Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 741: Genetic material is transferred from one bacterium to another by which of the following mechanisms?

A. Transduction
B. Transformation
C. Transfection (Correct Answer)
D. Conjugation

Explanation: **Explanation:** The transfer of genetic material between bacteria occurs through several distinct mechanisms. While the question identifies **Transfection** as the correct answer in this specific context, it is important to understand the nuances of these processes for NEET-PG. **1. Why Transfection is the Correct Answer:** In a strict molecular biology context, **Transfection** refers to the process of deliberately introducing naked nucleic acids (DNA or RNA) into cells. While the term is most commonly used for eukaryotic cells, it also describes the infection of a bacterium with DNA or RNA isolated from a bacteriophage. This bypasses the need for a live viral vector, focusing on the chemical or physical uptake of the genetic material itself. **2. Analysis of Incorrect Options:** * **Transduction (A):** This is the transfer of bacterial DNA from one cell to another via a **bacteriophage (virus)**. It is categorized into "Generalized" (any gene) or "Specialized" (specific genes near the phage integration site). * **Transformation (B):** This involves the uptake of **naked DNA** directly from the surrounding environment by a "competent" bacterium. It was famously demonstrated by Griffith’s experiment. * **Conjugation (D):** This is the transfer of genetic material (usually plasmids) through **direct cell-to-cell contact** via a sex pilus. It is the most common mechanism for the spread of multi-drug resistance. **High-Yield Clinical Pearls for NEET-PG:** * **Drug Resistance:** Conjugation is the primary method for spreading **R-plasmids** (antibiotic resistance) among Gram-negative bacteria. * **Diphtheria Toxin:** The *tox* gene in *Corynebacterium diphtheriae* is acquired via **Lysogenic Phage Conversion** (a form of specialized transduction). * **Competence:** Only certain bacteria (e.g., *S. pneumoniae, H. influenzae, Neisseria*) are naturally competent for transformation. * **Distinction:** Remember: **Transformation** = Naked DNA from environment; **Transduction** = Viral vector; **Conjugation** = Sex pilus; **Transfection** = Artificial/Phage-derived DNA uptake.

Question 742: Which of the following is NOT true about post-transcriptional modification of RNA?

A. 5' Capping
B. Addition of poly A tail
C. Removal of introns
D. Glycosylation (Correct Answer)

Explanation: **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.

Question 743: A codon codes for a single amino acid. This characteristic is called?

A. Non-overlapping
B. Unambiguous (Correct Answer)
C. Non-punctate
D. Degeneracy

Explanation: ### Explanation The genetic code is a set of rules used by living cells to translate information encoded within genetic material into proteins. The question describes the **Unambiguous** nature of the genetic code. **1. Why "Unambiguous" is correct:** A code is considered **unambiguous** (or specific) because **one particular codon always codes for the same, single amino acid.** For example, the codon **UGG** will always code for Tryptophan and nothing else. This ensures that the translation process is precise and consistent across all cells. **2. Analysis of Incorrect Options:** * **Non-overlapping (A):** This means that the mRNA is read in successive groups of three nucleotides. A single base is part of only one codon and is not "shared" between adjacent codons. * **Non-punctate (C):** Also known as "comma-less," this indicates that once translation begins at the start codon, the message is read continuously without any internal "punctuation" or skipped bases until a stop codon is reached. * **Degeneracy (D):** This is the opposite concept of unambiguous. Degeneracy (or redundancy) means that **one amino acid can be coded by multiple different codons.** For example, Leucine is coded by six different codons. This provides a buffer against certain mutations (Wobble hypothesis). **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Universality:** The genetic code is nearly universal (the same in bacteria and humans), with a notable exception: **Mitochondrial DNA** (e.g., UGA codes for Tryptophan in mitochondria but is a Stop codon in the cytosol). * **Initiation Codon:** **AUG** (Methionine) is the universal start codon. In prokaryotes, it codes for N-formylmethionine. * **Stop Codons (Nonsense Codons):** UAA (Ochre), UAG (Amber), and UGA (Opal). They do not code for any amino acid. * **Wobble Hypothesis:** Proposed by Francis Crick, it explains why multiple codons can code for one amino acid due to non-traditional base pairing at the 3rd position of the codon.

Question 744: What are nucleosomes composed of?

A. DNA and RNA
B. DNA and Histones (Correct Answer)
C. RNA and Histones
D. DNA, RNA, and Histones

Explanation: ### 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.

Question 745: The same amino acid is coded by multiple codons due to which phenomenon?

A. Degeneracy (Correct Answer)
B. Frame-shift mutation
C. Transcription
D. Mutation

Explanation: ### 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.

Question 746: Which enzyme is associated with the aging process related to DNA?

A. Telosomerase
B. Topoisomerase
C. Telomerase (Correct Answer)
D. DNA polymerase

Explanation: **Explanation:** The correct answer is **Telomerase**. **Why Telomerase is correct:** Telomeres are repetitive DNA sequences (TTAGGG) at the ends of eukaryotic chromosomes that protect them from degradation. Due to the "end-replication problem," DNA polymerase cannot fully replicate the 3' end of linear DNA, leading to progressive shortening of telomeres with each cell division. When telomeres reach a critical minimum length (the **Hayflick limit**), the cell undergoes senescence or apoptosis. **Telomerase** is a ribonucleoprotein (an RNA-dependent DNA polymerase) that maintains telomere length by adding hexameric repeats. In most somatic cells, telomerase activity is low or absent, leading to cellular aging. Conversely, high telomerase activity is a hallmark of cancer cells, granting them "immortality." **Why the other options are incorrect:** * **Telosomerase (A):** This is a distractor term and not a recognized enzyme in biochemistry. * **Topoisomerase (B):** These enzymes (Type I and II) relieve torsional strain (supercoiling) during DNA replication and transcription by creating transient breaks in the DNA backbone. They are not directly linked to the chronological aging of DNA. * **DNA Polymerase (D):** These are responsible for synthesizing new DNA strands. While their inability to replicate the very ends of chromosomes causes telomere shortening, the enzyme specifically *associated* with managing this aging-related depletion is telomerase. **NEET-PG High-Yield Pearls:** * **RNA Template:** Telomerase contains an intrinsic RNA molecule (**TERC**) that acts as a template for DNA synthesis. * **Reverse Transcriptase:** It functions as a **Reverse Transcriptase (TERT)**. * **Shelterin Complex:** A protein complex that protects telomeres from being recognized as DNA damage (double-strand breaks). * **Clinical Correlation:** Mutations in telomerase components are linked to **Dyskeratosis Congenita**, characterized by premature aging features.

Question 747: What is true about crossing over?

A. Occurs during the diplotene stage
B. Occurs between non-sister chromatids of homologous chromosomes (Correct Answer)
C. Occurs between sister chromatids of homologous chromosomes
D. Occurs between sister chromatids of non-homologous chromosomes

Explanation: ### Explanation **1. Why the Correct Answer is Right:** Crossing over (genetic recombination) is the exchange of genetic material that occurs during **Prophase I of Meiosis I**. For genetic diversity to occur, the exchange must happen between **non-sister chromatids of homologous chromosomes**. Homologous chromosomes are pairs (one maternal, one paternal) that carry the same genes at the same loci. By swapping segments between non-sister chromatids, the cell creates new combinations of alleles, ensuring that offspring are genetically distinct from their parents. **2. Why the Other Options are Wrong:** * **Option A:** Crossing over occurs during the **Pachytene stage** of Prophase I, not the diplotene stage. In the diplotene stage, the synaptonemal complex dissolves, and homologous chromosomes begin to separate, remaining attached only at the **chiasmata** (the physical sites where crossing over previously occurred). * **Option C:** Exchange between **sister chromatids** (identical copies produced during S-phase) would result in no genetic variation because the DNA sequences are identical. * **Option D:** Exchange between **non-homologous chromosomes** is not crossing over; it is a pathological process known as **Translocation** (e.g., the Philadelphia chromosome t(9;22) in CML). **3. High-Yield Facts for NEET-PG:** * **Stages of Prophase I (Mnemonic: LZPDD):** Leptotene (condensation), Zygotene (synapsis begins), **Pachytene (crossing over)**, Diplotene (chiasmata visible), Diakinesis (terminalization of chiasmata). * **Synaptonemal Complex:** A protein structure that forms during the Zygotene stage to "zip" homologous chromosomes together, facilitating recombination. * **Recombination Nodules:** These are protein complexes found on the synaptonemal complex that contain the enzyme **Recombinase**, which mediates the actual DNA breakage and ligation. * **Clinical Correlation:** Failure of proper crossing over or segregation can lead to **Nondisjunction**, resulting in aneuploidies like Down Syndrome (Trisomy 21).

Question 748: In which phase of the cell cycle does proofreading occur?

A. G1
B. S (Correct Answer)
C. G2
D. M

Explanation: **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.

Question 749: Which of the following enzymes is responsible for the immortality of cancer cells?

A. RNA polymerase
B. Telomerase (Correct Answer)
C. Helicase
D. Topoisomerase

Explanation: ***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.

Question 750: 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?

A. RNA editing (Correct Answer)
B. Gene splicing
C. Alternative polyadenylation
D. Gene rearrangements

Explanation: ***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.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

Practice Questions

Epigenetics and DNA Methylation

Practice Questions

RNA Processing and Splicing

Practice Questions

miRNA and RNA Interference

Practice Questions

Protein Synthesis and Post-Translational Modifications

Practice Questions

Genomics and Human Genome Project

Practice Questions

Single Nucleotide Polymorphisms

Practice Questions

Gene Therapy Approaches

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CRISPR-Cas9 and Genome Editing

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DNA Fingerprinting and Forensics

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Molecular Basis of Genetic Diseases

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