Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 681: What is true about alternative splicing?

A. It is the reason why the human genome is much more complex than other species.
B. It creates different proteins from a single gene. (Correct Answer)
C. It creates protein from multiple segments of DNA on different chromosomes.
D. It is not tissue specific.

Explanation: **Explanation:** **1. Why Option B is Correct:** Alternative splicing is a post-transcriptional modification process where different combinations of exons from a single **pre-mRNA** are joined together. By selectively including or excluding specific exons, a single gene can encode multiple distinct mRNA isoforms, which are then translated into different proteins (protein isoforms) with varying functions or properties. This process significantly increases the **proteomic diversity** of an organism without increasing the number of genes. **2. Analysis of Incorrect Options:** * **Option A:** While alternative splicing adds complexity, it is not the *sole* reason the human genome is more complex. Many species share similar splicing mechanisms. The complexity arises from a combination of gene regulation, non-coding DNA, and post-translational modifications. * **Option C:** Splicing occurs within a single primary transcript derived from one gene. The process of joining segments from different chromosomes is known as **trans-splicing**, which is rare in humans. * **Option D:** Alternative splicing is highly **tissue-specific** and developmentally regulated. For example, the *Calcitonin* gene produces Calcitonin in the thyroid but undergoes alternative splicing to produce Calcitonin Gene-Related Peptide (CGRP) in neural tissue. **Clinical Pearls for NEET-PG:** * **Proteomic Diversity:** Alternative splicing explains why humans have ~20,000 genes but over 100,000 different proteins. * **Spliceosome:** The machinery responsible for this process consists of **snRNPs** (small nuclear ribonucleoproteins, e.g., U1, U2, U4, U5, U6). * **Clinical Correlation:** Mutations in splice sites are responsible for diseases like **Beta-thalassemia** and **Spinal Muscular Atrophy (SMA)**. * **Systemic Lupus Erythematosus (SLE):** Patients often develop antibodies against snRNPs (Anti-Smith antibodies), which are highly specific for SLE.

Question 682: Genomic imprinting is associated with which of the following?

A. Silencing of the paternal chromosome
B. Silencing of the maternal chromosome
C. Angelman syndrome
D. All of the above (Correct Answer)

Explanation: **Explanation:** **Genomic Imprinting** is an epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner. Unlike most genes where both alleles are active, imprinted genes are "silenced" (usually via DNA methylation) on either the maternal or paternal chromosome. 1. **Why Option D is correct:** Genomic imprinting involves the selective silencing of genes from one parent. Depending on the specific gene locus, this can involve **silencing of the paternal chromosome** (Option A) or **silencing of the maternal chromosome** (Option B). When these normal imprinting patterns are disrupted or combined with genetic deletions, clinical disorders like **Angelman syndrome** (Option C) occur. 2. **Clinical Correlation (Prader-Willi vs. Angelman):** * Both syndromes involve a microdeletion on **Chromosome 15 (q11-q13)**. * **Prader-Willi Syndrome:** Occurs when the **Paternal** allele is deleted/absent (the maternal allele is normally silenced). Features include hyperphagia, obesity, and hypogonadism. * **Angelman Syndrome:** Occurs when the **Maternal** allele is deleted/absent (the paternal allele is normally silenced). Features include inappropriate laughter ("Happy Puppet"), seizures, and ataxia. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Primarily mediated by **DNA Methylation** (at CpG islands) and histone modification. * **Uniparental Disomy (UPD):** A condition where an individual receives two copies of a chromosome from one parent and none from the other; this is a common cause of imprinting disorders. * **Key Genes:** *UBE3A* (associated with Angelman) and *SNRPN* (associated with Prader-Willi). * **Reversibility:** Imprints are erased and re-established during gametogenesis to reflect the sex of the individual.

Question 683: All of the following statements about non-disjunction are true EXCEPT?

A. It may result in aneuploidy.
B. It is the failure of chromosomes to separate normally during cell division.
C. It may occur in meiosis or mitosis.
D. It cannot cause mosaicism. (Correct Answer)

Explanation: **Explanation:** **Non-disjunction** is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. **Why Option D is the correct answer (The Exception):** Non-disjunction **can** cause mosaicism. If non-disjunction occurs during **mitosis** in an early embryonic stage (post-zygotic), it leads to two or more cell lines with different genetic constitutions within the same individual. For example, some cells may have 46 chromosomes while others have 47 (e.g., Mosaic Down Syndrome). Therefore, the statement that it "cannot" cause mosaicism is false. **Analysis of other options:** * **Option A:** True. Non-disjunction leads to **aneuploidy** (an abnormal number of chromosomes), resulting in conditions like trisomy (2n+1) or monosomy (2n-1). * **Option B:** True. This is the fundamental definition of non-disjunction. It occurs when the spindle fibers fail to pull chromosomes to opposite poles. * **Option C:** True. It can occur in **Meiosis I** (failure of homologous chromosomes to separate), **Meiosis II** (failure of sister chromatids to separate), or **Mitosis**. **Clinical Pearls for NEET-PG:** 1. **Meiosis I non-disjunction** is the most common cause of Trisomy 21 (Down Syndrome), often associated with advanced maternal age. 2. **Mitotic non-disjunction** is the primary mechanism behind **Mosaicism**. 3. **Turner Syndrome (45, XO)** is the only viable monosomy in humans. 4. **Kleinfelter Syndrome (47, XXY)** is a classic example of sex chromosome aneuploidy caused by non-disjunction.

Question 684: Which of the following genetic disorders is currently treated or being researched for gene therapy?

A. All of the above (Correct Answer)
B. Sickle cell anemia
C. Thalassemia
D. Cystic fibrosis

Explanation: **Explanation:** Gene therapy involves the introduction, removal, or alteration of genetic material within a patient's cells to treat a disease. The correct answer is **"All of the above"** because all three conditions are classic targets for gene therapy due to their monogenic nature (caused by a mutation in a single gene). * **Sickle Cell Anemia & Thalassemia:** These hemoglobinopathies are being treated using **Ex-vivo gene therapy**. Patient hematopoietic stem cells are harvested and modified—either by inserting a functional $\beta$-globin gene or by using **CRISPR-Cas9** to knock out the *BCL11A* gene. This "silencing the silencer" approach restarts the production of **Fetal Hemoglobin (HbF)**, which compensates for the defective adult hemoglobin. * **Cystic Fibrosis (CF):** This was one of the first diseases targeted for gene therapy. Research focuses on delivering a functional copy of the **CFTR gene** to the respiratory epithelium using viral vectors (like Adenovirus or AAV) or liposomes via inhalation. **Why other options are "incorrect" as standalone choices:** While B, C, and D are individual targets, selecting any one of them would be incomplete. All three represent the current frontier of molecular medicine and are frequently tested together in the context of genetic engineering. **High-Yield Clinical Pearls for NEET-PG:** * **Vectors:** Viral vectors (Retrovirus, Lentivirus, Adenovirus) are the most common delivery vehicles. * **First Gene Therapy:** Successfully performed in 1990 for **ADA-SCID** (Adenosine Deaminase deficiency). * **Luxturna:** The first FDA-approved *in-vivo* gene therapy for an inherited retinal disease (RPE65 mutation). * **Zolgensma:** A landmark gene therapy for **Spinal Muscular Atrophy (SMA)**.

Question 685: Which of the following is not used as a vector in genetics?

A. Adenovirus
B. Proteasome (Correct Answer)
C. Liposome
D. Retrovirus

Explanation: **Explanation:** In genetics and gene therapy, a **vector** is a vehicle used to deliver genetic material (DNA or RNA) into a target cell. **Why Proteasome is the correct answer:** A **proteasome** is not a delivery vehicle; it is a large protein complex found in all eukaryotes and archaea. Its primary function is the **degradation of unneeded or damaged proteins** that have been tagged with ubiquitin (the Ubiquitin-Proteasome Pathway). It acts as the cell's "garbage disposal" rather than a genetic carrier. **Analysis of other options:** * **Adenovirus (Option A):** These are non-enveloped DNA viruses used as viral vectors. They are highly efficient at transducing both dividing and non-dividing cells. (e.g., used in certain COVID-19 vaccines). * **Liposome (Option C):** These are synthetic, spherical vesicles composed of a lipid bilayer. They are used as **non-viral vectors** to carry DNA or drugs across the hydrophobic cell membrane via endocytosis. * **Retrovirus (Option D):** These are RNA viruses that use reverse transcriptase to integrate their genetic payload into the host cell's genome, making them useful for long-term gene expression in gene therapy. **High-Yield Clinical Pearls for NEET-PG:** * **Bortezomib:** A clinical correlation to remember is that Bortezomib is a **proteasome inhibitor** used in the treatment of Multiple Myeloma. * **Viral vs. Non-viral:** Viral vectors (Adeno, Retro, Lentivirus) are generally more efficient but carry risks of immunogenicity or insertional mutagenesis. Non-viral vectors (Liposomes, Plasmids) are safer but less efficient. * **pBR322:** The first artificial cloning vector widely used in molecular biology.

Question 686: Which of the following statements regarding the Ames test is false?

A. Salmonella typhimurium is used in the test.
B. Mutated bacteria fail to grow in the absence of histidine. (Correct Answer)
C. It is used for testing mutagens.
D. It was developed by Bruce N. Ames in the 1970s.

Explanation: The **Ames Test** is a rapid biochemical assay used to screen chemicals for their potential to cause mutations (mutagenicity), which often correlates with carcinogenicity. ### **Explanation of the Correct Answer (Option B)** The statement in Option B is false because the test uses a **histidine-dependent (auxotrophic)** strain of *Salmonella typhimurium* that **already carries a mutation** preventing it from synthesizing histidine. * **Initial State:** These bacteria cannot grow on a histidine-free medium. * **The Mechanism:** When exposed to a mutagen, a **"reverse mutation" (back mutation)** occurs, restoring the bacteria's ability to synthesize histidine. * **The Result:** Therefore, it is the **reverted (newly mutated) bacteria** that **gain** the ability to grow in the absence of histidine. A high number of colonies indicates a strong mutagen. ### **Analysis of Other Options** * **Option A:** Correct. *Salmonella typhimurium* (specifically strains with pre-existing mutations in the *his* operon) is the standard organism used. * **Option C:** Correct. The primary purpose of the test is to identify substances that cause genetic damage (mutagens). * **Option D:** Correct. It was developed by Bruce Ames at UC Berkeley in the early 1970s. ### **High-Yield Clinical Pearls for NEET-PG** * **Rat Liver Extract (S9 Mix):** Many non-carcinogenic compounds become mutagenic only after metabolic activation in the liver. To simulate human metabolism, rat liver enzymes (S9 fraction) are added to the medium. * **Correlation:** Approximately 90% of known carcinogens test positive in the Ames test. * **Type of Mutation:** The test specifically detects **point mutations** (substitution, insertion, or deletion). * **Frameshift vs. Base-pair substitution:** Different strains of *Salmonella* are used to detect different types of mutations.

Question 687: Which of the following are components of a chromosome?

A. DNA and Histone (Correct Answer)
B. tRNA
C. DNA and tRNA
D. DNA and rRNA

Explanation: **Explanation:** The fundamental unit of eukaryotic chromatin is the **nucleosome**, which consists of double-stranded DNA wrapped around a protein core. A chromosome is essentially a highly condensed package of these nucleosomes. 1. **Why Option A is Correct:** Chromosomes are composed of **Chromatin**, which is a complex of **DNA** and **Histone proteins** (along with some non-histone proteins). The DNA (negatively charged due to phosphate groups) wraps approximately 1.65 times around an octamer of Histones (positively charged due to Lysine and Arginine). This "beads-on-a-string" structure further coils into solenoids and scaffolds to form the visible chromosome during cell division. 2. **Why Incorrect Options are Wrong:** * **tRNA (Options B & C):** Transfer RNA is a functional RNA molecule involved in translation (protein synthesis) in the cytoplasm. It is not a structural component of the chromosome. * **rRNA (Option D):** Ribosomal RNA combines with proteins to form ribosomes. While rRNA is transcribed from specific regions of DNA (Nucleolar Organizer Regions), it does not form the structural framework of the chromosome itself. **High-Yield Clinical Pearls for NEET-PG:** * **Histone Octamer:** Consists of two units each of **H2A, H2B, H3, and H4**. * **Linker Histone:** **H1** is the only histone not part of the nucleosome core; it binds to the linker DNA to stabilize the 30nm fiber. * **Acetylation vs. Methylation:** Histone **acetylation** (by HATs) neutralizes positive charges, relaxing chromatin (**Euchromatin**) and increasing transcription. Histone **deacetylation** (by HDACs) leads to condensation (**Heterochromatin**) and gene silencing. * **Charge:** DNA is acidic/negative; Histones are basic/positive.

Question 688: What process involves the excision of introns?

A. RNA splicing (Correct Answer)
B. RNA editing
C. Restriction endonuclease
D. DNAase

Explanation: **Explanation:** **RNA Splicing (Correct Answer):** In eukaryotes, genes are composed of coding sequences called **exons** and non-coding intervening sequences called **introns**. During transcription, a primary transcript (hnRNA) is formed containing both. **RNA splicing** is the post-transcriptional modification process where introns are precisely excised and exons are joined together by a complex called the **spliceosome** (composed of snRNPs). This results in a mature mRNA molecule ready for translation. **Why other options are incorrect:** * **RNA Editing:** This involves the alteration of specific nucleotide sequences within the RNA (e.g., C-to-U or A-to-I conversion) after transcription, changing the message without removing segments. A classic example is the production of ApoB-48 and ApoB-100 from the same gene. * **Restriction Endonucleases:** These are bacterial enzymes ("molecular scissors") used in recombinant DNA technology to cut **double-stranded DNA** at specific palindromic sequences. They are not involved in RNA processing. * **DNAase:** This is an enzyme that catalyzes the hydrolytic cleavage of phosphodiester linkages in the **DNA backbone**, degrading DNA into smaller units. **High-Yield Clinical Pearls for NEET-PG:** * **Splice Site Mutation:** Mutations at the conserved **GU (5' donor site)** or **AG (3' acceptor site)** can lead to aberrant splicing, a common cause of diseases like **β-thalassemia**. * **Alternative Splicing:** This allows a single gene to code for multiple proteins (isoforms), significantly increasing proteomic diversity. * **Autoimmunity:** Antibodies against snRNPs (e.g., **Anti-Smith antibodies**) are highly specific diagnostic markers for **Systemic Lupus Erythematosus (SLE)**.

Question 689: What activity does RNA polymerase possess?

A. Primase (Correct Answer)
B. Helicase
C. Ligase
D. Topoisomerase

Explanation: ### Explanation **1. Why Primase is the Correct Answer:** In the context of DNA replication, **Primase** is a specialized type of **DNA-dependent RNA polymerase**. DNA polymerase cannot initiate the synthesis of a new strand *de novo*; it requires a free 3'-OH group. Primase solves this by synthesizing a short RNA primer (approximately 10 nucleotides long). Because Primase creates an RNA sequence, it inherently possesses **RNA polymerase activity**. **2. Analysis of Incorrect Options:** * **Helicase:** This enzyme is responsible for unwinding the DNA double helix at the replication fork by breaking hydrogen bonds between complementary bases. It requires ATP but does not synthesize RNA. * **Ligase:** This enzyme acts as "molecular glue." It catalyzes the formation of a phosphodiester bond to seal nicks between DNA fragments (like Okazaki fragments), joining the 3'-OH end of one fragment to the 5'-phosphate end of another. * **Topoisomerase:** These enzymes (e.g., DNA Gyrase in prokaryotes) relieve the torsional strain and supercoiling caused by the unwinding of the DNA strand. They work by cutting and resealing the DNA backbone. **3. High-Yield Clinical Pearls for NEET-PG:** * **The Primosome:** In prokaryotes, the complex consisting of DnaB (Helicase) and DnaG (Primase) is known as the primosome. * **Eukaryotic Equivalent:** In eukaryotes, Primase is associated with **DNA Polymerase $\alpha$**, which initiates the synthesis of the primer. * **Directionality:** Like all polymerases, Primase synthesizes the RNA primer in the **5' to 3' direction**. * **Rifampicin Connection:** While Rifampicin inhibits bacterial RNA polymerase (transcription), it does not inhibit the RNA polymerase activity of Primase during replication.

Question 690: What is a leucine zipper complex?

A. B cell epitomes
B. Receptor ligand protein
C. DNA binding protein (Correct Answer)
D. Membrane attack complex

Explanation: **Explanation:** The **Leucine Zipper** is a common structural motif found in **DNA-binding proteins**, specifically transcription factors. It consists of an alpha-helix where the amino acid **Leucine** occurs at every seventh position (heptad repeat) along the hydrophobic face of the helix. This arrangement allows two such proteins to "zip" together through hydrophobic interactions, forming a dimer. This dimerization creates a Y-shaped structure where the basic regions of the protein (rich in Arginine and Lysine) can bind specifically to the major groove of DNA. Classic examples include transcription factors like **c-Jun, c-Fos, and CREB**. **Analysis of Options:** * **Option A (B cell epitopes):** These are specific parts of an antigen to which B cell receptors or antibodies bind. They are related to immunology, not DNA transcription motifs. * **Option B (Receptor ligand protein):** While some transcription factors are activated by ligands (like steroid receptors), the leucine zipper itself is a structural motif for DNA interaction, not a general term for receptor-ligand complexes. * **Option D (Membrane attack complex):** This is the end-product of the complement cascade (C5b-C9) that creates pores in bacterial cell membranes. **High-Yield Clinical Pearls for NEET-PG:** * **Other DNA-binding motifs:** Zinc fingers (e.g., Steroid receptors), Helix-turn-helix (e.g., Homeodomain proteins), and Helix-loop-helix (e.g., MYC). * **Oncogenic link:** Mutations in leucine zipper proteins like **c-Myc** or **c-Fos** are frequently associated with uncontrolled cell proliferation and various cancers. * **Key Feature:** The leucine zipper facilitates **dimerization**, which is essential for the protein to function as a transcription factor.

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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