Molecular Biology and Genomics — MCQs
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Which pattern of inheritance is characteristic of mitochondrial DNA?
The uvr ABC endonuclease is involved in which one of the following processes?
Which of the following enzymes transcribes 5S rRNA?
A 4-year-old girl presented with failure to thrive and megaloblastic anemia on peripheral blood smear. Despite Vitamin B12 and folate supplementation, her anemia did not improve. Enzyme assay from cultured PBMCs showed a deficiency of orotate phosphoribosyltransferase. What is the probable diagnosis?
DNA restriction fragments are separated by which method?
What are jumping genes also known as?
Apolipoprotein B48 and Apolipoprotein B100 are synthesized from the same mRNA. The difference between them is due to what mechanism?
How does RNA polymerase differ from DNA polymerase?
Nonsense codons are all of the following EXCEPT:
Which one of the following is the complementary sequence of 5'TTAAGCTAC3'?
Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs
Question 141: Which pattern of inheritance is characteristic of mitochondrial DNA?
- A. Father to son transmission
- B. Father to both son and daughter transmission
- C. Mother to daughter transmission
- D. Mother to both son and daughter transmission (Correct Answer)
Explanation: **Explanation:** **1. Why the correct answer is right:** Mitochondrial DNA (mtDNA) follows **Maternal Inheritance** (also known as Non-Mendelian or Extranuclear inheritance). During fertilization, the zygote receives almost all its cytoplasm and organelles from the **ovum**, as the sperm contributes only its nuclear genetic material. Since mitochondria are located in the cytoplasm, they are inherited exclusively from the mother. Therefore, an affected mother will transmit the mitochondrial trait or disease to **all her children** (both sons and daughters). **2. Why the incorrect options are wrong:** * **Options A & B:** These are incorrect because **paternal transmission** of mitochondrial DNA does not occur in humans. A father cannot pass a mitochondrial disease to his offspring because sperm mitochondria are generally degraded or excluded during fertilization. * **Option C:** This is incomplete. While a mother does pass mtDNA to her daughters, she also passes it to her sons. The inheritance is not sex-limited. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Heteroplasmy:** This is a key concept where a cell contains a mixture of both normal and mutated mtDNA. The severity of the disease depends on the ratio of mutant to normal mitochondria (Threshold effect). * **Replication:** mtDNA is circular, double-stranded, and lacks histones. It replicates independently of the cell cycle. * **Common Mitochondrial Diseases:** * **LHON** (Leber’s Hereditary Optic Neuropathy): Sudden painless loss of vision. * **MELAS** (Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like episodes). * **MERRF** (Myoclonic Epilepsy with Ragged Red Fibers). * **Muscle Biopsy:** Often shows "Ragged Red Fibers" on Gomori trichrome stain due to compensatory proliferation of mitochondria.
Question 142: The uvr ABC endonuclease is involved in which one of the following processes?
- A. DNA replication
- B. RNA splicing
- C. DNA repair (Correct Answer)
- D. DNA recombination
Explanation: **Explanation:** The **uvr ABC endonuclease** is a multi-enzyme complex in prokaryotes (like *E. coli*) that plays a central role in **Nucleotide Excision Repair (NER)**. This pathway is the primary mechanism for identifying and removing bulky DNA lesions, most notably **pyrimidine dimers** (thymine dimers) caused by Ultraviolet (UV) radiation. * **Why C is Correct:** The uvr ABC complex acts as an "excision nuclease" (exinuclease). **UvrA and UvrB** scan the DNA to detect distortions in the double helix. Once a lesion is found, **UvrC** (the endonuclease) cuts the damaged strand on both sides of the lesion. The damaged segment is then removed, and the gap is filled by DNA Polymerase I and sealed by DNA Ligase. * **Why A is Incorrect:** DNA replication involves enzymes like DNA Polymerases, Helicase, and Primase. While repair occurs on DNA, uvr ABC is specifically for damage correction, not genomic duplication. * **Why B is Incorrect:** RNA splicing is a post-transcriptional modification involving the removal of introns from pre-mRNA, mediated by the spliceosome (snRNPs). * **Why D is Incorrect:** DNA recombination (e.g., VDJ recombination or crossing over) involves the exchange of genetic material between strands, typically mediated by enzymes like RecA in bacteria. **High-Yield Clinical Pearls for NEET-PG:** * **Human Homolog:** In humans, the NER pathway is more complex but functionally similar. A genetic defect in the human equivalent of this repair system leads to **Xeroderma Pigmentosum (XP)**. * **Clinical Presentation of XP:** Patients exhibit extreme photosensitivity, severe sunburns, and a 1000-fold increased risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma) due to the inability to repair UV-induced damage. * **Key Step:** Remember that NER removes a **patch** of nucleotides (an oligonucleotide), whereas Base Excision Repair (BER) removes only a single damaged base.
Question 143: Which of the following enzymes transcribes 5S rRNA?
- A. RNA Polymerase I
- B. RNA Polymerase II
- C. RNA Polymerase III (Correct Answer)
- D. None of the above
Explanation: In eukaryotic cells, three distinct types of RNA polymerases (I, II, and III) are responsible for transcribing different classes of genes. **Correct Answer: RNA Polymerase III** RNA Polymerase III is responsible for transcribing small, stable RNAs. Its primary products include **5S rRNA**, **tRNA**, and **U6 snRNA**. It is unique because it is the only polymerase that transcribes a ribosomal RNA component (5S) outside of the nucleolus. **Analysis of Incorrect Options:** * **RNA Polymerase I:** This enzyme is located in the nucleolus and transcribes a single precursor (45S pre-rRNA), which is then processed into the **28S, 18S, and 5.8S rRNAs**. It does *not* transcribe 5S rRNA. * **RNA Polymerase II:** This is the most versatile polymerase. It transcribes **mRNA** (protein-coding genes), most **snRNA** (involved in splicing), and **microRNA** (miRNA). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (RMT):** Remember the order of products for Pol I, II, and III as **R-M-T** (**R**ibosomal, **M**essenger, **T**ransfer). * **Alpha-Amanitin Sensitivity:** This toxin (from *Amanita phalloides* mushrooms) inhibits RNA Polymerase II most strongly, followed by Pol III. Pol I is generally resistant. * **Mitochondrial RNA Polymerase:** Mitochondria have their own single RNA polymerase that resembles bacterial RNA polymerase. * **Location:** Pol I is the only one active in the **nucleolus**; Pol II and III function in the nucleoplasm.
Question 144: A 4-year-old girl presented with failure to thrive and megaloblastic anemia on peripheral blood smear. Despite Vitamin B12 and folate supplementation, her anemia did not improve. Enzyme assay from cultured PBMCs showed a deficiency of orotate phosphoribosyltransferase. What is the probable diagnosis?
- A. Orotate deficiency
- B. Gout
- C. Orotic aciduria (Correct Answer)
- D. Severe combined immunodeficiency (SCID)
Explanation: **Explanation:** The clinical presentation of **failure to thrive** and **refractory megaloblastic anemia** (anemia that does not respond to Vitamin B12 or folate) in a child is a classic hallmark of **Hereditary Orotic Aciduria**. **1. Why Orotic Aciduria is correct:** This is an autosomal recessive disorder of **pyrimidine synthesis**. It is caused by a deficiency in the bifunctional enzyme **UMP Synthase**, which possesses two activities: **Orotate phosphoribosyltransferase (OPRT)** and **Orotidylate decarboxylase (ODC)**. * **Mechanism:** The deficiency leads to an accumulation of orotic acid (excreted in urine) and a failure to produce UMP. * **Megaloblastic Anemia:** Without UMP, the body cannot produce pyrimidine nucleotides (CTP, TTP) required for DNA synthesis in erythroblasts, leading to megaloblastic changes. * **Treatment:** Administration of **Uridine** bypasses the metabolic block, providing a source for pyrimidine synthesis and feedback-inhibiting the pathway to reduce orotic acid production. **2. Why incorrect options are wrong:** * **Orotate deficiency:** This is the opposite of the pathology; there is an *excess* of orotic acid due to the metabolic block. * **Gout:** This involves disorders of **purine metabolism** (hyperuricemia), not pyrimidines, and typically presents with joint inflammation rather than megaloblastic anemia. * **SCID:** While Adenosine Deaminase (ADA) deficiency (a cause of SCID) involves nucleotide metabolism, it primarily affects lymphocyte development and presents with recurrent infections, not isolated refractory megaloblastic anemia. **Clinical Pearls for NEET-PG:** * **Differentiating Factor:** To distinguish Orotic Aciduria from **Ornithine Transcarbamylase (OTC) deficiency** (Urea cycle), look for **Ammonia levels**. OTC deficiency has high orotic acid + **Hyperammonemia**, whereas Hereditary Orotic Aciduria has high orotic acid + **Megaloblastic Anemia** (normal ammonia). * **Enzyme:** UMP Synthase is a single polypeptide with two catalytic domains (OPRT and ODC).
Question 145: DNA restriction fragments are separated by which method?
- A. Paper chromatography
- B. Agarose gel electrophoresis (Correct Answer)
- C. Thin-layer chromatography
- D. Ultracentrifugation
Explanation: **Explanation:** **Why Agarose Gel Electrophoresis is correct:** DNA molecules are negatively charged due to their phosphate backbone. In **Agarose Gel Electrophoresis**, DNA fragments are placed in a porous gel matrix and subjected to an electric field. The fragments migrate toward the positive electrode (anode). The gel acts as a molecular sieve; smaller fragments move faster and further through the pores, while larger fragments move slower. This allows for the precise separation of DNA restriction fragments based on their **molecular size (length in base pairs).** **Why the other options are incorrect:** * **Paper and Thin-layer Chromatography (A & C):** These techniques separate molecules (like amino acids or lipids) based on their differential solubility in a solvent and affinity for a stationary phase. They are not suitable for large, charged macromolecules like DNA. * **Ultracentrifugation (D):** This method separates particles based on their **density** or sedimentation rate. While used in the classic Meselson-Stahl experiment to distinguish between DNA isotopes ($^{14}N$ and $^{15}N$), it is not the standard method for separating restriction fragments by size. **High-Yield Clinical Pearls for NEET-PG:** * **Visualization:** DNA bands in the gel are typically visualized using **Ethidium Bromide (EtBr)**, which intercalates between bases and fluoresces orange under UV light. * **Pulsed-Field Gel Electrophoresis (PFGE):** A variation used to separate very large DNA fragments (e.g., whole chromosomes). * **Southern Blotting:** After electrophoresis, DNA is transferred to a membrane for hybridization; this is the gold standard for detecting specific DNA sequences (e.g., in Sickle Cell Anemia diagnosis). * **Polyacrylamide Gel Electrophoresis (PAGE):** Used for separating very small DNA fragments (differing by only 1 base pair), commonly used in DNA sequencing.
Question 146: What are jumping genes also known as?
- A. Intron
- B. Transposons (Correct Answer)
- C. Plasmids
- D. Exon
Explanation: **Explanation:** **Transposons** (Option B) are DNA sequences that can move from one location to another within the genome. Discovered by Barbara McClintock, they are colloquially termed **"jumping genes."** They move via a "cut-and-paste" or "copy-and-paste" mechanism using the enzyme **transposase**. In humans, they play a role in genetic diversity but can also cause mutations if they insert into functional genes. **Why other options are incorrect:** * **Introns (Option A):** These are non-coding sequences within a gene that are removed during RNA splicing. They do not move between genomic locations. * **Plasmids (Option C):** These are extrachromosomal, circular DNA molecules found primarily in bacteria. While they can transfer between cells (conjugation), they are not defined as "jumping genes" within a single genome. * **Exons (Option D):** These are the coding regions of a gene that remain in the mature mRNA and are translated into proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Medical Significance:** Transposons are a major mechanism for the spread of **antibiotic resistance** genes (e.g., in *Staphylococcus aureus*). * **Retrotransposons:** A subtype that moves via an RNA intermediate using **reverse transcriptase** (e.g., LINEs and SINEs/Alu elements in humans). * **Clinical Correlation:** Insertional mutagenesis by transposons is linked to diseases like **Hemophilia A** (L1 insertion in Factor VIII gene) and certain cancers. * **Barbara McClintock** received the Nobel Prize for this discovery in 1983.
Question 147: Apolipoprotein B48 and Apolipoprotein B100 are synthesized from the same mRNA. The difference between them is due to what mechanism?
- A. RNA splicing
- B. Allelic exclusion
- C. Deamination of cytidine to uridine (Correct Answer)
- D. Upstream repression
Explanation: ### Explanation The correct answer is **C. Deamination of cytidine to uridine.** This process is a classic example of **RNA Editing**, a post-transcriptional modification where the nucleotide sequence of the mRNA is altered after transcription but before translation. **Mechanism:** Both Apo B100 and Apo B48 are encoded by the same *APOB* gene. 1. **In the Liver:** The mRNA remains unedited, leading to the synthesis of the full-length **Apo B100** (used in VLDL and LDL). 2. **In the Intestine:** An enzyme called **Cytidine Deaminase** (specifically APOBEC-1) targets the mRNA. It deaminates a specific Cytidine (C) to Uridine (U) at codon 2153. This changes the codon **CAA** (which codes for Glutamine) into **UAA** (a **Stop Codon**). 3. **Result:** Translation terminates prematurely, producing a truncated protein that is 48% the length of the original, known as **Apo B48** (used in Chylomicrons). --- ### Why other options are incorrect: * **A. RNA Splicing:** This involves the removal of introns and joining of exons. While alternative splicing creates protein diversity, it is not the mechanism for Apo B diversity. * **B. Allelic Exclusion:** This is a process in B-lymphocytes where only one allele of an immunoglobulin gene is expressed to ensure antigen specificity. * **D. Upstream Repression:** This refers to transcriptional regulation where a repressor binds to an operator/enhancer site to inhibit gene expression; it does not alter the mRNA sequence. --- ### NEET-PG High-Yield Pearls: * **Apo B100:** Found in VLDL, IDL, and LDL. It acts as a ligand for the **LDL receptor**. * **Apo B48:** Found exclusively in **Chylomicrons** and chylomicron remnants. It lacks the LDL-receptor binding domain. * **Mnemonic:** **L**iver makes the **L**ong version (B100); **I**ntestine makes the **I**ncomplete version (B48). * **RNA Editing** is also seen in glutamate receptors in the brain (Adenosine to Inosine editing).
Question 148: How does RNA polymerase differ from DNA polymerase?
- A. It edits and synthesizes.
- B. It synthesizes RNA primers. (Correct Answer)
- C. It synthesizes only in the 5' to 3' direction.
- D. It uses RNA templates.
Explanation: **Explanation:** The fundamental difference between RNA polymerase (RNAP) and DNA polymerase (DNAP) lies in their requirement for a **primer**. DNA polymerase is incapable of initiating synthesis *de novo*; it requires a pre-existing 3'-OH group to add nucleotides. In contrast, RNA polymerase can initiate the synthesis of a polynucleotide chain without a primer. In the context of DNA replication, a specialized RNA polymerase called **Primase** synthesizes short **RNA primers**, providing the necessary 3'-OH terminus for DNA polymerase to begin elongation. **Analysis of Options:** * **Option A (Incorrect):** While DNA polymerase has 3' to 5' exonuclease activity (proofreading/editing), most RNA polymerases lack this robust editing capability, leading to a higher error rate in transcription. * **Option C (Incorrect):** This is a **similarity**, not a difference. Both enzymes synthesize exclusively in the **5' to 3' direction** by adding nucleotides to the 3' end of the growing chain. * **Option D (Incorrect):** Both enzymes typically use **DNA templates** (DNA-dependent). RNA-dependent RNA polymerase is found primarily in certain viruses, not as a standard feature of human cellular replication/transcription. **High-Yield Clinical Pearls for NEET-PG:** * **Rifampicin:** Inhibits bacterial DNA-dependent RNA polymerase (specifically the beta subunit), making it a cornerstone for TB treatment. * **α-Amanitin:** Found in *Amanita phalloides* (Death Cap mushroom); it potently inhibits **RNA Polymerase II**, halting mRNA synthesis and causing severe hepatotoxicity. * **Primase** is part of the "Primosome" complex in prokaryotes. In eukaryotes, the primase activity is associated with **DNA Polymerase α**.
Question 149: Nonsense codons are all of the following EXCEPT:
- A. UAA
- B. UAG
- C. UGA
- D. UCA (Correct Answer)
Explanation: **Explanation:** In molecular biology, the genetic code consists of 64 codons. Of these, 61 are **sense codons** (coding for amino acids), and 3 are **nonsense codons** (also known as **stop codons** or termination codons). **1. Why UCA is the correct answer:** **UCA** is a sense codon that codes for the amino acid **Serine**. It does not signal the termination of protein synthesis. Therefore, it is not a nonsense codon. **2. Why the other options are wrong (The Stop Codons):** The three nonsense codons do not code for any amino acid. Instead, they signal the ribosome to terminate translation and release the newly synthesized polypeptide chain. * **UAA (Ochre):** One of the three standard stop codons. * **UAG (Amber):** One of the three standard stop codons. * **UGA (Opal):** One of the three standard stop codons. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mnemonic to remember stop codons:** * **U** **A**re **A**way (UAA) * **U** **A**re **G**one (UAG) * **U** **G**o **A**way (UGA) * **Nonsense Mutation:** A point mutation that changes a sense codon into a nonsense codon, leading to premature termination of the protein and often resulting in a non-functional product (e.g., certain forms of β-thalassemia). * **Exceptions to the Universal Code:** In **mitochondria**, the genetic code differs slightly; for example, **UGA** codes for Tryptophan rather than acting as a stop codon. * **Initiation Codon:** **AUG** is the universal start codon (coding for Methionine in eukaryotes and Formyl-methionine in prokaryotes).
Question 150: Which one of the following is the complementary sequence of 5'TTAAGCTAC3'?
- A. 5' GTACGCTTAA 3' (Correct Answer)
- B. 5' AATTCGCATG 3'
- C. 5' CATGCGAATT 3'
- D. 5' TTAAGCGTAC 3'
Explanation: To solve this question, one must apply two fundamental rules of DNA structure: **Complementarity** and **Antiparallel orientation**. ### 1. The Underlying Concept * **Base Pairing:** Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). * **Antiparallel Nature:** DNA strands run in opposite directions. If the template is 5' to 3', the complementary strand is synthesized 3' to 5'. * **Standard Notation:** By convention, DNA sequences are always written in the **5' to 3' direction** unless specified otherwise. **Step-by-step derivation:** 1. **Template:** 5' T T A A G C T A C 3' 2. **Complementary (3' to 5'):** 3' A A T T C G A T G 5' 3. **Reverse to 5' to 3' (Correct Answer):** 5' G T A G C G T T A A 3' ### 2. Analysis of Options * **Option A (Correct):** This is the reverse complement. It correctly pairs the bases and flips the orientation to the standard 5'→3' format. * **Option B (Incorrect):** This is the "direct" complement (AATTCGATG) but written in the 5'→3' direction. This would imply a parallel orientation, which is biologically impossible. * **Option C (Incorrect):** This is a scrambled sequence that does not follow base-pairing rules for the given template. * **Option D (Incorrect):** This is almost identical to the template, representing a common distractor where only the last few bases are changed. ### 3. NEET-PG High-Yield Pearls * **Chargaff’s Rule:** In double-stranded DNA, A+G (purines) = T+C (pyrimidines). This rule does **not** apply to single-stranded RNA or DNA. * **Bonding:** A-T pairs have **2 hydrogen bonds**, while G-C pairs have **3 hydrogen bonds**. Therefore, DNA with high G-C content has a higher melting temperature (Tm). * **Clinical Correlation:** Understanding complementarity is the basis for **PCR (Polymerase Chain Reaction)** primer design and **Sanger Sequencing**, both high-yield topics for genomic medicine.
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
Practice Questions
CRISPR-Cas9 and Genome Editing
Practice Questions
DNA Fingerprinting and Forensics
Practice Questions
Molecular Basis of Genetic Diseases
Practice Questions
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