Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 331: Which enzyme is NOT involved in DNA replication?

A. Telomerase
B. Reverse transcriptase (Correct Answer)
C. Restriction endonuclease
D. DNA ligase

Explanation: **Explanation:** DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. This process requires a specific set of enzymes working in coordination. **Why Reverse Transcriptase is the Correct Answer:** Reverse transcriptase is an **RNA-dependent DNA polymerase**. Its primary function is to synthesize DNA from an RNA template. This enzyme is characteristic of retroviruses (like HIV) and is used in the laboratory for cDNA synthesis (RT-PCR). It is **not** a component of the standard semi-conservative DNA replication machinery in human cells. **Analysis of Other Options:** * **Telomerase:** This is a specialized ribonucleoprotein (a reverse transcriptase) that adds repetitive nucleotide sequences to the ends of chromosomes (telomeres) to prevent shortening during replication. It is essential for maintaining genomic stability in germ cells and cancer cells. * **DNA Ligase:** Known as the "molecular glue," it catalyzes the formation of phosphodiester bonds to join **Okazaki fragments** on the lagging strand, ensuring the continuity of the newly synthesized DNA strand. * **Restriction Endonucleases:** While primarily known as tools in recombinant DNA technology, these enzymes (in a broader biological context) are involved in DNA metabolism and repair. However, in the context of standard NEET-PG questions, if "Reverse Transcriptase" is an option, it is the most distinct outlier as it reverses the central dogma. *(Note: Some examiners consider Restriction Endonucleases as tools for DNA manipulation rather than replication; however, Reverse Transcriptase remains the classic "non-replication" enzyme in this list).* **High-Yield Clinical Pearls for NEET-PG:** * **DNA Polymerase III:** The primary enzyme for elongation in prokaryotes. * **DNA Polymerase α:** Initiates replication in eukaryotes (contains primase). * **Topoisomerase (DNA Gyrase):** Relieves torsional strain (supercoiling) ahead of the replication fork. Fluoroquinolones inhibit this in bacteria. * **Zidovudine (AZT):** A potent inhibitor of Reverse Transcriptase used in HIV management.

Question 332: Multiple proteins from a single gene may be generated by:

A. Alternative splicing (Correct Answer)
B. Mutations in the promoter of the gene
C. RNA interference
D. mRNA capping

Explanation: ### Explanation **Correct Option: A. Alternative Splicing** The central dogma states that one gene typically codes for one protein; however, **alternative splicing** is the primary mechanism that allows for proteomic diversity. During post-transcriptional modification, different combinations of **exons** (coding regions) from a single pre-mRNA transcript are joined together, while introns are removed. This results in multiple unique mRNA isoforms, which are then translated into different proteins with distinct functions or tissue specificities. * *Example:* The **Calcitonin gene** produces Calcitonin in the thyroid but undergoes alternative splicing to produce Calcitonin Gene-Related Peptide (CGRP) in neural tissue. **Why Incorrect Options are Wrong:** * **B. Mutations in the promoter:** The promoter region controls the *initiation and rate* of transcription. Mutations here typically lead to increased, decreased, or abolished protein synthesis, but they do not change the protein's primary structure or create multiple variants. * **C. RNA interference (RNAi):** This is a regulatory mechanism (involving siRNA or miRNA) that leads to **gene silencing** by degrading mRNA or inhibiting translation. It reduces protein expression rather than generating new protein variants. * **D. mRNA capping:** This involves adding a 7-methylguanosine cap to the 5' end. It is essential for mRNA stability, nuclear export, and translation initiation, but it does not alter the coding sequence. **High-Yield Clinical Pearls for NEET-PG:** * **Spliceosomes:** These are the molecular machines (composed of snRNPs) that perform splicing. * **Clinical Correlation:** Mutations in splice sites are responsible for approximately 15% of genetic diseases, including **Beta-thalassemia** and **Spinal Muscular Atrophy (SMA)**. * **Isoforms:** Different proteins produced from the same gene via alternative splicing are called isoforms.

Question 333: Defects in snRNPs cause which of the following genetic disorders?

A. Sickle cell anemia
B. Thalassemia (Correct Answer)
C. Marfan syndrome
D. Ehlers-Danlos syndrome

Explanation: ### Explanation **Correct Answer: B. Thalassemia** **Underlying Concept:** Small nuclear ribonucleoproteins (**snRNPs**, pronounced "snurps") are essential components of the **Spliceosome**. Their primary function is to remove introns from pre-mRNA and join exons together to form mature mRNA. Defects in the splicing process—specifically mutations at the splice donor or acceptor sites—lead to aberrant mRNA processing. In **$\beta$-thalassemia**, mutations often occur at these splice junctions, leading to the production of non-functional $\beta$-globin chains or a total absence of them, resulting in ineffective erythropoiesis. **Analysis of Incorrect Options:** * **A. Sickle cell anemia:** This is caused by a **missense mutation** (point mutation) in the $\beta$-globin gene where Glutamic acid is replaced by Valine at the 6th position. It does not involve splicing defects. * **C. Marfan syndrome:** This is an autosomal dominant disorder caused by mutations in the **FBN1 gene** (encoding Fibrillin-1), typically involving missense or nonsense mutations rather than snRNP-related splicing errors. * **D. Ehlers-Danlos syndrome:** This group of disorders results from defects in **collagen synthesis or processing** (e.g., mutations in COL5A1 or lysyl hydroxylase deficiency), not the splicing machinery. **High-Yield Clinical Pearls for NEET-PG:** * **Spliceosome Components:** Composed of snRNAs (U1, U2, U4, U5, U6) and specific proteins. * **Systemic Lupus Erythematosus (SLE):** Patients often develop **Anti-Smith (anti-Sm) antibodies**, which are directed against the proteins associated with snRNPs. This is a highly specific diagnostic marker for SLE. * **Spinal Muscular Atrophy (SMA):** Caused by a deficiency in the **SMN (Survival Motor Neuron) protein**, which is critical for the assembly of snRNPs. * **Rule of Thumb:** If a question mentions "splice site mutation" or "intron retention" in the context of anemia, think $\beta$-Thalassemia.

Question 334: Friedreich's ataxia is caused due to triplet repeats of which nucleotide sequence?

A. AGG
B. GAA (Correct Answer)
C. UAA
D. AUG

Explanation: **Explanation:** **Friedreich's Ataxia (FRDA)** is an autosomal recessive neurodegenerative disorder characterized by progressive ataxia, loss of deep tendon reflexes, and hypertrophic cardiomyopathy. **Why GAA is correct:** The molecular basis of Friedreich's Ataxia is the **expansion of a GAA trinucleotide repeat** located in the first intron of the **FXN gene** on chromosome 9. This gene encodes **Frataxin**, a mitochondrial protein involved in iron-sulfur cluster biogenesis. The expansion leads to transcriptional silencing (via heterochromatin formation), resulting in a deficiency of Frataxin. This causes mitochondrial iron overload, increased oxidative stress, and subsequent damage to the spinal cord (spinocerebellar tracts) and heart. **Analysis of Incorrect Options:** * **AGG:** This sequence is not typically associated with a major trinucleotide repeat disorder. However, **CGG** repeats are seen in Fragile X Syndrome. * **UAA:** This is a **stop codon** (Ochre). While mutations can create premature stop codons (nonsense mutations), it is not the basis for triplet repeat expansion diseases. * **AUG:** This is the **start codon** (codes for Methionine). It initiates translation and is not associated with expansion disorders. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Unlike most triplet repeat disorders (which are Autosomal Dominant), Friedreich's Ataxia is **Autosomal Recessive**. * **Anticipation:** It does *not* typically show significant clinical anticipation, unlike Huntington’s or Fragile X. * **Clinical Triad:** Progressive limb ataxia, absent knee/ankle jerks, and **Hypertrophic Cardiomyopathy** (the most common cause of death). * **Skeletal Deformities:** Kyphoscoliosis and **Pes Cavus** (high arched feet) are classic findings. * **Diabetes:** About 10-20% of patients develop Diabetes Mellitus due to pancreatic beta-cell dysfunction.

Question 335: Which of the following is a classic example of a missense mutation?

A. Thalassemia
B. Sickle cell disease (Correct Answer)
C. Sideroblastic anemia
D. Hemochromatosis

Explanation: **Explanation:** **Sickle Cell Disease (SCD)** is the classic prototype of a **missense mutation**. In SCD, a single base substitution occurs in the 6th codon of the $\beta$-globin gene (GAG $\rightarrow$ GTG). This point mutation results in the replacement of **Glutamic acid** (a polar, negatively charged amino acid) with **Valine** (a non-polar, hydrophobic amino acid). This single amino acid change causes the hemoglobin (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of red blood cells. **Analysis of Incorrect Options:** * **Thalassemia:** Most commonly results from **splice-site mutations** or **nonsense mutations** (leading to premature stop codons) in $\beta$-thalassemia, or large **gene deletions** in $\alpha$-thalassemia. It is a quantitative defect in globin chain synthesis, rather than a qualitative structural change. * **Sideroblastic Anemia:** This is primarily a defect in heme synthesis (often due to ALAS2 enzyme deficiency) rather than a specific point mutation in the globin gene. * **Hemochromatosis:** While the most common cause (HFE gene mutation, C282Y) is technically a missense mutation, it is not considered the "classic" teaching example for this genetic concept in medical biochemistry compared to Sickle Cell Disease. **High-Yield Clinical Pearls for NEET-PG:** * **Transition vs. Transversion:** The SCD mutation (A $\rightarrow$ T) is a **transversion** (purine to pyrimidine). * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because it loses the negative charge of glutamic acid. * **Sticky Patches:** The substitution of Valine creates "sticky" hydrophobic patches on the surface of the hemoglobin molecule.

Question 336: Which of the following is NOT a function of restriction endonucleases?

A. They cut both strands of double-stranded DNA.
B. The cut ends produced by restriction endonucleases can be sticky.
C. The cut ends produced by restriction endonucleases can be blunt.
D. They are non-specific to base pairs. (Correct Answer)

Explanation: **Explanation:** Restriction Endonucleases (REs), often called "molecular scissors," are enzymes primarily derived from bacteria where they serve as a defense mechanism against viral DNA. **Why Option D is the Correct Answer:** The defining characteristic of restriction endonucleases is their **high specificity**. They recognize and bind to specific DNA sequences, typically 4 to 8 base pairs long, known as **palindromic sequences** (reading the same 5'→3' on both strands). Because they only cut at these precise recognition sites, saying they are "non-specific" is factually incorrect, making it the right choice for this "NOT" question. **Analysis of Incorrect Options:** * **Option A:** REs function by hydrolyzing the phosphodiester bonds on **both strands** of the DNA double helix at specific points within or near the recognition site. * **Option B:** Many REs (like *EcoRI*) make staggered cuts, leaving short, single-stranded overhangs known as **sticky (cohesive) ends**. These are highly useful in recombinant DNA technology as they can easily hybridize with complementary sequences. * **Option C:** Some REs (like *SmaI*) cut straight across the DNA at the same position on both strands, resulting in **blunt ends**. These lack overhangs and are generally more difficult to ligate. **High-Yield Clinical Pearls for NEET-PG:** * **Type II REs** are the most commonly used in genetic engineering because they cut exactly at the recognition site and do not require ATP. * **Naming Convention:** The first letter is the Genus, the next two are the species, and the Roman numeral indicates the order of discovery (e.g., *EcoRI*: *Escherichia coli*, strain RY13, 1st enzyme). * **Restriction Fragment Length Polymorphism (RFLP):** A technique using REs to detect genetic variations/mutations, crucial in forensic medicine and prenatal diagnosis of diseases like Sickle Cell Anemia.

Question 337: What is the primary function of Type II restriction enzymes?

A. Methylation of specific DNA sequences.
B. Cleavage of specific palindromic DNA sequences. (Correct Answer)
C. Facilitation of protein digestion.
D. Maintenance of nascent protein unfolding.

Explanation: **Explanation:** **1. Why Option B is Correct:** Type II restriction enzymes (Restriction Endonucleases) are fundamental tools in molecular biology and recombinant DNA technology. Their primary function is to recognize specific, short nucleotide sequences—typically **4 to 8 base pairs long and palindromic** (reading the same 5' to 3' on both strands)—and cleave the phosphodiester backbone at or near that specific site. Unlike Type I or III, Type II enzymes do not require ATP and cut precisely, making them predictable for gene cloning and DNA mapping. **2. Why the Other Options are Incorrect:** * **Option A:** Methylation is performed by **Methyltransferases**. In bacteria, this serves as a defense mechanism (Restriction-Modification System) to protect host DNA from being degraded by its own restriction enzymes. * **Option C:** Protein digestion is the role of **proteases** (e.g., pepsin, trypsin), not restriction enzymes, which act exclusively on nucleic acids. * **Option D:** Maintaining protein unfolding is the function of **molecular chaperones** (e.g., Heat Shock Proteins like HSP70), which prevent premature folding or aggregation. **3. NEET-PG High-Yield Pearls:** * **Nomenclature:** The first letter is the genus, the next two are the species, and the Roman numeral denotes the order of discovery (e.g., *EcoRI* from *E. coli*). * **Blunt vs. Sticky Ends:** Some enzymes (like *HpaI*) produce blunt ends, while others (like *EcoRI*) produce cohesive "sticky" ends, which are more efficient for ligation in genetic engineering. * **Clinical Application:** Restriction Fragment Length Polymorphism (**RFLP**) uses these enzymes to detect mutations (e.g., Sickle Cell Anemia, where a mutation abolishes a *MstII* recognition site).

Question 338: Which of the following is required for certain types of eukaryotic protein synthesis but not for prokaryotic protein synthesis?

A. Ribosomal RNA
B. Messenger RNA
C. Signal recognition particle (Correct Answer)
D. Peptidyl transferase

Explanation: ### Explanation The correct answer is **Signal Recognition Particle (SRP)**. **1. Why SRP is the Correct Answer:** In eukaryotes, proteins destined for secretion, integration into the plasma membrane, or lysosomal enzymes require **co-translational translocation** into the Rough Endoplasmic Reticulum (RER). The **Signal Recognition Particle (SRP)** is a cytosolic ribonucleoprotein that recognizes the N-terminal signal sequence of a nascent polypeptide. It temporarily halts translation and docks the ribosome-mRNA complex to the SRP receptor on the RER membrane. While prokaryotes have simplified SRP-like homologs (like Ffh/4.5S RNA), the classic SRP-dependent pathway involving a membrane-bound ER system is a hallmark of **eukaryotic** protein targeting. In prokaryotes, most translation occurs freely in the cytoplasm, and protein secretion (via the Sec pathway) often occurs post-translationally. **2. Analysis of Incorrect Options:** * **A & B (rRNA and mRNA):** These are fundamental components of the translation machinery in **all** living organisms. Both prokaryotes (70S) and eukaryotes (80S) require mRNA as a template and rRNA to form the structural and catalytic core of the ribosome. * **D (Peptidyl transferase):** This is the primary enzyme activity (a ribozyme) responsible for peptide bond formation. It is located in the large ribosomal subunit (23S in prokaryotes; 28S in eukaryotes) and is essential for protein synthesis in both domains. **3. High-Yield Clinical Pearls for NEET-PG:** * **I-Cell Disease (Mucolipidosis II):** A clinical correlation of protein targeting. It is caused by a deficiency in *N-acetylglucosaminyl-1-phosphotransferase*, leading to a failure to tag lysosomal enzymes with **Mannose-6-Phosphate**. Consequently, enzymes are secreted extracellularly rather than being targeted to lysosomes. * **SRP Composition:** In eukaryotes, it consists of **7S RNA** and six different proteins. * **Antibiotic Target:** Many antibiotics (e.g., Macrolides, Tetracyclines) exploit the structural differences between prokaryotic (70S) and eukaryotic (80S) ribosomes to achieve selective toxicity.

Question 339: Termination of translation is caused by all of the following except:

A. Peptidyl transferase (Correct Answer)
B. 48S complex
C. RF-1
D. UAA

Explanation: ### Explanation **1. Why Peptidyl Transferase is the Correct Answer:** Peptidyl transferase is an enzyme (specifically a ribozyme) integrated into the large ribosomal subunit. Its primary role occurs during the **elongation phase** of translation, where it catalyzes the formation of peptide bonds between amino acids. While it does play a role in the final hydrolysis of the peptide chain during termination, it is **not a cause** of termination. Termination is triggered by the presence of a stop codon and the binding of release factors, not the enzyme itself. **2. Analysis of Incorrect Options:** * **UAA (Option D):** This is one of the three **stop codons** (UAA, UAG, UGA). Termination begins when one of these codons enters the 'A' site of the ribosome, as there are no tRNAs with matching anticodons. * **RF-1 (Option C):** **Release Factors (RFs)** are proteins that recognize stop codons. RF-1 specifically recognizes UAA and UAG. They mimic the shape of tRNA, enter the A-site, and trigger the release of the completed polypeptide chain. * **48S Complex (Option B):** This is an **initiation complex** (formed by the 40S subunit, mRNA, and Met-tRNA). Since the question asks which of the following is *not* a cause of termination, the 48S complex is technically a distractor because it belongs to the **initiation phase**, not termination. However, in the context of standard medical entrance exams, Peptidyl transferase is the most "functional" enzyme associated with elongation, making it the primary answer. **3. High-Yield Clinical Pearls for NEET-PG:** * **Stop Codons:** UAA (Ochre), UAG (Amber), UGA (Opal). * **Ribozyme:** In eukaryotes, the **28S rRNA** acts as the peptidyl transferase; in prokaryotes, it is the **23S rRNA**. * **Energy Requirement:** Translation is energetically expensive; **GTP** is required for initiation, translocation (elongation), and termination. * **Antibiotic Link:** Chloramphenicol inhibits prokaryotic peptidyl transferase, preventing peptide bond formation.

Question 340: What are the small DNA segments termed that are produced during discontinuous DNA replication?

A. Okazaki fragments (Correct Answer)
B. Crick strands
C. Watson fragments
D. Tsuneko strands

Explanation: **Explanation:** DNA replication is **semi-discontinuous** because DNA polymerase can only synthesize DNA in the **5' to 3' direction**. While the leading strand is synthesized continuously toward the replication fork, the lagging strand must be synthesized in the opposite direction (away from the fork). **1. Why Okazaki fragments is correct:** To accommodate the 5' to 3' synthesis requirement on the lagging strand, DNA is synthesized in short, discrete segments. These segments are called **Okazaki fragments** (named after Reiji and Tsuneko Okazaki). Each fragment begins with an RNA primer synthesized by **Primase**, which is later removed and replaced with DNA by DNA Polymerase I, and the nicks are sealed by **DNA Ligase**. **2. Why other options are incorrect:** * **Crick strands / Watson fragments:** These are distractors named after Francis Crick and James Watson, who discovered the double-helix structure of DNA. There are no specific "fragments" or "strands" named after them in the context of replication. * **Tsuneko strands:** While Tsuneko Okazaki was the co-discoverer of these fragments, the term "Tsuneko strands" is not standard biological nomenclature. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **DNA Ligase:** The enzyme responsible for joining Okazaki fragments. A deficiency in DNA ligase can lead to impaired DNA repair and replication (e.g., Bloom Syndrome). * **Directionality:** Remember that both strands are *read* 3'→5' but *synthesized* 5'→3'. * **Length:** In eukaryotes, Okazaki fragments are typically 100–200 nucleotides long, whereas in prokaryotes, they are much longer (1000–2000 nucleotides). * **Topoisomerase:** Relieves torsional strain (supercoiling) ahead of the replication fork; targeted by drugs like **Fluoroquinolones** (DNA Gyrase) and **Etoposide/Irinotecan**.

Practice by Chapter

DNA Replication and Repair Mechanisms

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Transcription Factors and Gene Regulation

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Epigenetics and DNA Methylation

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RNA Processing and Splicing

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miRNA and RNA Interference

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Protein Synthesis and Post-Translational Modifications

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Genomics and Human Genome Project

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Single Nucleotide Polymorphisms

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Gene Therapy Approaches

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CRISPR-Cas9 and Genome Editing

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DNA Fingerprinting and Forensics

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Molecular Basis of Genetic Diseases

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