET-PG, this question typically highlights that **Option D** is the false statement. ### Explanation **Why Option D is the correct answer (The False Statement):** Northern blotting is used to analyze **RNA**. Unlike DNA, RNA molecules are already short enough to be separated by size via electrophoresis and do not possess the double-stranded stability required for consistent recognition by restriction enzymes. Therefore, **Restriction Endonucleases are NOT required** for Northern blotting; they are a hallmark of Southern blotting (DNA analysis). **Analysis of Other Options:** * **Option A (Electrophoresis):** True. RNA fragments are separated based on size using gel electrophoresis (usually formaldehyde-agarose gel to prevent RNA secondary structure formation). * **Option B (Hybridization Probes):** True. After transferring RNA to a membrane (nitrocellulose or nylon), a labeled complementary DNA or RNA probe is used to identify specific sequences. * **Option C (Detects RNA):** True. Northern blotting is the gold standard for measuring gene expression by detecting specific mRNA levels. ### High-Yield Clinical Pearls for NEET-PG: To remember the blotting techniques, use the **SNOW DROP** mnemonic: * **S**outhern = **D**NA (Requires Restriction Endonucleases) * **N**orthern = **R**NA (No Restriction Endonucleases) * **O** = **O** (Nothing) * **W**estern = **P**roteins (Uses Antibodies) * **Southwestern Blotting:** Used to detect **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Eastern Blotting:** Used to detect post-translational modifications of proteins.

Q: The genetic defect associated with decreased response to interferon therapy for Hepatitis C is:

IL28B. **Explanation:** The correct answer is **IL28B** (Interleukin 28B). **1. Why IL28B is correct:** IL28B is a gene that encodes **Interferon-lambda 3 (IFN-λ3)**. Extensive genome-wide association studies (GWAS) have identified that single nucleotide polymorphisms (SNPs) near the IL28B gene are the strongest predictors of a patient's response to **Pegylated Interferon-alpha and Ribavirin** therapy in Chronic Hepatitis C (Genotype 1). Specifically, the **CC genotype** is associated with a high rate of sustained virologic response (SVR) and spontaneous viral clearance, whereas the **CT or TT genotypes** are associated with a significantly decreased response to therapy. **2. Why the other options are incorrect:** * **IL15:** This is a cytokine involved in the proliferation of T-cells and Natural Killer (NK) cells. While important in the immune response, it is not a genetic predictor for Hepatitis C treatment outcomes. * **c-KIT:** This is a proto-oncogene encoding a receptor tyrosine kinase. Mutations in c-KIT are associated with **Gastrointestinal Stromal Tumors (GIST)** and mastocytosis, not interferon response. * **NAT2 (N-acetyltransferase 2):** This gene is responsible for the "Acetylation" pathway of drug metabolism. Polymorphisms in NAT2 categorize individuals into **Fast or Slow Acetylators**, which is high-yield for drugs like **Isoniazid (INH)**, Hydralazine, and Procainamide, but irrelevant to Hepatitis C. **High-Yield Clinical Pearls for NEET-PG:** * **IL28B polymorphism** is the most important host genetic factor for predicting SVR in HCV Genotype 1. * **HCV Genotype 1** is generally more resistant to interferon than Genotypes 2 or 3. * With the advent of **Direct-Acting Antivirals (DAAs)** like Sofosbuvir, the clinical reliance on IL28B testing has decreased, but it remains a classic, high-yield biochemistry/genetics question.

Q: A family, after consuming wild mushrooms during a picnic, became ill. The youngest child exhibited the most severe symptoms. The family's illness is attributed to a primary cellular inability to accomplish which of the following?

Synthesize proteins. **Explanation:** The clinical scenario describes **Amanita phalloides (Death Cap mushroom) poisoning**. This is a classic high-yield topic in biochemistry and toxicology. **1. Why "Synthesize proteins" is correct:** The primary toxin in these mushrooms is **$\alpha$-amanitin**. This potent toxin acts by specifically binding to and inhibiting **RNA Polymerase II** in eukaryotic cells. RNA Polymerase II is responsible for the transcription of DNA into **messenger RNA (mRNA)**. When mRNA synthesis is blocked, the cell loses the template required for translation at the ribosomes, leading to a complete cessation of **protein synthesis**. This results in massive cell death, particularly in the liver (centrilobular necrosis) and kidneys, leading to fulminant hepatic failure. **2. Why the incorrect options are wrong:** * **Synthesize lipids (B):** While lipid metabolism occurs in the liver, it is not the primary target of $\alpha$-amanitin. * **Synthesize DNA (C):** DNA synthesis (replication) involves DNA polymerases. $\alpha$-amanitin specifically targets RNA Polymerase II, not DNA polymerase. * **Synthesize carbohydrates (D):** While hypoglycemia is a common clinical finding in mushroom poisoning due to liver failure, it is a secondary effect of hepatocyte destruction, not the primary molecular mechanism of the toxin. **3. Clinical Pearls for NEET-PG:** * **RNA Polymerase I:** Synthesizes rRNA (located in the nucleolus). * **RNA Polymerase II:** Synthesizes mRNA and snRNA (Inhibited by $\alpha$-amanitin). * **RNA Polymerase III:** Synthesizes tRNA and 5S rRNA. * **Clinical Presentation:** Typically involves a latent period (6–24 hours), followed by severe GI distress, and eventually "Phase 3" characterized by hepatic and renal failure. * **Antidote:** Silibinin (milk thistle extract) or N-acetylcysteine are often used in management.

Q: Cells involved in active protein synthesis characteristically exhibit an increase in which of the following organelles?

Rough endoplasmic reticulum. **Explanation:** The correct answer is **Rough Endoplasmic Reticulum (RER)**. Protein synthesis occurs via ribosomes. When cells are specialized for high-volume protein production (such as pancreatic acinar cells secreting enzymes or plasma cells secreting antibodies), they possess an extensive network of RER. The RER is "rough" because its cytosolic surface is studded with ribosomes, which are the primary sites for the translation of mRNA into proteins destined for secretion, membrane integration, or lysosomal enzymes. **Analysis of Incorrect Options:** * **Mitochondria:** These are the "powerhouses" of the cell, responsible for ATP production via oxidative phosphorylation. While protein synthesis requires energy, an increase in mitochondria is more characteristic of metabolically active cells with high energy demands, such as cardiac muscle or proximal convoluted tubule cells. * **Golgi Apparatus:** The Golgi functions in the modification, sorting, and packaging of proteins received from the RER. While it is prominent in secretory cells, the actual *synthesis* occurs in the RER. * **Lysosomes:** These contain hydrolytic enzymes for intracellular digestion. An increase in lysosomes is seen in phagocytic cells (like macrophages) rather than those primarily focused on protein synthesis. **High-Yield NEET-PG Pearls:** * **Nissl Bodies:** In neurons, the RER is visualized as Nissl bodies; their disappearance (chromatolysis) indicates axonal injury. * **Free Ribosomes:** Synthesize proteins for *intracellular* use (e.g., hemoglobin, mitochondrial enzymes), whereas **RER-bound ribosomes** synthesize proteins for *export*. * **Smooth ER (SER):** Involved in lipid synthesis, steroid hormone production (adrenal cortex/gonads), and detoxification (liver).

Question 1

Which of the following tests is NOT used in epigenetics?

Accepted Answer

HPLC

Answer

ChIP-on-ChIP

Answer

Bisulfite sequencing

Answer

Methylation-sensitive restriction enzyme digestion

Question 2

In which of the following techniques are oligonucleotide primers used?

Accepted Answer

Polymerase Chain Reaction (PCR)

Answer

Restriction Fragment Length Polymorphism (RFLP)

Answer

Fluorescence In Situ Hybridization (FISH)

Answer

Chromosomal walking

Question 3

Which of the following is not common to the synthesis of both leading and lagging strands?

Accepted Answer

DNA ligase is repeatedly used to join fragments of DNA along the growing strand

Answer

RNA primer is needed

Answer

Nucleoside monophosphates are added in the 5' to 3' direction along the growing DNA chain

Answer

DNA polymerase III synthesizes DNA

Question 4

What is the typical size of a microsatellite repeat sequence?

Accepted Answer

2-6 base pairs (bp)

Answer

Less than 1 kilobase (kb)

Answer

1-3 kilobases (kb)

Answer

Greater than 3 kilobases (kb)

Question 5

All of the following proteins are synthesized by free ribosomes, EXCEPT?

Accepted Answer

Lysosomal enzymes

Answer

Proteins that remain in the cytosol

Answer

Mitochondrial proteins

Answer

Peroxisomal enzymes

Question 6

Which of the following is not required for PCR, a cell-free test tube method for amplifying a target sequence of DNA?

Accepted Answer

Radiolabeled DNA probe

Answer

Taq polymerase

Answer

Deoxynucleotide triphosphates (dNTPs)

Answer

Primer

Question 7

All are true about the Northern blotting technique EXCEPT:

Accepted Answer

It is used to detect RNA molecules.

Answer

It involves electrophoresis.

Answer

It requires hybridization probes.

Answer

It requires restriction endonucleases.

Question 8

The genetic defect associated with decreased response to interferon therapy for Hepatitis C is:

Accepted Answer

IL28B

Answer

IL15

Answer

c-KIT

Answer

NAT2

Question 9

A family, after consuming wild mushrooms during a picnic, became ill. The youngest child exhibited the most severe symptoms. The family's illness is attributed to a primary cellular inability to accomplish which of the following?

Accepted Answer

Synthesize proteins

Answer

Synthesize lipids

Answer

Synthesize DNA

Answer

Synthesize carbohydrates

Question 10

Cells involved in active protein synthesis characteristically exhibit an increase in which of the following organelles?

Accepted Answer

Rough endoplasmic reticulum

Answer

Mitochondria

Answer

Golgi apparatus

Answer

Lysosomes

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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