Molecular Biology and Genomics Practice Questions

Q: Which is the most processive DNA polymerase?

DNA Polymerase III. **Explanation:** **DNA Polymerase III** is the primary enzyme responsible for prokaryotic DNA replication. Its high **processivity**—defined as the ability of an enzyme to catalyze consecutive reactions without releasing its substrate—is its defining characteristic. 1. **Why DNA Polymerase III is correct:** DNA Pol III is a complex holoenzyme. Its high processivity (adding thousands of nucleotides per binding event) is attributed to the **$\beta_2$ clamp (sliding clamp)** subunit. This ring-shaped protein encircles the DNA strand, tethering the catalytic core to the template and preventing it from dissociating. This allows for the rapid and continuous synthesis of the leading strand and long fragments of the lagging strand. 2. **Why other options are incorrect:** * **DNA Polymerase I:** It has low processivity (adding only 20–50 nucleotides). Its primary roles are **primer removal** (via 5'→3' exonuclease activity) and filling short gaps during DNA repair and lagging strand maturation. * **DNA Polymerase II:** It is mainly involved in **DNA repair** (SOS response) when the replication fork is stalled. It has intermediate processivity but is significantly lower than Pol III. **High-Yield Clinical Pearls for NEET-PG:** * **Eukaryotic Counterpart:** In eukaryotes, **DNA Polymerase $\delta$ (delta)** and **$\epsilon$ (epsilon)** are the highly processive enzymes, utilizing the **PCNA** (Proliferating Cell Nuclear Antigen) as their sliding clamp. * **Exonuclease Activity:** DNA Pol III possesses **3'→5' exonuclease activity** for proofreading, ensuring high fidelity, but lacks the 5'→3' exonuclease activity found in Pol I. * **Speed:** DNA Pol III can add nucleotides at a rate of approximately 1,000 per second.

Q: On which chromosome is the ABO gene located?

9q. The ABO blood group system is determined by a single gene located on the **long arm (q) of chromosome 9**, specifically at the locus **9q34.2**. ### Explanation of the Correct Answer The ABO gene encodes a glycosyltransferase enzyme. Depending on the allele inherited (A, B, or O), this enzyme modifies the H antigen on the surface of red blood cells. * The **A allele** encodes α-1,3-N-acetylgalactosaminyltransferase. * The **B allele** encodes α-1,3-galactosyltransferase. * The **O allele** is a result of a frameshift mutation (deletion of Guanine at position 258), resulting in a non-functional protein. ### Analysis of Incorrect Options * **10p:** No major blood group systems are mapped to the short arm of chromosome 10. * **11q:** While chromosome 11 contains important clusters like the **Beta-globin gene**, it does not house the ABO locus. * **12p:** Chromosome 12 contains genes related to Von Willebrand Factor (12p13), but not the ABO system. ### High-Yield Clinical Pearls for NEET-PG * **H-Substance:** The precursor for A and B antigens is the H antigen, which is regulated by the **FUT1 gene** located on **Chromosome 19**. * **Bombay Phenotype:** Occurs when there is a deficiency of the H antigen (h/h genotype). These individuals type as "O" but have potent anti-H antibodies. * **Inheritance:** ABO blood groups follow **Codominance** (A and B are codominant) and **Mendelian inheritance**. * **Secretor Status:** Regulated by the **FUT2 gene**, also located on **Chromosome 19**.

Q: Which radiolabeled marker is best for studying DNA replication?

Thymidine. ### Explanation **Correct Option: B. Thymidine** The study of DNA replication requires a marker that is **exclusive to DNA**. Thymidine is a nucleoside containing the nitrogenous base **Thymine**, which is found only in DNA and not in RNA. When cells are incubated with radiolabeled thymidine (usually $^3$H-Thymidine), it is incorporated into the newly synthesized DNA strands during the **S-phase** of the cell cycle. This allows researchers to measure the rate of DNA synthesis and visualize chromosomal replication via autoradiography. **Why the other options are incorrect:** * **A. Ribose:** This is the pentose sugar found in **RNA**. DNA contains deoxyribose. Using ribose would label RNA molecules, not DNA. * **C. Phosphate:** Phosphorus is present in the backbone of **both DNA and RNA**, as well as in phospholipids and ATP. It lacks the specificity required to isolate DNA replication specifically. * **D. Uracil:** This nitrogenous base is **unique to RNA** (replacing Thymine). Radiolabeled Uracil is the marker of choice for studying **transcription** (RNA synthesis), not DNA replication. **High-Yield Clinical Pearls for NEET-PG:** * **S-Phase Specificity:** Tritiated thymidine ($^3$H-TdR) is the gold standard for calculating the **Labeling Index**, which indicates the proliferative activity of a tumor. * **5-Fluorouracil (5-FU):** A common chemotherapy agent that acts by inhibiting **Thymidylate Synthase**, thereby blocking the synthesis of thymidine and halting DNA replication. * **Modern Alternative:** In contemporary labs, **BrdU (Bromodeoxyuridine)** is often used instead of radioactive thymidine; it is a thymidine analogue detected via immunohistochemistry.

Q: Which enzymes are involved in Base-excision repair?

DNA polymerase I. **Explanation:** **Base Excision Repair (BER)** is the primary mechanism for repairing non-bulky DNA damage, such as deaminated bases (e.g., Uracil), oxidized bases, or alkylated bases. The process follows a specific sequence: 1. **DNA Glycosylase:** Recognizes and removes the damaged base, creating an **AP site** (Apurinic/Apyrimidinic). 2. **AP Endonuclease:** Cleaves the phosphodiester backbone at the AP site. 3. **DNA Polymerase I:** In prokaryotes, this enzyme performs "nick translation." It uses its 5'→3' exonuclease activity to remove the damaged segment and its polymerase activity to fill the gap with correct nucleotides. (In eukaryotes, DNA Polymerase β performs this role). 4. **DNA Ligase:** Seals the final nick. **Analysis of Options:** * **Option A (DNA Helicase II):** Involved in **Methyl-directed Mismatch Repair (MMR)** in *E. coli* (MutHLS pathway) to unwind the DNA strand for degradation. * **Option B (ABC Excinuclease):** The hallmark enzyme of **Nucleotide Excision Repair (NER)**. It removes bulky lesions like pyrimidine dimers caused by UV light. * **Option D (DNA Photolyase):** Involved in **Direct Reversal** (Photoreactivation) of UV-induced damage. This enzyme is notably absent in placental mammals. **Clinical Pearls for NEET-PG:** * **Mnemonic for BER:** "GEL P" (Glycosylase, Endonuclease, Ligase, Polymerase). * **Defect in BER:** Mutations in *MUTYH* glycosylase lead to **MUTYH-associated polyposis (MAP)**, increasing colorectal cancer risk. * **Key distinction:** BER repairs **single base** damage, while NER repairs **bulky adducts** (distorting the helix).

Q: Which cytoplasmic process is involved during the processing of RNA precursors?

Attachment of CCA in tRNA. **Explanation:** The processing of RNA precursors (post-transcriptional modification) occurs in different cellular compartments depending on the type of RNA and the specific modification involved. **Why Option D is Correct:** The addition of the **CCA sequence** to the 3' end of tRNA is a crucial maturation step. While some organisms encode this in the DNA, in eukaryotes, it is added post-transcriptionally by the enzyme **nucleotidyltransferase**. While initial tRNA processing begins in the nucleus, the final "quality control" and the attachment/repair of the CCA tail can occur in the **cytoplasm**. This CCA tail is essential because it serves as the attachment site for amino acids during translation (aminoacylation). **Analysis of Incorrect Options:** * **A. 5' capping:** This occurs exclusively in the **nucleus** shortly after transcription begins (co-transcriptional). It involves adding a 7-methylguanosine cap to protect mRNA from degradation. * **B. Poly (A) tailing:** This involves the addition of ~200 adenine nucleotides to the 3' end of mRNA. This process is mediated by Poly(A) polymerase and occurs in the **nucleus** before the mRNA is exported. * **C. Methylation of tRNA:** Base modifications, including methylation (e.g., producing thymidine or pseudouridine), primarily occur in the **nucleus** during the early stages of tRNA maturation. **High-Yield NEET-PG Pearls:** * **CCA Sequence:** Remember the mnemonic **"Can Carry Amino acids"** for the CCA tail at the 3' end. * **RNA Polymerases:** Know your types—Pol I (rRNA), Pol II (mRNA), Pol III (tRNA). * **Splicing:** Another major nuclear processing event involving snRNPs (Small Nuclear Ribonucleoproteins). * **Clinical Correlation:** Defects in tRNA processing are linked to mitochondrial encephalopathies (e.g., MELAS syndrome).

Q: Which of the following organelles is involved in the formation of N-glycosylated products?

Golgi apparatus. **Explanation:** Protein glycosylation is a post-translational modification essential for protein folding, stability, and cell signaling. It occurs primarily in the **Endoplasmic Reticulum (ER)** and the **Golgi apparatus**. **1. Why Golgi apparatus is correct:** N-glycosylation begins in the Rough ER, where a pre-formed oligosaccharide (dolichol-linked) is attached to an Asparagine residue. However, the **processing, trimming, and final maturation** of these N-linked glycans occur within the **Golgi apparatus**. The Golgi contains specific glycosyltransferases that add complex sugar chains (like galactose and sialic acid) to the core structure. Note: O-glycosylation (attachment to Serine/Threonine) occurs *exclusively* in the Golgi. **2. Why other options are incorrect:** * **Nucleolus:** This is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly. It is not involved in protein modification or glycosylation. * **Smooth Endoplasmic Reticulum (SER):** The SER is primarily involved in lipid synthesis, steroid hormone production, and detoxification (Cytochrome P450 system). While the *Rough* ER initiates N-glycosylation, the SER does not play a significant role in this process. **High-Yield Clinical Pearls for NEET-PG:** * **I-Cell Disease (Inclusion Cell Disease):** A deficiency in *N-acetylglucosaminyl-1-phosphotransferase* in the Golgi. This prevents the phosphorylation of mannose residues (Mannose-6-Phosphate) on lysosomal enzymes, causing them to be secreted extracellularly rather than sent to lysosomes. * **Dolichol Phosphate:** The lipid carrier in the ER membrane required for the initial step of N-glycosylation. It is inhibited by the antibiotic **Tunicamycin**. * **N-linked** = Asparagine; **O-linked** = Serine/Threonine.

Q: Which of the following techniques can be used to differentiate the chromosomal abnormalities between normal and cancer cells?

Comparative Genomic Hybridization (CGH). **Explanation:** The correct answer is **Comparative Genomic Hybridization (CGH)**. **1. Why CGH is the correct answer:** CGH is a molecular cytogenetic technique used to detect **copy number variations (CNVs)**—specifically gains (amplifications) or losses (deletions) of whole chromosomes or specific chromosomal regions. In this method, DNA from a "test" sample (cancer cell) and a "reference" sample (normal cell) are labeled with different fluorescent dyes (usually green and red) and hybridized to a normal metaphase spread or a microarray. By comparing the fluorescence ratio, clinicians can identify unbalanced chromosomal abnormalities characteristic of malignancies, such as oncogene amplification or tumor suppressor gene deletion. **2. Why other options are incorrect:** * **PCR:** Primarily used to amplify specific DNA sequences. While it can detect point mutations or small indels, it is not the standard tool for global chromosomal differentiation between cell types. * **Karyotyping:** While it visualizes chromosomes, it has low resolution (5–10 Mb) and requires living, dividing cells in metaphase. CGH provides much higher resolution and can be performed on archived or non-dividing tissue. * **Western Blotting:** This technique detects and quantifies specific **proteins**, not chromosomal or genomic DNA abnormalities. **Clinical Pearls for NEET-PG:** * **Array-CGH (aCGH):** The modern version using microarrays; it is the first-line investigation for children with developmental delays and multiple congenital anomalies. * **Limitation of CGH:** It cannot detect **balanced** chromosomal abnormalities like reciprocal translocations, inversions, or polyploidy, as there is no net change in the amount of DNA. * **FISH vs. CGH:** FISH requires a specific suspected probe; CGH scans the entire genome without prior knowledge of the specific abnormality.

Q: True about RNA Polymerase is:

No proofreading activity. **Explanation:** The correct answer is **C. No proofreading activity.** Unlike DNA Polymerase, **RNA Polymerase (RNAP)** lacks the 3'→5' exonuclease activity required for proofreading. While RNAP has a very limited ability to pause and remove mismatched bases, it is significantly less efficient than DNA Polymerase. This results in a much higher error rate (approx. 1 in $10^4$ to $10^5$ nucleotides), which is evolutionarily tolerable because RNA is transient and not the permanent repository of genetic information. **Analysis of Incorrect Options:** * **A. Requires primers:** RNA Polymerase is capable of **de novo** synthesis. It can initiate the formation of a phosphodiester bond between two free nucleotides without a pre-existing 3'-OH primer, unlike DNA Polymerase. * **B. RNA dependent DNA Polymerase:** This describes **Reverse Transcriptase** (found in retroviruses like HIV). RNA Polymerase is a **DNA-dependent RNA polymerase**, as it uses a DNA template to synthesize RNA. * **D. Recognizes Shine-Dalgarno sequence:** The Shine-Dalgarno sequence is recognized by the **16S rRNA of the 30S ribosomal subunit** during the initiation of translation (protein synthesis), not by RNA polymerase during transcription. **High-Yield Clinical Pearls for NEET-PG:** * **Rifampicin:** Inhibits bacterial RNA Polymerase by binding to the $\beta$-subunit; used in Tuberculosis. * **$\alpha$-Amanitin:** Found in *Amanita phalloides* (death cap mushroom); it specifically inhibits **RNA Polymerase II**, leading to severe hepatotoxicity. * **Sigma ($\sigma$) Factor:** A subunit of prokaryotic RNAP holoenzyme required for **initiation** and recognition of the promoter site (Pribnow box). * **Eukaryotic RNAP Types:** * I: rRNA (except 5S) * II: mRNA (most sensitive to $\alpha$-amanitin) * III: tRNA and 5S rRNA.

Question 1

All of the following conditions are characterized by trinucleotide repeat expansion affecting the coding regions, except?

Accepted Answer

Huntington disease

Answer

Friedreich ataxia

Answer

Fragile X syndrome

Answer

Myotonic dystrophy

Question 2

Which is the most processive DNA polymerase?

Accepted Answer

DNA Polymerase III

Answer

DNA Polymerase I

Answer

DNA Polymerase II

Answer

None

Question 3

On which chromosome is the ABO gene located?

Accepted Answer

9q

Answer

10p

Answer

11q

Answer

12p

Question 4

Which radiolabeled marker is best for studying DNA replication?

Accepted Answer

Thymidine

Answer

Ribose

Answer

Phosphate

Answer

Uracil

Question 5

Which enzymes are involved in Base-excision repair?

Accepted Answer

DNA polymerase I

Answer

DNA helicase II

Answer

ABC excinuclease

Answer

DNA photolyase

Question 6

Which cytoplasmic process is involved during the processing of RNA precursors?

Accepted Answer

Attachment of CCA in tRNA

Answer

5' capping

Answer

Poly (A) tailing

Answer

Methylation of tRNA

Question 7

Which of the following organelles is involved in the formation of N-glycosylated products?

Accepted Answer

Golgi apparatus

Answer

Nucleolus

Answer

Smooth Endoplasmic Reticulum

Answer

All of the above

Question 8

Which of the following techniques can be used to differentiate the chromosomal abnormalities between normal and cancer cells?

Accepted Answer

Comparative Genomic Hybridization (CGH)

Answer

Polymerase Chain Reaction (PCR)

Answer

Karyotyping

Answer

Western Blotting

Question 9

True about RNA Polymerase is:

Accepted Answer

No proofreading activity

Answer

Requires primers

Answer

RNA dependent DNA Polymerase

Answer

Recognizes Shine-Dalgarno sequence

Question 10

What is the function of micro-RNA?

Accepted Answer

Modulates the translation of target mRNAs into their corresponding proteins

Answer

Encodes proteins

Answer

Regulation of gene expression

Answer

mRNA splicing

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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