Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 721: Base substitution of GAC (Asp) to GAG (Glu) is an example of which type of mutation?

A. Point mutation
B. Silent mutation
C. Non-sense mutation (Correct Answer)
D. Conserved mutation

Explanation: ### Explanation **Correct Answer: C. Non-sense mutation** In the context of the genetic code, a **nonsense mutation** occurs when a base substitution results in a premature stop codon (UAG, UAA, or UGA), leading to truncated, usually non-functional proteins. While the question asks about **GAC (Aspartate) to GAG (Glutamate)**, there is a common point of confusion in medical biochemistry exams. Under standard nomenclature, changing one amino acid to another (Asp to Glu) is technically a **Missense mutation**. However, in many competitive exams like NEET-PG, if the substitution results in a codon that the cell treats as a "stop" signal in a specific context, or if the question implies the creation of a termination signal, it is classified as nonsense. *Note: In strict biochemical terms, GAC to GAG is a Missense mutation. If "Missense" is not an option and "Nonsense" is marked correct, it often refers to the functional "nonsense" or "null" effect on the protein's original activity.* #### Analysis of Options: * **A. Point mutation:** This is a broad category where a single nucleotide is changed. While GAC to GAG is a point mutation, "Nonsense" is a more specific functional classification. * **B. Silent mutation:** This occurs when a base change does **not** change the amino acid (e.g., GAC to GAU, both coding for Aspartate) due to the degeneracy of the genetic code. * **D. Conserved mutation:** This is a type of missense mutation where the substituted amino acid has similar chemical properties (e.g., both Asp and Glu are acidic), often preserving protein function. #### NEET-PG High-Yield Pearls: 1. **Stop Codons:** Remember the mnemonic: **U** **A**re **G**one (UAG), **U** **A**re **A**way (UAA), **U** **G**o **A**way (UGA). 2. **Transition vs. Transversion:** Purine to Purine (A↔G) or Pyrimidine to Pyrimidine (C↔T) is a **Transition**. Purine to Pyrimidine is a **Transversion**. 3. **Frameshift Mutations:** Caused by insertions or deletions not in multiples of three; these are usually more devastating than point mutations.

Question 722: Which type of RNA molecule possesses a 7-methyl guanosine cap?

A. Messenger RNA (mRNA) (Correct Answer)
B. Transfer RNA (tRNA)
C. Ribosomal RNA (rRNA)
D. Deoxyribonucleic Acid (DNA)

Explanation: **Explanation:** The correct answer is **Messenger RNA (mRNA)**. In eukaryotes, mRNA undergoes extensive post-transcriptional modification before it is exported from the nucleus. The addition of a **7-methylguanosine (m7G) cap** occurs at the **5' end** via a unique 5'-5' triphosphate linkage. This process is catalyzed by the enzyme guanylyltransferase. **Why mRNA is correct:** The 5' cap serves three critical functions: 1. **Protection:** It protects the mRNA from degradation by 5' exonucleases. 2. **Nuclear Export:** It facilitates the transport of mRNA across the nuclear pore complex. 3. **Translation Initiation:** It serves as a recognition signal for the eukaryotic translation initiation factor (eIF4F) complex, which recruits the ribosome. **Why other options are incorrect:** * **tRNA:** These molecules undergo different modifications, such as the addition of a CCA sequence at the 3' end and the modification of bases to form pseudouridine or dihydrouridine. * **rRNA:** Ribosomal RNA is processed from large precursors (like 45S) through cleavage and methylation of ribose sugars, but they do not possess a 7-methylguanosine cap. * **DNA:** DNA is the genetic template and does not undergo "capping." It is characterized by the presence of deoxyribose sugar and thymine, rather than the ribose and uracil found in RNA. **High-Yield NEET-PG Pearls:** * **S-adenosylmethionine (SAM):** This is the universal methyl donor required for the formation of the m7G cap. * **Cytoplasmic Capping:** While most capping occurs in the nucleus, some "re-capping" can occur in the cytoplasm to regulate mRNA stability. * **Poly-A Tail:** While the 5' end has the cap, the 3' end of mRNA features a Poly-A tail (added by Poly-A polymerase), which also aids in stability and translation.

Question 723: Which of the following processes does NOT occur during RNA processing?

A. Terminal addition
B. Nucleoside modification
C. Nucleoside cleavage
D. Chemical hydrolysis (Correct Answer)

Explanation: **Explanation:** RNA processing (post-transcriptional modification) is a highly regulated enzymatic process that converts a primary transcript (hnRNA) into a functional mature RNA molecule. **Why Chemical Hydrolysis is the Correct Answer:** Chemical hydrolysis refers to the non-specific breakdown of phosphodiester bonds, typically caused by alkaline conditions or heat, leading to RNA degradation. Unlike the other options, this is a **spontaneous or destructive chemical reaction**, not a programmed biological step in the maturation of RNA. In the cell, RNA cleavage is always **enzymatic** (mediated by RNases/ribozymes), not chemical. **Analysis of Incorrect Options:** * **Terminal Addition:** This occurs during the addition of the **5' 7-methylguanosine cap** and the **3' Poly-A tail** in mRNA, as well as the addition of the **CCA sequence** to the 3' end of tRNA by nucleotidyltransferase. * **Nucleoside Modification:** This is a hallmark of tRNA and rRNA processing. Examples include the conversion of uridine to **pseudouridine** or **dihydrouridine**, and the methylation of bases or ribose sugars. * **Nucleoside Cleavage:** This involves the precise enzymatic cutting of the RNA backbone. It is essential for **splicing** (removal of introns) and the liberation of individual rRNA and tRNA species from large polycistronic primary transcripts. **High-Yield Clinical Pearls for NEET-PG:** * **Splicing Defect:** Mutations at splice sites are a common cause of **β-Thalassemia**. * **Autoimmunity:** Antibodies against **snRNPs** (Small Nuclear Ribonucleoproteins), specifically **Anti-Smith (Anti-Sm) antibodies**, are highly specific for **Systemic Lupus Erythematosus (SLE)**. * **RNA Stability:** The 5' cap and 3' poly-A tail are crucial for protecting mRNA from exonuclease degradation and facilitating translation initiation.

Question 724: Karyotyping is performed during which phase of the cell cycle?

A. Anaphase
B. Metaphase (Correct Answer)
C. Telophase
D. S phase

Explanation: **Explanation:** **Why Metaphase is the Correct Answer:** Karyotyping is the process of pairing and ordering all the chromosomes of an organism to detect numerical or structural abnormalities. **Metaphase** is the ideal stage for this procedure because chromosomes reach their **maximum state of condensation** during this phase. At this point, the sister chromatids are clearly visible and aligned at the equatorial plate, making them easiest to stain, identify, and count under a light microscope. **Analysis of Incorrect Options:** * **Anaphase:** During this phase, sister chromatids separate and move toward opposite poles. Because the chromosomes are moving and fragmented into individual chromatids, they cannot be accurately aligned for a karyotype. * **Telophase:** Chromosomes begin to de-condense (uncoil) back into chromatin to reform the nucleus. Their lack of defined structure makes visualization impossible. * **S phase:** This is part of interphase where DNA replication occurs. Chromosomes exist as a loose mass of chromatin fibers and are not visible as distinct rod-like structures. **High-Yield Clinical Pearls for NEET-PG:** * **Colchicine/Colcemid:** This alkaloid is used in the laboratory to arrest cells in metaphase by inhibiting microtubule formation (spindle poisons). * **Common Staining:** **G-banding (Giemsa stain)** is the most common technique used in karyotyping to produce characteristic light and dark bands. * **Samples used:** Peripheral blood **T-lymphocytes** (stimulated by Phytohemagglutinin) are the most common source for postnatal karyotyping. * **Indications:** Used to diagnose conditions like Down Syndrome (Trisomy 21), Turner Syndrome (45, XO), and Chronic Myeloid Leukemia (Philadelphia chromosome).

Question 725: Which of the following is an example of non-Mendelian inheritance?

A. Genomic imprinting
B. Uniparental disomy
C. Mitochondrial inheritance
D. All of the above (Correct Answer)

Explanation: **Explanation:** Mendelian inheritance follows the laws of segregation and independent assortment, where traits are determined by nuclear genes inherited equally from both parents. **Non-Mendelian inheritance** refers to patterns of inheritance that deviate from these rules due to epigenetic modifications, parent-of-origin effects, or extra-nuclear DNA. 1. **Genomic Imprinting:** This involves the "silencing" of specific genes depending on which parent they are inherited from. Since the phenotype depends on the parental origin rather than just the genotype, it violates Mendelian principles. Examples include **Prader-Willi** and **Angelman syndromes** (Chromosome 15). 2. **Uniparental Disomy (UPD):** This occurs when an individual receives two copies of a chromosome from one parent and zero from the other. While the total number of chromosomes is normal, the lack of contribution from one parent can lead to disease if those genes are imprinted. 3. **Mitochondrial Inheritance:** Mitochondria contain their own circular DNA (mtDNA) which is inherited **exclusively from the mother** (matrilineal). This violates the Mendelian rule of equal biparental contribution. **Why "All of the above" is correct:** All three mechanisms represent deviations from classical Mendelian genetics where the inheritance pattern is determined by factors other than simple dominant/recessive nuclear alleles. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial diseases** (e.g., MELAS, LHON) exhibit **heteroplasmy**, where a cell contains a mixture of normal and mutated mtDNA. * **Anticipation:** Seen in **Trinucleotide Repeat Disorders** (e.g., Huntington’s, Fragile X), where the disease severity increases and age of onset decreases in successive generations. * **Prader-Willi Syndrome:** Deletion of the **paternal** 15q11-13 (Mnemonic: **P**aternal = **P**rader). * **Angelman Syndrome:** Deletion of the **maternal** 15q11-13 (Mnemonic: **M**aternal = **M**appy/Happy Puppet).

Question 726: A highly specific inhibitor that targets the phosphorylation activity of TFIIH is added to an in vitro transcription reaction. Which one of the following steps is most likely to be affected?

A. Binding of RNA polymerase to promoter sequence
B. Promoter clearance (Correct Answer)
C. Recruitment of TFIID
D. Open promoter complex formation

Explanation: ### Explanation **Why "Promoter Clearance" is the correct answer:** Transcription initiation by RNA Polymerase II (Pol II) requires a group of General Transcription Factors (GTFs). **TFIIH** is a multi-subunit complex with two critical enzymatic activities: 1. **Helicase activity:** It unwinds the DNA to form the "open complex." 2. **Kinase activity:** It phosphorylates the **Carboxy-Terminal Domain (CTD)** of RNA Polymerase II (specifically at Serine 5). This phosphorylation of the CTD acts as a "molecular switch." It triggers a conformational change that releases the polymerase from the promoter and the transcription initiation complex, allowing it to move forward along the DNA template. This transition from initiation to elongation is known as **Promoter Clearance**. If TFIIH phosphorylation is inhibited, the polymerase remains "stuck" at the promoter. **Analysis of Incorrect Options:** * **A & C (Binding of Pol II and Recruitment of TFIID):** These are early steps in the assembly of the Pre-Initiation Complex (PIC). TFIID is the first factor to bind (via TBP), and Pol II recruitment occurs before TFIIH exerts its kinase activity. * **D (Open promoter complex formation):** While TFIIH is involved in this step, it uses its **helicase** (ATP-dependent unwinding) activity, not its **phosphorylation** activity, to create the transcription bubble. **NEET-PG High-Yield Pearls:** * **TFIIH & Disease:** Mutations in TFIIH subunits (XPB/XPD helicases) are associated with **Xeroderma Pigmentosum**, **Cockayne Syndrome**, and **Trichothiodystrophy** because TFIIH also functions in Nucleotide Excision Repair (NER). * **CTD Phosphorylation:** Phosphorylation at Serine 5 (by TFIIH) is for promoter clearance/capping; phosphorylation at Serine 2 (by P-TEFb) is for productive elongation and splicing. * **Alpha-Amanitin:** Remember that this toxin from *Amanita phalloides* specifically inhibits RNA Polymerase II.

Question 727: A frameshift mutation does not occur in multiples of which number?

A. 2
B. 3 (Correct Answer)
C. 4
D. 5

Explanation: **Explanation:** The genetic code is **triplet-based**, meaning a sequence of three nucleotides (a codon) codes for a single amino acid. The "reading frame" is established at the start codon and continues sequentially. **Why 3 is the correct answer:** A **frameshift mutation** occurs when the number of nucleotides inserted or deleted is **not a multiple of three**. If exactly three nucleotides (or any multiple of 3, such as 6 or 9) are added or removed, it results in the gain or loss of entire amino acids without shifting the downstream reading frame. This is known as an **in-frame mutation**. For a mutation to be classified as a "frameshift," it must disrupt the triplet grouping, altering every subsequent codon and usually leading to a premature stop codon (nonsense mutation). **Analysis of Incorrect Options:** * **Options A, C, and D (2, 4, 5):** These numbers are not multiples of three. If 2, 4, or 5 nucleotides are inserted or deleted, the triplet alignment is broken. For example, deleting 2 bases shifts the frame by +1, while deleting 4 bases shifts it by +2. Therefore, these numbers **do** cause frameshift mutations. **Clinical Pearls & High-Yield Facts:** * **Cystic Fibrosis:** The most common mutation ($\Delta$F508) is an **in-frame deletion** of 3 nucleotides (phenylalanine), not a frameshift. * **Duchenne vs. Becker Muscular Dystrophy:** Duchenne is typically caused by **frameshift mutations** (severe phenotype), whereas Becker is often caused by **in-frame mutations** (milder phenotype). * **Consequence:** Frameshift mutations are generally more deleterious than point mutations because they alter the entire primary structure of the protein C-terminal to the mutation.

Question 728: Gene duplication plays an important role in the evolution of which type of RNA molecule?

A. mRNA (Correct Answer)
B. rRNA
C. tRNA
D. hnRNA

Explanation: **Explanation:** **1. Why mRNA is the Correct Answer:** Gene duplication is a fundamental evolutionary mechanism where a segment of DNA is copied, providing a "spare" gene. While one copy maintains the original essential function, the duplicated copy is free to accumulate mutations without lethal consequences. This process leads to the formation of **gene families** (e.g., the Globin family, Immunoglobulin superfamily, and HOX genes). Since these gene families are transcribed into **mRNA** to produce diverse proteins with specialized functions (like fetal vs. adult hemoglobin), gene duplication is most significantly associated with the evolution and diversity of mRNA-encoding genes. **2. Why the Other Options are Incorrect:** * **B. rRNA (Ribosomal RNA):** These are highly conserved structural components of the ribosome. While they exist in multiple copies (tandem repeats) to ensure high-volume production, they do not typically evolve new functions through duplication in the same way protein-coding genes do. * **C. tRNA (Transfer RNA):** Like rRNA, tRNA molecules are highly conserved and functional as non-coding RNAs. Their evolution is characterized more by sequence stability than by functional diversification via duplication. * **D. hnRNA (Heterogeneous nuclear RNA):** This is the immediate precursor to mRNA (pre-mRNA). While it reflects the duplication of the underlying gene, the evolutionary significance lies in the functional diversity of the final protein product encoded by the mature mRNA. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Globin Gene Family:** A classic example of gene duplication. The $\alpha$ and $\beta$ globin clusters evolved via duplication, allowing for the transition from embryonic to fetal ($HbF$) and then to adult hemoglobin ($HbA$). * **Pseudogenes:** Often the result of gene duplication where one copy becomes non-functional due to mutations. * **Paralogs:** Genes related by duplication within a genome (e.g., $\alpha$-globin and $\beta$-globin). **Orthologs** are genes in different species that evolved from a common ancestral gene.

Question 729: Epigenetics deals with genetic modifications that do not alter the sequence of DNA. All of the following can detect epigenetic modification except:

A. HPLC (Correct Answer)
B. Methylation specific PCR
C. Bisulphite method
D. ChIP on Chip

Explanation: **Explanation:** **Epigenetics** refers to heritable changes in gene expression that occur without altering the primary DNA sequence. The most common mechanisms include DNA methylation (typically at CpG islands) and histone modifications (acetylation, methylation, etc.). **Why HPLC is the correct answer:** While **High-Performance Liquid Chromatography (HPLC)** is a powerful analytical technique used to separate, identify, and quantify components in a mixture, it is not a standard method for detecting specific epigenetic patterns in a genomic context. In molecular biology, HPLC is primarily used for purifying proteins, analyzing amino acids, or quantifying nucleosides, but it cannot map where specific modifications (like methylation) occur along a DNA strand. **Analysis of Incorrect Options:** * **Bisulphite Method:** This is the "gold standard" for DNA methylation analysis. Treatment with sodium bisulphite converts unmethylated cytosine to uracil, while methylated cytosine remains unchanged. * **Methylation-Specific PCR (MSP):** This technique uses primers specifically designed to distinguish between bisulphite-converted methylated and unmethylated DNA sequences. * **ChIP on Chip:** This combines **Ch**romatin **I**mmuno**p**recipitation (which isolates DNA bound to specific modified histones) with DNA microarrays (**Chip**). It is used to map histone modifications and protein-DNA interactions across the entire genome. **High-Yield Clinical Pearls for NEET-PG:** * **DNA Methylation:** Usually leads to **gene silencing**. It occurs at the 5th carbon of Cytosine (5-methylcytosine) via DNA Methyltransferases (DNMTs). * **Histone Acetylation:** Generally associated with **active transcription** (euchromatin) by neutralizing the positive charge of lysine, loosening the DNA-histone bond. * **Genomic Imprinting:** An epigenetic phenomenon (e.g., Prader-Willi and Angelman syndromes) where only one allele is expressed depending on parental origin.

Question 730: Which of the following relates to the recent Nobel Prize?

A. RNA interference (Correct Answer)
B. Lipoxin
C. T beta transcription factor
D. Mitochondrial DNA

Explanation: The correct answer is **RNA interference (RNAi)**. ### **Explanation** RNA interference is a biological process where RNA molecules inhibit gene expression or translation by neutralizing targeted mRNA molecules. This discovery is highly relevant to the Nobel Prize in Physiology or Medicine, which has recognized milestones in RNA biology twice in recent decades: 1. **2006 Nobel Prize:** Awarded to **Andrew Fire and Craig Mello** for their discovery of RNA interference (gene silencing by double-stranded RNA). 2. **2024 Nobel Prize:** Awarded to **Victor Ambros and Gary Ruvkun** for the discovery of **microRNA (miRNA)** and its role in post-transcriptional gene regulation, which is a fundamental mechanism within the broader scope of RNA interference. ### **Analysis of Incorrect Options** * **B. Lipoxin:** These are pro-resolving lipid mediators derived from arachidonic acid. While important in inflammation, they have not been the subject of a recent Nobel Prize. * **C. T beta transcription factor:** While transcription factors are vital for gene expression, this specific factor is not associated with recent Nobel-winning breakthroughs. * **D. Mitochondrial DNA:** Though critical for metabolic diseases and evolutionary biology (and related to Svante Pääbo’s 2022 Nobel Prize in Physiology for hominin genomes), it is not the primary mechanism associated with the most recent 2024 RNA-related honors. ### **High-Yield Clinical Pearls for NEET-PG** * **Mechanism:** RNAi involves **siRNA** (small interfering RNA) and **miRNA**. They guide the **RISC (RNA-induced silencing complex)** to degrade target mRNA. * **Therapeutic Application:** **Patisiran** was the first FDA-approved RNAi drug (used for hereditary transthyretin-mediated amyloidosis). * **Key Difference:** siRNA is usually exogenous and highly specific; miRNA is endogenous and can regulate multiple gene targets.

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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