Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 41: A ten-year-old child presents with aggressive behavior, poor concentration, joint pain, reduced urinary output, and a history of self-mutilative behavior, specifically mutilating his fingers. Which of the following enzymes is likely to be deficient in this child?

A. Hypoxanthine-guanine phosphoribosyltransferase (HGPrtase) (Correct Answer)
B. Adenosine deaminase
C. Alkaline phosphatase (APase)
D. Acid maltase

Explanation: ### Explanation The clinical presentation described—**self-mutilation** (biting fingers/lips), **aggressive behavior**, and **joint pain** (gouty arthritis)—is classic for **Lesch-Nyhan Syndrome**. **1. Why Option A is Correct:** Lesch-Nyhan Syndrome is an X-linked recessive disorder caused by a complete deficiency of **Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)**. This enzyme is crucial for the **Purine Salvage Pathway**, where it converts Hypoxanthine to IMP and Guanine to GMP. * **Pathophysiology:** When HGPRT is deficient, PRPP levels increase and purines cannot be salvaged. This leads to massive overproduction of uric acid (hyperuricemia), causing joint pain (gout) and renal issues (reduced urinary output/stones). The neurological symptoms and self-mutilation are linked to dopamine dysfunction in the basal ganglia, though the exact mechanism remains complex. **2. Why the Other Options are Incorrect:** * **Option B (Adenosine Deaminase):** Deficiency leads to **Severe Combined Immunodeficiency (SCID)**. Patients present with recurrent infections due to T and B cell dysfunction, not self-mutilation. * **Option C (Alkaline Phosphatase):** Low levels are seen in Hypophosphatasia (bone mineralization defects); high levels are seen in bone diseases or cholestasis. It has no link to purine metabolism. * **Option D (Acid Maltase):** Also known as α-1,4-glucosidase. Its deficiency causes **Pompe Disease** (GSD Type II), characterized by cardiomegaly and muscle weakness. **3. Clinical Pearls for NEET-PG:** * **Inheritance:** X-linked Recessive (usually affects males). * **Mnemonic (HGPRT):** **H**yperuricemia, **G**out, **P**issed off (aggression/self-mutilation), **R**etardation (intellectual disability), dys**T**onia. * **Biochemical Marker:** Elevated serum uric acid and "orange sand" (sodium urate crystals) in the diaper of infants. * **Treatment:** Allopurinol or Febuxostat (to manage uric acid), but these do not improve neurological symptoms.

Question 42: Uric acid is converted to allantoin in:

A. Catabolism of pyrimidines
B. Catabolism of purines (Correct Answer)
C. Synthesis of pyrimidines
D. Synthesis of purines

Explanation: **Explanation:** **1. Why the correct answer is right:** The catabolism of **purines** (Adenine and Guanine) involves their conversion into hypoxanthine and xanthine, which are eventually oxidized into **uric acid** by the enzyme **xanthine oxidase**. In humans and higher primates, uric acid is the final excretory product. However, in most other mammals, the enzyme **urate oxidase (uricase)** further oxidizes uric acid into **allantoin**, a more water-soluble compound. While humans lack this enzyme, the concept is high-yield for understanding comparative biochemistry and the mechanism of certain drugs. **2. Why the incorrect options are wrong:** * **A & C (Pyrimidine Metabolism):** Pyrimidine catabolism (Cytosine, Uracil, Thymine) does not produce uric acid. Instead, it leads to highly soluble end-products like **β-alanine** and **β-aminoisobutyrate**, which are excreted or converted to CO₂ and NH₃. * **D (Purine Synthesis):** Purine synthesis (De novo or Salvage pathways) focuses on building the purine ring (IMP, AMP, GMP) from precursors like glycine, glutamine, and aspartate. It does not involve the breakdown products like uric acid or allantoin. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Gout:** Caused by hyperuricemia (excess uric acid). Humans lack uricase, making us prone to gout if uric acid production exceeds renal excretion. * **Rasburicase:** A recombinant version of the enzyme **urate oxidase** used clinically to treat **Tumor Lysis Syndrome**. It works by converting uric acid into the soluble allantoin, preventing renal crystals. * **Xanthine Oxidase Inhibitors:** Drugs like **Allopurinol** and **Febuxostat** inhibit the production of uric acid from xanthine/hypoxanthine. * **Final Products:** Remember: Purines → Uric acid; Pyrimidines → β-amino acids.

Question 43: Where does the synthesis of rRNA take place?

A. Cytosol
B. Nucleus
C. Nucleolus (Correct Answer)
D. Mitochondria

Explanation: **Explanation:** The **nucleolus** is a non-membrane-bound sub-compartment within the nucleus and is the primary site for **ribosomal RNA (rRNA) synthesis** and ribosome biogenesis. Specifically, RNA Polymerase I transcribes the 45S precursor rRNA, which is then processed into the 18S, 5.8S, and 28S subunits within the nucleolus. (Note: The 5S rRNA is the only exception, as it is transcribed by RNA Polymerase III in the nucleoplasm). **Analysis of Options:** * **A. Cytosol:** This is the site of **translation** (protein synthesis) and the synthesis of tRNA and mRNA occurs in the nucleus before being exported here. * **B. Nucleus:** While the nucleolus is located inside the nucleus, the question asks for the specific site. The nucleoplasm is where mRNA and tRNA are synthesized, but rRNA is specifically localized to the nucleolus. * **C. Nucleolus (Correct):** The "ribosome factory" of the cell where rRNA transcription and assembly of ribosomal subunits occur. * **D. Mitochondria:** Mitochondria have their own DNA and synthesize their own specific mtrRNA, but the bulk of cellular rRNA required for cytoplasmic ribosomes is produced in the nucleolus. **High-Yield NEET-PG Pearls:** * **RNA Polymerase I:** Transcribes most rRNA (18S, 5.8S, 28S). *Mnemonic: R-M-T (Pol I-II-III).* * **Nucleolar Organizer Regions (NORs):** These are chromosomal regions (on chromosomes 13, 14, 15, 21, and 22) that contain the genes for rRNA and form the nucleolus. * **Clinical Link:** The size of the nucleolus increases in highly metabolically active cells or malignant cells due to increased protein synthesis demands.

Question 44: A 12-year-old child presents with mental retardation and a history of self-mutilation. Which of the following tests will help in determining the diagnosis?

A. Serum Lead Levels
B. Serum Alkaline Phosphatase Levels
C. Serum Lactate Dehydrogenase Levels
D. Serum Uric Acid Levels (Correct Answer)

Explanation: ### Explanation The clinical presentation of **mental retardation** and **self-mutilation** (such as biting of lips and fingers) in a young child is a classic hallmark of **Lesch-Nyhan Syndrome**. **1. Why Serum Uric Acid Levels is the Correct Answer:** Lesch-Nyhan Syndrome is an X-linked recessive disorder caused by a complete deficiency of the enzyme **Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT)**. This enzyme is crucial for the **Purine Salvage Pathway**. When HGPRT is deficient: * Hypoxanthine and Guanine cannot be recycled into IMP and GMP. * This leads to an accumulation of PRPP (Phosphoribosyl pyrophosphate) and increased *de novo* purine synthesis. * The excess purines are shunted into the catabolic pathway, resulting in severe **Hyperuricemia** (elevated serum uric acid). High uric acid levels lead to gouty arthritis, nephrolithiasis, and the characteristic neurological symptoms. **2. Why the Other Options are Incorrect:** * **A. Serum Lead Levels:** While lead poisoning causes neurological deficits and irritability, it does not typically present with the specific compulsive self-mutilation seen in this case. * **B. Serum Alkaline Phosphatase (ALP):** ALP is a marker for bone turnover or hepatobiliary obstruction and has no diagnostic value for purine metabolism disorders. * **C. Serum Lactate Dehydrogenase (LDH):** LDH is a non-specific marker of cell turnover or hemolysis and does not point toward a specific neurogenetic diagnosis. **3. High-Yield Clinical Pearls for NEET-PG:** * **Enzyme Deficiency:** HGPRT (converts Hypoxanthine → IMP and Guanine → GMP). * **Mnemonic (HGPRT):** **H**yperuricemia, **G**out, **P**issed off (self-mutilation/aggression), **R**etardation, **T**one (dystonia). * **Diagnostic Clue:** "Orange sand" crystals (sodium urate) found in the diapers of affected infants. * **Treatment:** Allopurinol or Febuxostat (to manage uric acid), but these do not reverse neurological symptoms.

Question 45: At physiological pH, what is the charge of DNA?

A. Acidic
B. Negatively charged
C. Amphipathic
D. All of the above (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. All of the above** because DNA (Deoxyribonucleic Acid) possesses distinct chemical properties dictated by its structure at physiological pH (~7.4). 1. **Negatively Charged:** DNA contains a sugar-phosphate backbone. Each phosphate group has a pKa near 1.0, meaning that at physiological pH, the hydroxyl groups are deprotonated, leaving a net negative charge on each phosphate. 2. **Acidic:** Due to the presence of these phosphoric acid derivatives (phosphate groups) that donate protons, DNA is chemically classified as an acid. This is reflected in its name: Deoxyribonucleic *Acid*. 3. **Amphipathic:** DNA exhibits dual solubility characteristics. The exterior sugar-phosphate backbone is **hydrophilic** (polar), allowing it to interact with the aqueous cellular environment. Conversely, the nitrogenous bases (adenine, guanine, cytosine, thymine) are **hydrophobic** (non-polar) and are stacked in the interior of the double helix to avoid water. **Why other options are part of the whole:** While B is the most commonly cited characteristic in basic biology, DNA is simultaneously acidic by definition and amphipathic by structural arrangement. Therefore, "All of the above" is the most comprehensive description. **High-Yield Clinical Pearls for NEET-PG:** * **Histone Interaction:** The negative charge of DNA is crucial for its interaction with **Histones**, which are rich in basic amino acids like **Lysine and Arginine** (positively charged). * **Electrophoresis:** In agarose gel electrophoresis, DNA migrates toward the **Anode** (positive electrode) because of its inherent negative charge. * **Hyperchromicity:** When DNA is denatured (melted), its UV light absorption at 260 nm increases; this is due to the disruption of the hydrophobic stacking of bases.

Question 46: Orotic aciduria is due to deficiency of which enzyme?

A. Decarboxylase (Correct Answer)
B. Isomerase
C. Tyrosinase
D. Homogentisate oxidase

Explanation: **Explanation:** **Orotic Aciduria** is a rare autosomal recessive disorder of **pyrimidine synthesis**. It is caused by a deficiency in the bifunctional enzyme **UMP Synthase**, which possesses two distinct catalytic activities: **Orotate phosphoribosyltransferase (OPRT)** and **Orotidylate decarboxylase**. 1. **Why Option A is correct:** In the de novo pyrimidine pathway, Orotic acid is converted to Orotidine monophosphate (OMP) by OPRT. Subsequently, OMP is converted to Uridine monophosphate (UMP) by the enzyme **Orotidylate decarboxylase**. A deficiency in this decarboxylase activity leads to the accumulation of orotic acid in the blood and its excretion in urine (Orotic aciduria). 2. **Why other options are incorrect:** * **Isomerase:** These enzymes catalyze structural rearrangements (e.g., Phosphohexose isomerase in glycolysis) and are not involved in the orotic acid pathway. * **Tyrosinase:** Deficiency of this enzyme leads to **Albinism**, as it is required for melanin synthesis from tyrosine. * **Homogentisate oxidase:** Deficiency of this enzyme leads to **Alkaptonuria**, characterized by the accumulation of homogentisic acid, resulting in dark urine and ochronosis. **Clinical Pearls for NEET-PG:** * **Presentation:** Patients typically present with **megaloblastic anemia** that is refractory to Vitamin B12 and Folate therapy, along with growth retardation and orotic acid crystals in the urine. * **Treatment:** Administration of **Uridine** (Uridine triacetate) bypasses the metabolic block, providing the necessary pyrimidines for DNA/RNA synthesis and feedback-inhibiting the pathway to reduce orotic acid production. * **Differential Diagnosis:** Distinguish from **Ornithine Transcarbamylase (OTC) deficiency** (Urea cycle disorder), which also shows orotic aciduria but is accompanied by **hyperammonemia** and decreased BUN.

Question 47: Azaserine inhibits which of the following enzymes?

A. Glycinamide ribonucleotide synthetase
B. Glycinamide ribonucleotide transformylase
C. Formyl glycinamide ribonucleotide amidotransferase (Correct Answer)
D. Inosine monophosphate synthase

Explanation: ### Explanation **Correct Option: C. Formyl glycinamide ribonucleotide (FGAR) amidotransferase** Azaserine (and its analog, 6-diazo-5-oxo-L-norleucine or DON) acts as a potent **glutamine antagonist**. In the *de novo* purine synthesis pathway, several steps require glutamine as a nitrogen donor. Azaserine structurally mimics glutamine and irreversibly binds to the enzymes that utilize it. The conversion of **Formyl glycinamide ribonucleotide (FGAR)** to **Formyl glycinamide ribosylamine (FGAM)** is catalyzed by **FGAR amidotransferase**, which requires glutamine. By inhibiting this enzyme, azaserine halts purine nucleotide synthesis, making it a significant tool in biochemical research and a potential (though toxic) anti-tumor agent. **Analysis of Incorrect Options:** * **A. Glycinamide ribonucleotide (GAR) synthetase:** This is the second step of purine synthesis. It requires glycine and ATP, not glutamine; therefore, it is not inhibited by azaserine. * **B. Glycinamide ribonucleotide (GAR) transformylase:** This enzyme catalyzes the transfer of a formyl group from N10-formyl tetrahydrofolate. It is inhibited by folate analogs (like methotrexate), not glutamine antagonists. * **D. Inosine monophosphate (IMP) synthase:** This refers to the final steps of the pathway (bifunctional ATIC enzyme). It involves cyclohydrolase and transformylase activities, neither of which is the primary target for azaserine. **High-Yield Clinical Pearls for NEET-PG:** * **Glutamine Antagonists:** Azaserine, DON, and Acivicin inhibit three major steps in purine synthesis: 1. PRPP Amidotransferase (the rate-limiting step). 2. **FGAR Amidotransferase (the most commonly tested target).** 3. GMP Synthetase (conversion of XMP to GMP). * **Rate-Limiting Step:** Glutamine-PRPP amidotransferase is the committed step of purine synthesis, inhibited by IMP, AMP, and GMP (feedback inhibition). * **Source of Atoms:** Remember that Glutamine provides **N3 and N9** atoms to the purine ring.

Question 48: PRPP participates in purine biosynthesis in which step?

A. Serves as a scaffold for assembly of the purine ring (Correct Answer)
B. Post assembly of the purine ring
C. Both
D. None

Explanation: **Explanation:** In **Purine biosynthesis**, the assembly of the heterocyclic ring occurs **directly onto** a pre-existing ribose-5-phosphate molecule. This is a fundamental distinction from pyrimidine synthesis. **Why Option A is correct:** The process begins with **PRPP (5-Phosphoribosyl-1-pyrophosphate)**. In the first committed step, the enzyme *PRPP glutamyl amidotransferase* replaces the pyrophosphate group of PRPP with an amino group from glutamine. This ribose-5-phosphate unit then acts as the **scaffold (foundation)** upon which atoms from various donors (glycine, aspartate, glutamine, folate, and $CO_2$) are added sequentially to build the Inosine Monophosphate (IMP) ring. **Why other options are incorrect:** * **Option B:** This describes **Pyrimidine biosynthesis**. In pyrimidines, the base (Orotic acid) is synthesized first as a free ring and is only *subsequently* attached to PRPP. * **Option C & D:** These are incorrect because the biochemical sequence is specific to the "scaffold" method for purines. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** PRPP glutamyl amidotransferase (inhibited by AMP, GMP, and IMP via feedback inhibition). * **PRPP Synthetase:** The enzyme that forms PRPP from Ribose-5-P. Overactivity of this enzyme leads to **Hyperuricemia and Gout** due to overproduction of purines. * **Von Gierke’s Disease:** Increased G-6-P shunts into the HMP pathway, increasing Ribose-5-P and PRPP levels, which explains the associated hyperuricemia. * **Lesch-Nyhan Syndrome:** Failure of the salvage pathway (HGPRT deficiency) leads to an accumulation of PRPP, which further stimulates *de novo* purine synthesis.

Question 49: Which of the following is not a pyrimidine?

A. Uracil
B. Thymine
C. Adenine (Correct Answer)
D. Cytosine

Explanation: **Explanation:** The nitrogenous bases in nucleic acids are classified into two categories based on their chemical structure: **Purines** and **Pyrimidines**. **Why Adenine is the correct answer:** Adenine is a **Purine**, not a pyrimidine. Purines are heterocyclic aromatic organic compounds consisting of a **double-ring structure** (a six-membered pyrimidine ring fused to a five-membered imidazole ring). The two primary purines found in DNA and RNA are Adenine (A) and Guanine (G). A helpful mnemonic to remember this is: *"**Pure** **A**s **G**old"* (Purines = Adenine, Guanine). **Analysis of Incorrect Options:** * **Uracil (A):** A pyrimidine base found exclusively in **RNA**, where it replaces thymine and pairs with adenine. * **Thymine (B):** A pyrimidine base found exclusively in **DNA**. It is also known as 5-methyluracil. * **Cytosine (D):** A pyrimidine base found in both DNA and RNA. It pairs with guanine via three hydrogen bonds. **High-Yield NEET-PG Pearls:** 1. **Structure:** Pyrimidines have a **single-ring** structure. Mnemonic: *"**CUT** the **PY**"* (Cytosine, Uracil, and Thymine are Pyrimidines). 2. **Metabolism:** Purine catabolism leads to the production of **Uric Acid**. Excess uric acid results in Gout. Pyrimidine catabolism, conversely, produces highly soluble products like β-alanine and β-aminoisobutyrate. 3. **De novo Synthesis:** The amino acids required for purine synthesis are **Glycine, Aspartate, and Glutamine**, whereas pyrimidine synthesis primarily requires **Aspartate and Glutamine**. 4. **Drug Link:** **5-Fluorouracil (5-FU)** is a pyrimidine analog used in cancer chemotherapy to inhibit thymidylate synthase.

Question 50: Which type of RNA has the highest percentage of modified bases?

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

Explanation: **Explanation:** **Why tRNA is the correct answer:** Transfer RNA (tRNA) contains the highest percentage of modified bases, accounting for approximately **10–15%** of its total nucleotides. These modifications occur post-transcriptionally and are essential for the structural stability, L-shaped folding, and accurate codon-anticodon recognition. Common modified bases include **Pseudouridine (ψ)**, **Dihydrouridine (D)**, **Inosine (I)**, and **Ribothymidine (T)**. These modifications are concentrated in specific regions like the D-loop and TψC loop, which are critical for the tRNA's interaction with the ribosome and aminoacyl-tRNA synthetases. **Analysis of Incorrect Options:** * **mRNA (Messenger RNA):** Contains the least amount of modified bases. While it undergoes 5' capping (7-methylguanosine) and 3' polyadenylation, the internal sequence remains largely unmodified to ensure accurate translation of the genetic code. * **rRNA (Ribosomal RNA):** While rRNA does undergo some modifications (like ribose methylation and pseudouridylation), they are significantly less frequent than those found in tRNA. rRNA is the most **abundant** RNA in the cell (80%), but not the most modified. * **snRNA (Small Nuclear RNA):** Involved in splicing (spliceosomes), snRNAs contain some modifications to assist in RNA-protein interactions, but the density does not match that of tRNA. **High-Yield Clinical Pearls for NEET-PG:** * **Abundance Rule:** **r**RNA is the most **r**ampant (80%), **t**RNA is the **t**iniest/smallest, and **m**RNA is the **m**essy/most heterogeneous in size. * **Pseudouridine:** Known as the "fifth nucleotide," it is a hallmark of tRNA and is excreted in urine; elevated levels can be a biomarker for high cell turnover (e.g., leukemia). * **Wobble Hypothesis:** Inosine (a modified base) at the 5' end of the tRNA anticodon allows for non-traditional base pairing, enabling one tRNA to recognize multiple codons.

Practice by Chapter

Nucleotide Structure and Function

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DNA Structure and Replication

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RNA Structure and Types

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Transcription: RNA Synthesis

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Post-Transcriptional Modifications

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Translation: Protein Synthesis

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Genetic Code and Codon Usage

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Regulation of Gene Expression

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Mutations and DNA Repair

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Purine Metabolism and Disorders

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Pyrimidine Metabolism and Disorders

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Nucleotide Degradation and Salvage Pathways

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