Molecular Biology and Genomics Practice Questions

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.

Q: In which type of RNA is the anticodon found?

Transfer RNA (t-RNA). ### Explanation **Correct Answer: A. Transfer RNA (t-RNA)** The **anticodon** is a specific sequence of three nucleotides located on the anticodon loop of **t-RNA**. Its primary function is to recognize and base-pair with a complementary sequence of three nucleotides, known as the **codon**, found on the m-RNA. This interaction ensures that the correct amino acid (carried at the 3' end of the t-RNA) is incorporated into the growing polypeptide chain during translation. **Why other options are incorrect:** * **B. Messenger RNA (m-RNA):** m-RNA contains the **codon**, not the anticodon. It serves as the template that carries genetic information from DNA to the ribosome. * **C. Ribosomal RNA (r-RNA):** r-RNA is a structural and catalytic component of ribosomes. It facilitates the alignment of m-RNA and t-RNA and catalyzes peptide bond formation (peptidyl transferase activity) but does not contain anticodons. --- ### High-Yield Facts for NEET-PG: * **Wobble Hypothesis:** Proposed by Francis Crick, it states that the base pairing between the 3rd base of the codon and the 1st base of the anticodon is less stringent, allowing one t-RNA to recognize multiple codons. * **t-RNA Structure:** * **Secondary structure:** Cloverleaf model. * **Tertiary structure:** L-shaped. * **Acceptor Arm:** The 3' end of t-RNA always ends in the sequence **CCA**, where the amino acid attaches to the terminal Adenosine. * **DHU Loop:** Contains dihydrouridine; responsible for recognition by the enzyme *aminoacyl t-RNA synthetase*. * **TψC Loop:** Contains pseudouridine; involved in binding the t-RNA to the ribosomal surface.

Q: The human insulin gene receptor is located on which chromosome?

11. **Explanation:** The human **insulin gene (INS)** is located on the **short arm of chromosome 11 (11p15.5)**. This is a high-yield fact in medical biochemistry, as insulin is the primary hormone regulating glucose homeostasis. It is synthesized by the beta cells of the pancreatic islets of Langerhans as preproinsulin, which is then processed into proinsulin and finally active insulin and C-peptide. **Analysis of Options:** * **A. Chromosome 11 (Correct):** Houses the insulin gene (*INS*). Additionally, the gene for the **beta-globin chain** of hemoglobin is also located on chromosome 11, making it a "hotspot" for metabolic and hematologic disorders. * **B. Chromosome 15:** Associated with conditions like Marfan syndrome (Fibrillin-1 gene), Prader-Willi, and Angelman syndromes. * **C. Chromosome 19:** This is a common distractor. While the insulin gene is on chromosome 11, the **Insulin Receptor gene (INSR)** is located on **chromosome 19 (19p13.2)**. Mutations here lead to severe insulin resistance syndromes like Donohue syndrome (Leprechaunism). * **D. Chromosome 21:** Famous for Down Syndrome (Trisomy 21) and the Amyloid Precursor Protein (APP) gene associated with Alzheimer’s disease. **NEET-PG High-Yield Pearls:** * **Insulin Gene:** Chromosome 11. * **Insulin Receptor Gene:** Chromosome 19. * **Glut-4:** The insulin-dependent glucose transporter is primarily found in skeletal muscle and adipose tissue. * **C-peptide:** Secreted in equimolar amounts with insulin; used as a marker for endogenous insulin production (distinguishes Type 1 DM from Type 2 DM or exogenous insulin surreptitious use).

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.

Question 1

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 2

True about RNA Polymerase is:

Accepted Answer

No proofreading activity

Answer

Requires primers

Answer

RNA dependent DNA Polymerase

Answer

Recognizes Shine-Dalgarno sequence

Question 3

A young boy presents with difficulty in rising from a sitting position, and is diagnosed with Duchenne's muscular dystrophy. Which statement is true regarding mutations in the promoter region of the dystrophin gene?

Accepted Answer

Initiation of transcription of the dystrophin gene would be affected.

Answer

Capping of the mRNA of the dystrophin gene would be affected.

Answer

Tailing of the mRNA of the dystrophin gene would be affected.

Answer

Premature termination of translation would occur.

Question 4

In which type of RNA is the anticodon found?

Accepted Answer

Transfer RNA (t-RNA)

Answer

Messenger RNA (m-RNA)

Answer

Ribosomal RNA (r-RNA)

Answer

None of the above

Question 5

Which enzymatic mutation is responsible for immortality of cancer cells?

Accepted Answer

Telomerase

Answer

DNA reverse transcriptase

Answer

RNA polymerase

Answer

DNA polymerase

Question 6

For karyotyping, the dividing cells are arrested by the addition of colchicines in which mitotic phase?

Accepted Answer

Metaphase

Answer

Telophase

Answer

Anaphase

Answer

Prophase

Question 7

The human insulin gene receptor is located on which chromosome?

Accepted Answer

11

Answer

15

Answer

19

Answer

21

Question 8

Which of the following statements about peptidyl transferase is true?

Accepted Answer

It is used in elongation and causes the attachment of the peptide chain to the A-site of tRNA.

Answer

It is used in elongation and causes the attachment of the peptide chain to the P-site of tRNA.

Answer

It is used in initiation and causes 43S complex formation.

Answer

It is used in initiation and causes 48S complex formation.

Question 9

Which of the following is true about telomerase?

Accepted Answer

Ribonucleoprotein

Answer

Transcriptional factor activation

Answer

Membrane receptor

Answer

DNA binding domain

Question 10

Which of the following toxins inhibits the peptidyl transferase?

Accepted Answer

Ricin

Answer

Diphtheria toxin

Answer

Pertussis toxin

Answer

Amanitin

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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