Molecular Biology and Genomics Practice Questions

Q: In the lac operon, transcription is inhibited by binding into which site?

Operator site. ### Explanation The **lac operon** is a classic model of prokaryotic gene regulation, specifically an **inducible system** used by *E. coli* to metabolize lactose. **Why the Operator site is correct:** The **Operator (O site)** is a segment of DNA located between the promoter and the structural genes. In the absence of lactose, a **repressor protein** (encoded by the *lacI* gene) binds specifically to the operator. This binding creates physical steric hindrance, preventing **RNA polymerase** from moving forward from the promoter to the structural genes (*lacZ, lacY, lacA*), thereby inhibiting transcription. This is known as **negative control**. **Analysis of Incorrect Options:** * **A. Promoter site:** This is the binding site for **RNA polymerase**. While transcription *starts* here, inhibition occurs because the repressor blocks the polymerase's progress at the operator, not because the repressor binds to the promoter itself. * **C. CAP site:** The Catabolite Activator Protein (CAP) binds here to **stimulate** transcription when glucose levels are low (via cAMP). This represents **positive control**, not inhibition. * **D. None of the above:** Incorrect, as the operator is the definitive site of repressor-mediated inhibition. ### High-Yield Clinical Pearls for NEET-PG: * **Inducer:** Allolactose (an isomer of lactose) is the natural inducer that binds to the repressor, causing it to detach from the operator. * **IPTG:** A synthetic, non-metabolizable inducer often used in laboratory experiments. * **Constitutive Expression:** Mutations in the *i* gene (repressor) or the operator site lead to "constitutive" expression, where enzymes are produced regardless of lactose presence. * **Glucose Effect:** High glucose levels inhibit Adenylate Cyclase, leading to low cAMP and decreased CAP binding, ensuring the cell prefers glucose over lactose (Catabolite repression).

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.

Question 1

In the lac operon, transcription is inhibited by binding into which site?

Accepted Answer

Operator site

Answer

Promoter site

Answer

CAP site

Answer

None of the above

Question 2

Somatic mutations involve:

Accepted Answer

Frame-shift alteration in coding nucleotides

Answer

Deletion

Answer

Plasmid mediated

Answer

None

Question 3

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 4

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 5

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 6

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 7

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 8

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 9

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 10

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

Accepted Answer

IL28B

Answer

IL15

Answer

c-KIT

Answer

NAT2

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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