Molecular Biology and Genomics Practice Questions

Q: Which of the following is NOT true about post-transcriptional modification of RNA?

Glycosylation. **Explanation:** Post-transcriptional modifications are chemical alterations that convert a primary RNA transcript (hnRNA) into a functional mature mRNA molecule. This process occurs exclusively in the **nucleus** of eukaryotes. **Why Glycosylation is the correct answer:** **Glycosylation** is a **post-translational modification**, not post-transcriptional. It involves the addition of carbohydrate chains to proteins (forming glycoproteins) and occurs in the **Endoplasmic Reticulum (ER) and Golgi apparatus**. It is essential for protein folding, stability, and cell signaling, but it has no role in RNA processing. **Why the other options are incorrect (Post-transcriptional processes):** * **5' Capping (Option A):** Addition of 7-methylguanosine to the 5' end via a 5'-5' triphosphate linkage. It protects mRNA from exonucleases and is vital for translation initiation. * **Addition of poly A tail (Option B):** Polyadenylation involves adding ~200 adenine residues to the 3' end by the enzyme Poly(A) Polymerase. This regulates mRNA stability and nuclear export. * **Removal of introns (Option C):** Splicing removes non-coding sequences (introns) and joins coding sequences (exons) together, catalyzed by the spliceosome (snRNPs). **High-Yield Clinical Pearls for NEET-PG:** * **RNA Editing:** Another form of post-transcriptional modification (e.g., ApoB-100 in liver vs. ApoB-48 in intestine via C-to-U editing). * **Splicing Inhibitors:** Antibodies against snRNPs (Anti-Smith antibodies) are highly specific for **Systemic Lupus Erythematosus (SLE)**. * **Mushroom Poisoning:** $\alpha$-amanitin (from *Amanita phalloides*) inhibits **RNA Polymerase II**, blocking mRNA synthesis.

Q: What are nucleosomes composed of?

DNA and Histones. ### Explanation **1. Why the Correct Answer is Right:** A **nucleosome** is the fundamental structural unit of chromatin, often described as "beads on a string." It consists of a segment of **DNA** (approximately 147 base pairs) wrapped 1.65 times around a protein core called the **histone octamer**. This octamer is composed of two copies each of four core histones: **H2A, H2B, H3, and H4**. The DNA is negatively charged (due to phosphate groups), while histones are positively charged (rich in Lysine and Arginine), allowing for a tight electrostatic bond that facilitates DNA packaging within the nucleus. **2. Why the Other Options are Wrong:** * **Option A & C:** RNA is not a structural component of the nucleosome core. While RNA is involved in transcription and regulation, the nucleosome is strictly a DNA-protein complex. * **Option D:** Although RNA may be present in the overall chromatin environment during transcription, it is not a constituent of the nucleosome unit itself. **3. NEET-PG High-Yield Facts & Clinical Pearls:** * **Linker Histone:** **Histone H1** is known as the "linker histone." It sits outside the nucleosome core and helps stabilize the 30-nm chromatin fiber. It is *not* part of the octamer. * **Epigenetics:** Acetylation of histones (by HATs) neutralizes their positive charge, relaxing chromatin (**Euchromatin**) and increasing transcription. Deacetylation (by HDACs) leads to condensed **Heterochromatin** and gene silencing. * **Drug Link:** Sodium valproate (anti-epileptic) acts as a Histone Deacetylase (HDAC) inhibitor. * **Amino Acid Richness:** Histones are exceptionally rich in **Lysine and Arginine**, which is a frequent MCQ point regarding their basic nature.

Q: The same amino acid is coded by multiple codons due to which phenomenon?

Degeneracy. ### Explanation **1. Why Degeneracy is Correct:** The genetic code consists of 64 possible codons (4³ combinations of A, U, G, C) but only 20 standard amino acids. **Degeneracy** (or redundancy) refers to the fact that a single amino acid can be specified by more than one codon. For example, Leucine is coded by six different codons. This phenomenon is primarily explained by the **Wobble Hypothesis**, which states that the base pairing at the third position of the codon is less stringent, allowing a single tRNA to recognize multiple codons. This provides a protective mechanism against silent mutations. **2. Why Other Options are Incorrect:** * **Frame-shift mutation:** This occurs when the addition or deletion of nucleotides (not in multiples of three) shifts the reading frame of the mRNA, usually resulting in a completely different protein sequence or a premature stop codon. * **Transcription:** This is the biological process of copying a segment of DNA into RNA by the enzyme RNA polymerase. It is a step in gene expression, not a property of the genetic code. * **Mutation:** This is a general term for any permanent alteration in the DNA sequence. While mutations can lead to changes in codons, they do not define the inherent redundancy of the genetic code. **3. High-Yield Clinical Pearls for NEET-PG:** * **Universal Code:** The genetic code is nearly universal across all species, with minor exceptions in **Mitochondrial DNA** (e.g., UGA codes for Tryptophan instead of a Stop codon). * **Non-overlapping & Commaless:** The code is read sequentially without skipping bases or sharing nucleotides between adjacent codons. * **Initiation Codon:** **AUG** (Methionine) is the start codon. In prokaryotes, it codes for N-formylmethionine. * **Stop Codons (Nonsense Codons):** UAA (Ochre), UAG (Amber), and UGA (Opal). These do not code for any amino acid.

Q: In which phase of the cell cycle does proofreading occur?

S. **Explanation:** **Why S Phase is Correct:** DNA replication occurs exclusively during the **S (Synthesis) phase** of the cell cycle. Proofreading is an intrinsic function of **DNA Polymerases** (specifically Pol $\delta$ and Pol $\epsilon$ in eukaryotes), which possess **3' to 5' exonuclease activity**. This activity allows the enzyme to immediately identify, remove, and replace mismatched nucleotides as they are being incorporated. Since proofreading is a "real-time" correction mechanism coupled with DNA synthesis, it must occur during the S phase. **Why Other Options are Incorrect:** * **G1 Phase:** This is a pre-synthetic gap phase focused on cell growth and preparation for DNA replication. No new DNA is synthesized here, so proofreading does not occur. * **G2 Phase:** This is the post-synthetic gap phase. While **Mismatch Repair (MMR)** and other repair mechanisms (like homologous recombination) are active here to fix errors missed during replication, the specific process of "proofreading" by DNA polymerase has already concluded. * **M Phase:** This is the Mitosis phase where sister chromatids segregate. The DNA is highly condensed into chromosomes, making it inaccessible for replication-linked proofreading. **NEET-PG High-Yield Pearls:** * **Proofreading Direction:** Always **3' $\rightarrow$ 5' exonuclease activity**. * **Synthesis Direction:** Always **5' $\rightarrow$ 3' polymerase activity**. * **Clinical Correlation:** Defects in **Mismatch Repair (MMR)**—which acts as a "spell-check" after proofreading—lead to **Lynch Syndrome (HNPCC)**, characterized by microsatellite instability. * **Key Enzyme:** In eukaryotes, **DNA Polymerase $\gamma$** is responsible for proofreading mitochondrial DNA.

Q: Which of the following enzymes is responsible for the immortality of cancer cells?

Telomerase. ***Telomerase*** **Telomerase** is a **ribonucleoprotein enzyme** that adds repetitive DNA sequences (TTAGGG) to the ends of chromosomes (**telomeres**), counteracting the natural shortening that occurs during cell division. The activation of **telomerase** is characteristic of most cancer cells, granting them the ability to divide indefinitely, fulfilling the hallmark of **unlimited replicative potential** (immortality). *Topoisomerase* This enzyme is crucial for relieving the **torsional strain** (supercoiling) in the DNA helix that arises ahead of the replication fork or during transcription. Its primary role is managing DNA structure and integrity, not determining cellular lifespan or **immortality** through **telomere** maintenance. *Helicase* **Helicases** are motor proteins that use energy from ATP hydrolysis to **unwind nucleic acid duplexes** (like the DNA double helix) in fundamental processes such as DNA replication, repair, and transcription. While essential for replication, it does not prevent the gradual loss of terminal DNA sequences (**telomeres**) required for cancer cell immortality. *RNA polymerase* This enzyme is responsible for **transcription**, the process of synthesizing an **RNA molecule** from a DNA template. Its function is focused on gene expression (protein synthesis) and is not directly involved in maintaining the length of **telomeres** or conferring replicative immortality upon cells.

Q: In the liver, the Apo-B gene is completely translated to ApoB-100; in the intestine, it is translated to ApoB-48. Which of the following mechanism explains this?

RNA editing. ***RNA editing (Correct Answer)*** - This post-transcription modification involves a specific **cytidine deaminase** enzyme (APOBEC-1) found primarily in the intestine. - This enzyme converts a **CAA codon** (coding for Glutamine) into a **UAA stop codon** within the *ApoB* mRNA, truncating the protein from ApoB-100 to **ApoB-48**. *Gene splicing* - Gene splicing, including **alternative splicing**, involves differential removal of **introns** and joining of **exons** to create various mRNA transcripts from a single gene. - However, gene splicing does not involve the direct **nucleotide change** (C to U) necessary to create the premature stop codon responsible for shortening ApoB. *Alternative polyadenylation* - This process selects different cleavage sites towards the 3' end of the mRNA, influencing the length of the **3' untranslated region** and mRNA stability. - While it affects mRNA processing, it does not involve a **base conversion** that fundamentally alters the coding sequence by introducing a stop codon. *Gene rearrangements* - Gene rearrangements involve physical changes to the **genomic DNA** sequence itself (e.g., V(D)J recombination in immunoglobulins) and are typically irreversible. - The distinction between ApoB-100 and ApoB-48 is purely a **post-transcriptional** event changing the mRNA, not an alteration of the ApoB gene structure.

Question 1

Genetic material is transferred from one bacterium to another by which of the following mechanisms?

Accepted Answer

Transfection

Answer

Transduction

Answer

Transformation

Answer

Conjugation

Question 2

Which of the following is NOT true about post-transcriptional modification of RNA?

Accepted Answer

Glycosylation

Answer

5' Capping

Answer

Addition of poly A tail

Answer

Removal of introns

Question 3

A codon codes for a single amino acid. This characteristic is called?

Accepted Answer

Unambiguous

Answer

Non-overlapping

Answer

Non-punctate

Answer

Degeneracy

Question 4

What are nucleosomes composed of?

Accepted Answer

DNA and Histones

Answer

DNA and RNA

Answer

RNA and Histones

Answer

DNA, RNA, and Histones

Question 5

The same amino acid is coded by multiple codons due to which phenomenon?

Accepted Answer

Degeneracy

Answer

Frame-shift mutation

Answer

Transcription

Answer

Mutation

Question 6

Which enzyme is associated with the aging process related to DNA?

Accepted Answer

Telomerase

Answer

Telosomerase

Answer

Topoisomerase

Answer

DNA polymerase

Question 7

What is true about crossing over?

Accepted Answer

Occurs between non-sister chromatids of homologous chromosomes

Answer

Occurs during the diplotene stage

Answer

Occurs between sister chromatids of homologous chromosomes

Answer

Occurs between sister chromatids of non-homologous chromosomes

Question 8

In which phase of the cell cycle does proofreading occur?

Accepted Answer

S

Answer

G1

Answer

G2

Answer

M

Question 9

Which of the following enzymes is responsible for the immortality of cancer cells?

Accepted Answer

Telomerase

Answer

RNA polymerase

Answer

Helicase

Answer

Topoisomerase

Question 10

In the liver, the Apo-B gene is completely translated to ApoB-100; in the intestine, it is translated to ApoB-48. Which of the following mechanism explains this?

Accepted Answer

RNA editing

Answer

Gene splicing

Answer

Alternative polyadenylation

Answer

Gene rearrangements

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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