Molecular Biology and Genomics Practice Questions

Q: Alcohol acts as a substrate inducer for which gene?

Pol III gene. **Explanation:** **Why Pol III gene is correct:** RNA Polymerase III (Pol III) is responsible for the transcription of small non-coding RNAs, including **tRNA and 5S rRNA**, which are essential for protein synthesis and cell growth. Alcohol (ethanol) acts as a potent substrate inducer for Pol III. It stimulates the activity of the transcription factor **Brf1** (a subunit of TFIIIB), which specifically recruits Pol III to its target genes. This induction leads to an increase in tRNA and 5S rRNA levels, promoting protein synthesis and contributing to the cellular hypertrophy and increased metabolic activity often seen in chronic alcohol consumption. **Why the other options are incorrect:** * **Myc Oncogene:** While Myc is a transcription factor that regulates cell proliferation and can influence Pol III indirectly, it is not directly induced by alcohol as a primary substrate mechanism in this context. * **P53:** Known as the "Guardian of the Genome," P53 is a tumor suppressor gene. Alcohol-induced oxidative stress may damage DNA and lead to P53 activation, but alcohol does not act as a specific substrate inducer for its transcription. * **NF1 (Neurofibromin 1):** This is a tumor suppressor gene associated with Neurofibromatosis type 1. It functions as a GTPase-activating protein (GAP) for Ras and has no direct regulatory relationship with alcohol induction. **High-Yield Clinical Pearls for NEET-PG:** * **RNA Pol I:** Transcribes 45S pre-rRNA (located in the nucleolus). * **RNA Pol II:** Transcribes mRNA, miRNA, and snRNA (inhibited by $\alpha$-amanitin). * **RNA Pol III:** Transcribes tRNA, 5S rRNA, and U6 snRNA. * **Alcohol & Cancer:** The induction of Pol III by alcohol is one proposed mechanism for why chronic alcohol consumption increases the risk of certain cancers (e.g., liver, upper aerodigestive tract), as elevated Pol III products drive uncontrolled cell growth.

Q: Which of the following inhibits transcription?

Actinomycin D. **Explanation:** The correct answer is **Actinomycin D**. This drug inhibits transcription by binding to the DNA template. It intercalates between adjacent **Guanine-Cytosine (G-C) base pairs**, creating a stable complex that physically obstructs the movement of RNA polymerase along the DNA strand, thereby preventing the synthesis of RNA in both prokaryotes and eukaryotes. **Analysis of Options:** * **Amanitin (specifically $\alpha$-amanitin):** While this toxin from the *Amanita phalloides* mushroom also inhibits transcription, it does so by specifically binding to and inhibiting **RNA Polymerase II** in eukaryotes. In many competitive exams, if both are present, Actinomycin D is the classic general inhibitor of transcription via DNA intercalation. * **Chloramphenicol:** This is an inhibitor of **translation** (protein synthesis). It binds to the **50S ribosomal subunit** in bacteria, preventing the action of peptidyl transferase. * **Streptomycin:** This is an aminoglycoside that inhibits **translation** initiation. It binds to the **30S ribosomal subunit**, causing misreading of mRNA and inhibiting the formation of the initiation complex. **High-Yield Clinical Pearls for NEET-PG:** * **Rifampicin:** Inhibits transcription by binding to the $\beta$-subunit of bacterial **DNA-dependent RNA polymerase**. (Used in TB). * **Actinomycin D (Dactinomycin):** Clinically used as a chemotherapy agent for Wilms tumor and Ewing sarcoma. * **Translation Inhibitors Mnemonic:** **"Buy AT 30, CELL at 50"** * **30S:** **A**minoglycosides, **T**etracyclines. * **50S:** **C**hloramphenicol, **E**rythromycin (Macrolides), **L**incomycin/Clindamycin, **L**inezolid.

Q: Which of the following are examples of non-coding RNAs?

All of the above. **Explanation:** The central dogma of molecular biology states that DNA is transcribed into RNA, which is then translated into proteins. However, only about 2% of the human genome codes for proteins (**mRNA**). The remaining majority consists of **non-coding RNAs (ncRNAs)**—functional RNA molecules that are transcribed from DNA but are not translated into proteins. 1. **tRNA (Transfer RNA):** These are classic examples of **housekeeping ncRNAs**. They act as adapter molecules that carry specific amino acids to the ribosome during translation. 2. **miRNA (microRNA):** These are small (approx. 22 nucleotides), single-stranded regulatory RNAs. They play a crucial role in **post-transcriptional gene silencing** by binding to target mRNAs, leading to their degradation or inhibition of translation. 3. **siRNA (Small Interfering RNA):** These are double-stranded regulatory RNAs involved in the **RNA interference (RNAi)** pathway. They are highly specific and are often used in research and therapeutics to "knock down" specific gene expression. Since all three options (tRNA, miRNA, and siRNA) function as RNA molecules without being translated into proteins, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **rRNA (Ribosomal RNA):** The most abundant type of RNA in the cell (80%). * **snRNA (Small Nuclear RNA):** Involved in **splicing** (removal of introns); deficiency is linked to Spinal Muscular Atrophy. * **OncomiRs:** miRNAs that are dysregulated in cancer (e.g., miR-21 is often overexpressed in tumors). * **RNA Polymerase III** is responsible for transcribing tRNA and 5S rRNA.

Q: What is the approximate number of different types of tRNA molecules present in human cells?

23. **Explanation:** The correct answer is **23** (Option A). This number refers specifically to the distinct types of tRNA molecules required to translate the genetic code in human **mitochondria**. 1. **Why 23 is correct:** While the standard genetic code consists of 61 sense codons, human cells do not require 61 different tRNAs due to the **Wobble Hypothesis**. In human mitochondria, the translation system is highly streamlined. There are exactly **22 types of mitochondrial tRNAs** (encoded by mtDNA) plus the **initiator tRNA**, totaling 23 functional types. This minimal set is sufficient because mitochondrial tRNAs exhibit "super-wobbling," where a single tRNA can recognize an entire four-codon family. (Note: In the human cytoplasm/nucleus, there are approximately 48–50 distinct tRNA isoacceptors, but in the context of standard medical examinations like NEET-PG, "23" is the classic high-yield figure associated with the mitochondrial genome's requirement). 2. **Why other options are incorrect:** * **Options B, C, and D (25, 28, 30):** These numbers do not correspond to any established biological constant in human genomics. They exceed the minimal requirement for mitochondrial translation and fall significantly short of the ~50 types found in the cytoplasm. **High-Yield Clinical Pearls for NEET-PG:** * **Wobble Hypothesis:** Proposed by Francis Crick; states that the base at the 5' end of the tRNA anticodon can form non-standard hydrogen bonds with the 3' base of the mRNA codon. * **Mitochondrial DNA (mtDNA):** It is circular, double-stranded, and encodes 13 polypeptides, 2 rRNAs, and **22 tRNAs**. * **Clinical Correlation:** Mutations in mitochondrial tRNA genes (e.g., *MT-TL1*) are linked to disorders like **MELAS** (Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes). * **Inosine:** Often found in the wobble position of tRNA, it can pair with A, U, or C.

Q: What enzyme is used for mapping hypersensitive sites in recombinant DNA research?

DNase I. **Explanation:** **DNase I (Deoxyribonuclease I)** is the correct answer because it is the specific endonuclease used to identify **DNase-hypersensitive sites (DHSs)** in chromatin. 1. **Mechanism:** In eukaryotic cells, DNA is tightly wrapped around histones. However, active regulatory regions (like promoters and enhancers) are "open" or nucleosome-free to allow transcription factor binding. These open regions are highly sensitive to cleavage by low concentrations of DNase I. By digesting chromatin with DNase I and performing Southern blotting or sequencing, researchers can map these hypersensitive sites to identify transcriptionally active areas of the genome. **Analysis of Incorrect Options:** * **DNA Ligase:** This enzyme functions as "molecular glue" that joins two DNA fragments by forming phosphodiester bonds. It is used in DNA replication and repair, not for mapping structural sensitivity. * **DNA Polymerase I:** Primarily involved in prokaryotic DNA replication and repair (filling gaps). In research, its "Klenow fragment" is used for DNA labeling or sequencing, but it cannot identify open chromatin sites. * **Polynucleotide Kinase (PNK):** This enzyme transfers a phosphate group from ATP to the 5' end of a DNA or RNA strand. It is used for end-labeling nucleic acids, not for structural mapping. **High-Yield Clinical Pearls for NEET-PG:** * **Hypersensitivity = Activity:** DNase I hypersensitive sites are hallmarks of **transcriptionally active DNA**. * **Heterochromatin vs. Euchromatin:** Heterochromatin (tightly packed) is DNase-resistant, while Euchromatin (loosely packed) contains DNase-hypersensitive sites. * **Diagnostic Use:** DNase I is also used clinically as a nebulized drug (**Dornase alfa**) in **Cystic Fibrosis** to digest the thick DNA-rich mucus in the lungs, reducing viscosity.

Q: What is the term for the difference in gene expression between siblings, inherited from the father?

Mosaicism. **Explanation:** The correct answer is **Mosaicism**. In the context of medical genetics, mosaicism refers to the presence of two or more populations of cells with different genotypes in one individual who has developed from a single fertilized egg. When a mutation occurs post-zygotically (after fertilization), it can lead to varying degrees of gene expression among offspring. Specifically, **Germline Mosaicism** occurs when a parent (in this case, the father) carries a mutation in their germ cells but not in their somatic cells. This explains why he may be phenotypically normal but can pass the mutation to multiple siblings, leading to different clinical expressions or recurrence of a disease in a family where no previous history exists. **Analysis of Incorrect Options:** * **Anticipation:** Refers to the phenomenon where a genetic disorder (typically triplet repeat expansions like Huntington’s or Fragile X) becomes more severe or appears at an earlier age with each succeeding generation. * **Mutation:** This is a broad term for any permanent change in the DNA sequence. While it is the underlying cause of genetic variation, it does not specifically describe the inheritance pattern between siblings from a single parent. * **Replication:** This is the biological process of producing two identical replicas of DNA from one original DNA molecule; it is a physiological process, not a pattern of inheritance or expression. **High-Yield Clinical Pearls for NEET-PG:** * **Germline Mosaicism** is a classic explanation for why healthy parents have multiple children with **Osteogenesis Imperfecta** or **Duchenne Muscular Dystrophy**. * **Lyonization** (X-inactivation) is a form of functional mosaicism in females. * **Confined Placental Mosaicism** can lead to discrepancies between CVS results and the actual fetal karyotype.

Q: Which statement is true regarding DNA replication?

All of the above.. **Explanation:** DNA replication is a fundamental biological process occurring during the **S-phase** of the cell cycle, ensuring that genetic information is accurately transmitted to daughter cells. 1. **Semi-conservative (Option A):** As proven by the Meselson-Stahl experiment, each daughter DNA molecule consists of one original "parental" strand and one newly synthesized "daughter" strand. This mechanism ensures high fidelity in genetic transmission. 2. **Sister Chromatid Formation (Option B):** Following DNA replication, the single chromosome consists of two identical DNA molecules known as sister chromatids, held together at the centromere. These are eventually separated during mitosis (Anaphase). 3. **Base Pairing Rules (Option C):** Replication relies on Chargaff’s rules of complementarity. DNA polymerase adds nucleotides based on the template strand: Adenine (A) pairs with Thymine (T) via two hydrogen bonds, and Guanine (G) pairs with Cytosine (C) via three hydrogen bonds. Since all three statements accurately describe the process, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** DNA synthesis always occurs in the **5' to 3' direction**. * **Enzymes:** **Helicase** unwinds the helix; **Topoisomerase (DNA Gyrase in prokaryotes)** relieves torsional strain. Fluoroquinolones (e.g., Ciprofloxacin) act by inhibiting DNA Gyrase. * **Telomerase:** An RNA-dependent DNA polymerase that maintains chromosomal ends. It is highly active in cancer cells and germ cells but absent in somatic cells. * **Inhibitors:** Drugs like **Cytarabine** and **5-Fluorouracil** target DNA synthesis and are crucial in chemotherapy.

Q: What is the effect of histone acetylation on chromatin structure?

Increased euchromatin formation. **Explanation:** **1. Why Option B is Correct:** Histone acetylation is a key epigenetic modification catalyzed by **Histone Acetyltransferases (HATs)**. Histones are rich in basic amino acids like Lysine and Arginine, giving them a strong positive charge that binds tightly to the negatively charged DNA phosphate backbone. * **Mechanism:** Acetylation adds an acetyl group to the lysine residues on histone tails, neutralizing their positive charge. * **Result:** This weakens the electrostatic attraction between histones and DNA, causing the chromatin to "relax" or decondense. This open, transcriptionally active state is known as **Euchromatin**, which allows RNA polymerase and transcription factors to access the DNA. **2. Why Other Options are Incorrect:** * **Option A:** **Histone Deacetylation** (via HDACs) or certain types of methylation lead to heterochromatin (tightly packed, inactive DNA). * **Option C:** Methylation typically occurs on **Cytosine** residues (not cystine) in CpG islands of DNA, which usually leads to gene silencing, not chromatin relaxation. * **Option D:** While chromatin must be remodeled for replication to occur, histone acetylation is specifically a regulatory mechanism for **transcription** rather than the process of DNA replication itself. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **HATs vs. HDACs:** HATs (Histone Acetyltransferases) make DNA "Active" (Euchromatin); HDACs (Histone Deacetylases) make DNA "Silent" (Heterochromatin). * **Drug Link:** **Valproic acid** (anti-epileptic) acts as an HDAC inhibitor, promoting an open chromatin state. * **Mnemonic:** **A**cetylation **A**ctivates transcription; **M**ethylation **M**utes DNA (usually). * **Huntington’s Disease:** Pathogenesis involves the inhibition of HATs, leading to decreased transcription of neuroprotective genes.

Question 1

Alcohol acts as a substrate inducer for which gene?

Accepted Answer

Pol III gene

Answer

Myc Oncogene

Answer

P53

Answer

NF1

Question 2

Which of the following inhibits transcription?

Accepted Answer

Actinomycin D

Answer

Amanitin

Answer

Chloramphenicol

Answer

Streptomycin

Question 3

Which of the following are examples of non-coding RNAs?

Accepted Answer

All of the above

Answer

siRNA

Answer

miRNA

Answer

tRNA

Question 4

What is the approximate number of different types of tRNA molecules present in human cells?

Accepted Answer

23

Answer

25

Answer

28

Answer

30

Question 5

What enzyme is used for mapping hypersensitive sites in recombinant DNA research?

Accepted Answer

DNase I

Answer

DNA ligase

Answer

DNA polymerase I

Answer

Polynucleotide kinase

Question 6

What is the term for the difference in gene expression between siblings, inherited from the father?

Accepted Answer

Mosaicism

Answer

Anticipation

Answer

Mutation

Answer

Replication

Question 7

Which of the following techniques is not used for the detection of specific aneuploidy?

Accepted Answer

Microarray

Answer

FISH

Answer

RT-PCR

Answer

QF-PCR

Question 8

Which antibiotic is structurally similar to aminoacyl tRNA and inhibits protein synthesis in both prokaryotes and eukaryotes?

Accepted Answer

Puromycin

Answer

Azithromycin

Answer

Tetracycline

Answer

Chloramphenicol

Question 9

Which statement is true regarding DNA replication?

Accepted Answer

All of the above.

Answer

It is semi-conservative.

Answer

Sister chromatids are formed.

Answer

It follows base pairing rules.

Question 10

What is the effect of histone acetylation on chromatin structure?

Accepted Answer

Increased euchromatin formation

Answer

Increased heterochromatin formation

Answer

Methylation of cystine

Answer

DNA replication

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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