Molecular Biology and Genomics Practice Questions

Q: What is the macromolecular complex that associates with introns during mRNA splicing?

Spliceosome. **Explanation:** The correct answer is **Spliceosome**. mRNA splicing is a critical post-transcriptional modification in eukaryotes where non-coding sequences (**introns**) are removed and coding sequences (**exons**) are joined together to form mature mRNA. 1. **Why Spliceosome is correct:** The spliceosome is a large macromolecular complex composed of **snRNPs** (small nuclear ribonucleoproteins, pronounced "snurps")—specifically U1, U2, U4, U5, and U6—and additional protein factors. These snRNPs recognize specific consensus sequences at the 5' splice site (GU) and 3' splice site (AG) to facilitate the transesterification reactions required for splicing. 2. **Why other options are incorrect:** * **Dicer:** An enzyme involved in the **RNA interference (RNAi)** pathway. It cleaves double-stranded RNA or pre-miRNA into short fragments (siRNA/miRNA) to silence gene expression. * **Nuclear body:** These are non-membrane-bound structures within the nucleus (e.g., Cajal bodies, Nucleoli) that serve various organizational functions but are not the specific machinery for intron removal. * **Splicer:** This is a distractor term and not a recognized biological complex in molecular biology. **Clinical Pearls for NEET-PG:** * **Systemic Lupus Erythematosus (SLE):** Patients often develop **anti-Smith (anti-Sm) antibodies**, which are directed against the proteins in snRNPs. This is a highly specific diagnostic marker for SLE. * **Spinal Muscular Atrophy (SMA):** Caused by a defect in the SMN1 gene, leading to impaired assembly of snRNPs. * **Beta-Thalassemia:** Some forms are caused by mutations at splice sites, leading to improper intron removal and non-functional hemoglobin chains.

Q: What is the attachment site of miRNA to inhibit gene expression?

3' end of mRNA. **Explanation:** MicroRNAs (miRNAs) are small, non-coding RNA molecules (typically 21–25 nucleotides long) that play a crucial role in post-transcriptional gene regulation. **Why Option A is Correct:** The primary mechanism of miRNA action involves binding to the **3' Untranslated Region (3' UTR)** of the target mRNA. This binding occurs via base-pairing between the "seed sequence" (nucleotides 2–7) of the miRNA and the complementary sequence on the 3' end of the mRNA. Once attached, the miRNA-induced silencing complex (miRISC) inhibits translation or promotes the degradation of the mRNA, thereby "silencing" the gene. **Why Other Options are Incorrect:** * **Option B (5' end of mRNA):** While the 5' end is involved in translation initiation, it is not the standard physiological target site for miRNA-mediated inhibition. * **Option C (Sigma factor):** Sigma factors are prokaryotic transcription factors required for RNA polymerase binding to promoters. miRNAs are eukaryotic regulatory elements and do not interact with sigma factors. * **Option D (Cap binding protein):** Proteins like eIF4E bind to the 5' cap to initiate translation. While miRNAs may interfere with the function of these proteins indirectly, they do not physically attach to them to inhibit gene expression. **High-Yield NEET-PG Pearls:** * **Biogenesis:** miRNAs are processed by enzymes **Drosha** (in the nucleus) and **Dicer** (in the cytosol). * **Mechanism:** If the complementarity is perfect, mRNA is cleaved; if it is partial (common in humans), translation is repressed. * **Clinical Link:** Dysregulation of miRNAs is linked to various cancers (acting as "oncomiRs") and cardiovascular diseases. * **RNA Interference (RNAi):** Unlike siRNA (which is exogenous and double-stranded), miRNA is endogenous and single-stranded.

Q: A 2-month-old, breast-fed baby presents with gastrointestinal problems and cirrhosis of the liver. Molecular analysis indicates a normal amount of galactose-1-phosphate uridyl transferase (GALT) mRNA, but no observable enzyme activity. What is the most likely explanation for this observation?

Nonsense mutation. ### Explanation The clinical presentation of gastrointestinal distress and cirrhosis in a 2-month-old infant suggests **Classic Galactosemia**, typically caused by a deficiency of **Galactose-1-phosphate uridyl transferase (GALT)**. **Why Nonsense Mutation is Correct:** The key to this question lies in the molecular findings: **normal mRNA levels but zero enzyme activity.** * A **nonsense mutation** involves a single nucleotide substitution that creates a premature stop codon (UAG, UAA, or UGA) within the coding region. * The transcription process remains unaffected, resulting in a **normal amount of mRNA**. * However, during translation, the ribosome dissociates early, leading to a truncated, non-functional protein that is often rapidly degraded. This explains the complete absence of enzyme activity despite the presence of mRNA. **Why the Other Options are Incorrect:** * **A. Gene Deletion:** If the gene were deleted, there would be no template for transcription, resulting in an **absence of mRNA**. * **C. Premature Transcription Termination:** This occurs at the DNA level (e.g., a mutation creating a poly-A signal too early). This would result in **truncated/abnormal mRNA**, not a "normal amount" of full-length mRNA. * **D. Promoter Mutation:** The promoter is responsible for the initiation of transcription. A mutation here would typically lead to **decreased or absent mRNA synthesis**. **Clinical Pearls for NEET-PG:** * **Classic Galactosemia (Type I):** Deficiency of GALT. Symptoms appear once breastfeeding starts (lactose = glucose + galactose). * **Key Findings:** Oil-drop cataracts, hepatosplenomegaly, jaundice, and *E. coli* sepsis. * **Diagnosis:** Reducing sugars in urine (Benedict's test positive) but negative glucose oxidase test. * **Management:** Immediate withdrawal of milk; switch to soy-based or lactose-free formula.

Q: Which step in prokaryotic translation is inhibited by Tetracycline?

Binding of aminoacyl-tRNA to the A site on the ribosome. ### Explanation **Mechanism of Action (The Correct Answer):** Tetracyclines (e.g., Doxycycline, Minocycline) are bacteriostatic antibiotics that inhibit protein synthesis by binding reversibly to the **30S subunit** of the bacterial ribosome. Specifically, they block the **A site (aminoacyl site)**, preventing the attachment of the incoming **aminoacyl-tRNA** to the mRNA-ribosome complex. Since the tRNA cannot bind, no new amino acids can be added to the growing polypeptide chain, effectively halting translation. **Analysis of Incorrect Options:** * **A. Initiation:** This step involves the assembly of the 30S and 50S subunits with mRNA and fMet-tRNA. While Aminoglycosides (like Streptomycin) interfere with initiation by causing mRNA misreading, Tetracyclines act after the initiation complex is formed. * **C. Peptide-bond formation:** This step is catalyzed by peptidyl transferase (part of the 50S subunit). It is specifically inhibited by **Chloramphenicol**. * **D. Translocation:** This involves the movement of the ribosome along the mRNA. This step is inhibited by **Macrolides** (e.g., Erythromycin) and **Clindamycin**, which bind to the 50S subunit. **High-Yield Clinical Pearls for NEET-PG:** * **Resistance:** Primarily occurs via **efflux pumps** (encoded by *tet* genes) that push the drug out of the bacterial cell. * **Selectivity:** They are selective for bacteria because eukaryotic cells lack the specific energy-dependent transport system required for tetracycline uptake. * **Side Effects:** Look for "teeth discoloration" and "bone growth retardation" (due to calcium chelation) and "photosensitivity." * **Mnemonic:** **"Buy AT 30, CELL at 50"** * **30S inhibitors:** **A**minoglycosides, **T**etracyclines. * **50S inhibitors:** **C**hloramphenicol, **E**rythromycin (Macrolides), **L**inezolid, **L**incosamides (Clindamycin).

Q: What is the name of the enzyme that synthesizes a double-stranded DNA copy from a single-stranded RNA template molecule?

Reverse transcriptase. **Explanation:** The correct answer is **Reverse transcriptase**. This enzyme is an **RNA-dependent DNA polymerase**. In the standard "Central Dogma" of molecular biology, genetic information flows from DNA to RNA. However, reverse transcriptase reverses this process by using a single-stranded RNA template to synthesize a complementary DNA (cDNA) strand, which then becomes double-stranded DNA. **Analysis of Options:** * **DNA polymerase:** These are DNA-dependent DNA polymerases. They synthesize a new DNA strand using an existing **DNA** template during replication and repair. * **RNA polymerase:** These are DNA-dependent RNA polymerases. They synthesize RNA from a **DNA** template during the process of transcription. * **Phosphokinase (Kinases):** These enzymes are involved in signal transduction and metabolism; they catalyze the transfer of a phosphate group from high-energy donor molecules (like ATP) to specific substrates. **Clinical Pearls for NEET-PG:** * **Retroviruses:** Reverse transcriptase is a hallmark of retroviruses like **HIV**. It allows the viral RNA genome to integrate into the host's DNA as a "provirus." * **Pharmacology Link:** Nucleoside Reverse Transcriptase Inhibitors (**NRTIs**) like Zidovudine and Abacavir, and **NNRTIs** like Efavirenz, target this specific enzyme to treat HIV. * **Biotechnology:** In the lab, reverse transcriptase is essential for **RT-PCR** (Reverse Transcription Polymerase Chain Reaction), used to detect RNA viruses (e.g., SARS-CoV-2) or measure gene expression. * **Telomerase:** This enzyme, which maintains chromosomal ends, is a specialized type of reverse transcriptase that carries its own RNA template.

Q: What was the first successful somatic gene therapy performed for?

Severe Combined Immunodeficiency (SCID). ### Explanation **Correct Answer: A. Severe Combined Immunodeficiency (SCID)** The first successful somatic gene therapy was performed in **1990** at the National Institutes of Health (NIH) on a four-year-old girl, Ashanti DeSilva, suffering from **Adenosine Deaminase (ADA) deficiency**, a form of SCID. **Medical Concept:** ADA deficiency leads to the accumulation of deoxyadenosine, which is toxic to T and B lymphocytes, causing profound immunodeficiency. In this landmark procedure, white blood cells (T-lymphocytes) were extracted from the patient, a functional copy of the ADA gene was inserted using a **retroviral vector**, and the genetically corrected cells were infused back into the patient. This restored her immune function significantly. **Analysis of Incorrect Options:** * **B. Phenylketonuria (PKU):** While PKU is a classic target for gene therapy research (targeting the liver), successful clinical application lagged behind SCID due to the complexity of targeting hepatocytes effectively. * **C. Thalassemia:** Gene therapy for β-thalassemia (targeting hematopoietic stem cells) has seen recent success (e.g., Zynteglo), but these breakthroughs occurred decades after the initial SCID trials. * **D. Cystic Fibrosis:** Early trials in the 1990s using adenovirus vectors to deliver the CFTR gene to lung epithelium were largely unsuccessful due to transient expression and inflammatory immune responses. **High-Yield Clinical Pearls for NEET-PG:** * **Vector used:** Retroviruses are commonly used for SCID gene therapy because they integrate the therapeutic gene into the host genome. * **ADA Deficiency:** It is the second most common cause of SCID (the most common is X-linked SCID due to IL-2 receptor γ-chain mutation). * **Biochemical marker:** Look for increased **dATP** levels in RBCs, which inhibits ribonucleotide reductase and halts DNA synthesis.

Q: All of the following are true about DNA methylation except:

It has no role in carcinogenesis.. **Explanation** DNA methylation is a key epigenetic mechanism that involves the addition of a methyl group (–CH₃) to DNA, typically at the 5th carbon of a cytosine ring. **Why Option D is the correct answer (The Exception):** DNA methylation plays a **critical role in carcinogenesis**. Aberrant methylation patterns are hallmarks of cancer: * **Hypermethylation** of promoter regions in tumor suppressor genes (e.g., *p16*, *BRCA1*) leads to gene silencing, allowing uncontrolled cell growth. * **Global Hypomethylation** can lead to genomic instability and the activation of oncogenes. Therefore, stating it has "no role" is factually incorrect. **Analysis of Incorrect Options:** * **Option A:** Methylation primarily occurs at **cytosine residues** within **CpG islands** (regions with a high frequency of Cytosine-guanine phosphodiester bonds). This is catalyzed by DNA methyltransferases (DNMTs). * **Option B:** It is a major regulator of **gene expression**. Generally, high levels of methylation in promoter regions correlate with **transcriptional silencing** (gene "switching off"). * **Option C:** It is the fundamental mechanism behind **genomic imprinting**, where certain genes are expressed in a parent-of-origin-specific manner (e.g., Prader-Willi and Angelman syndromes). **High-Yield Clinical Pearls for NEET-PG:** * **S-Adenosylmethionine (SAM):** The universal methyl donor for DNA methylation. * **5-Azacytidine:** A hypomethylating agent used in the treatment of Myelodysplastic Syndrome (MDS). * **Maintenance vs. De Novo:** DNMT1 maintains methylation patterns during replication, while DNMT3a/3b are responsible for new (de novo) methylation. * **Gene Silencing:** Remember: **Methylation = Mute**; **Acetylation = Active** (referring to histones).

Q: What is an Okazaki fragment?

A DNA fragment with an RNA primer. ### Explanation **Correct Option: C (A DNA fragment with an RNA primer)** **Concept:** DNA replication is **semidiscontinuous**. While the leading strand is synthesized continuously, the **lagging strand** is synthesized in short, discontinuous segments known as **Okazaki fragments**. DNA polymerase requires a free 3'-OH group to initiate synthesis, which is provided by an **RNA primer** (synthesized by the enzyme Primase). Therefore, each Okazaki fragment begins with a short RNA primer (approx. 10 nucleotides) followed by a stretch of newly synthesized DNA (approx. 100–200 nucleotides in eukaryotes). Eventually, the RNA primers are removed by DNA Polymerase I (in prokaryotes) or RNase H/FEN1 (in eukaryotes) and replaced with DNA. **Why other options are incorrect:** * **Option A & B:** These are incomplete. An Okazaki fragment is a hybrid molecule during the replication process; it is not purely DNA or purely RNA until the maturation phase. * **Option D:** This is biologically reversed. Primers are always RNA (in natural replication), and the elongation is always DNA. --- ### High-Yield Clinical Pearls for NEET-PG: * **Directionality:** Okazaki fragments are synthesized in the **5' → 3' direction**, even though the lagging strand overall grows in the 3' → 5' direction relative to the replication fork. * **DNA Ligase:** This enzyme is responsible for joining Okazaki fragments by forming phosphodiester bonds. It is the "glue" of the lagging strand. * **Clinical Correlation:** Deficiencies in enzymes involved in Okazaki fragment processing (like **DNA Ligase I**) can lead to clinical conditions like **Bloom Syndrome** (characterized by genomic instability and sister chromatid exchanges). * **Length:** Okazaki fragments are significantly shorter in eukaryotes (100–200 bp) compared to prokaryotes (1000–2000 bp).

Question 1

What is the macromolecular complex that associates with introns during mRNA splicing?

Accepted Answer

Spliceosome

Answer

Splicer

Answer

Dicer

Answer

Nuclear body

Question 2

What is the attachment site of miRNA to inhibit gene expression?

Accepted Answer

3' end of mRNA

Answer

5' end of mRNA

Answer

Binding to sigma factor

Answer

Cap binding protein

Question 3

A 2-month-old, breast-fed baby presents with gastrointestinal problems and cirrhosis of the liver. Molecular analysis indicates a normal amount of galactose-1-phosphate uridyl transferase (GALT) mRNA, but no observable enzyme activity. What is the most likely explanation for this observation?

Accepted Answer

Nonsense mutation

Answer

Gene deletion

Answer

Premature transcription termination sequence in the DNA

Answer

Promoter mutation

Question 4

Which step in prokaryotic translation is inhibited by Tetracycline?

Accepted Answer

Binding of aminoacyl-tRNA to the A site on the ribosome

Answer

Initiation

Answer

Peptide-bond formation

Answer

Translocation

Question 5

What is the name of the enzyme that synthesizes a double-stranded DNA copy from a single-stranded RNA template molecule?

Accepted Answer

Reverse transcriptase

Answer

DNA polymerase

Answer

RNA polymerase

Answer

Phosphokinase

Question 6

Which of the following statements is true regarding the lac operon?

Accepted Answer

Catabolite repression is mediated by CRP.

Answer

The repressor is a dimer and a positive regulator.

Answer

CRP is a gratuitous inducer.

Answer

Lactose is a positive regulator.

Question 7

What was the first successful somatic gene therapy performed for?

Accepted Answer

Severe Combined Immunodeficiency (SCID)

Answer

Phenylketonuria

Answer

Thalassemia

Answer

Cystic fibrosis

Question 8

All of the following are true about DNA methylation except:

Accepted Answer

It has no role in carcinogenesis.

Answer

It usually occurs at cytosine residues.

Answer

It can alter gene expression patterns in cells.

Answer

It plays a role in genomic imprinting.

Question 9

What technique is used to identify the exact location of a genetic loci?

Accepted Answer

Fluorescent in-situ hybridization

Answer

Polymerase chain reaction

Answer

Chromosome painting

Answer

Comparative genomic hybridization

Question 10

What is an Okazaki fragment?

Accepted Answer

A DNA fragment with an RNA primer

Answer

A DNA fragment

Answer

An RNA fragment

Answer

An RNA fragment with a DNA primer

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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