Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 121: An RNA molecule is extracted from a culture of Gram-negative bacteria. This extract contained dihydrouracil, pseudouridine, and thymidine residues. Which of the following is the most likely composition of the 3' end of this molecule?

A. AUG
B. CCA (Correct Answer)
C. UAG
D. PolyA

Explanation: **Explanation:** The presence of modified bases such as **dihydrouracil (D), pseudouridine (ψ), and ribothymidine (T)** is the classic biochemical hallmark of **Transfer RNA (tRNA)**. These bases are formed post-transcriptionally and are essential for the structural integrity and function of the tRNA molecule (e.g., the TψC loop and the D-loop). All mature tRNA molecules, whether in prokaryotes or eukaryotes, end with the sequence **5’-CCA-3’** at their **3’ terminus**. This is known as the **acceptor stem**. The amino acid is covalently attached to the 3’-hydroxyl group of the adenosine (A) residue by the enzyme aminoacyl-tRNA synthetase. **Analysis of Options:** * **B (CCA): Correct.** This is the universal attachment site for amino acids at the 3' end of tRNA. * **A (AUG): Incorrect.** This is the universal **start codon** found on mRNA, which codes for Methionine. * **C (UAG): Incorrect.** This is the **Amber stop codon**, one of the three codons that signal the termination of translation on mRNA. * **D (PolyA): Incorrect.** Poly-A tails are added to the 3' end of **eukaryotic mRNA** for stability; they are not found in tRNA or typically in bacterial mRNA. **High-Yield NEET-PG Pearls:** * **Post-transcriptional modification:** In prokaryotes, the CCA sequence may be encoded genetically, but in eukaryotes, it is added post-transcriptionally by the enzyme **tRNA nucleotidyltransferase**. * **Charging:** The process of adding an amino acid to the CCA end is called "charging" or aminoacylation. * **Cloverleaf vs. L-shape:** tRNA has a 2D "cloverleaf" secondary structure but a 3D "L-shaped" tertiary structure.

Question 122: What is the first product of purine metabolism?

A. Uric acid
B. Xanthine (Correct Answer)
C. P-alanine
D. CO2

Explanation: **Explanation:** The catabolism of purine nucleotides (AMP and GMP) involves several enzymatic steps that converge at a common intermediate before the final excretory product is formed. **Why Xanthine is correct:** Purine degradation follows two pathways that meet at **Xanthine**: 1. **AMP pathway:** Adenosine is converted to Inosine, then to Hypoxanthine, which is oxidized to **Xanthine** by the enzyme *Xanthine Oxidase*. 2. **GMP pathway:** Guanosine is converted to Guanine, which undergoes deamination to directly form **Xanthine**. Therefore, Xanthine is considered the first common intermediate/product where both purine arms meet during catabolism. **Why other options are incorrect:** * **A. Uric acid:** This is the **final** end product of purine metabolism in humans, formed by the oxidation of xanthine. * **C. P-alanine (Beta-alanine):** This is a catabolic end product of **pyrimidine** metabolism (specifically Uracil and Cytosine), not purines. * **D. CO2:** While CO2 is a byproduct of many metabolic pathways, it is a major end product of **pyrimidine** catabolism (along with ammonia), whereas purines are excreted as uric acid. **High-Yield Clinical Pearls for NEET-PG:** * **Xanthine Oxidase:** The key regulatory enzyme that converts Hypoxanthine → Xanthine → Uric acid. It is inhibited by **Allopurinol** (a suicide inhibitor) used in Gout. * **Gout:** Characterized by hyperuricemia leading to the deposition of monosodium urate crystals in joints. * **Von Gierke’s Disease:** Associated with secondary hyperuricemia due to increased pentose phosphate pathway activity, leading to increased purine synthesis and breakdown. * **Lesch-Nyhan Syndrome:** Deficiency of HGPRT (salvage pathway) leads to excessive de novo purine synthesis and massive uric acid production.

Question 123: Which amino acid is used in the synthesis of purines?

A. Glycine (Correct Answer)
B. Ornithine
C. Alanine
D. Threonine

Explanation: **Explanation:** The synthesis of the purine ring (Adenine and Guanine) is a complex process that occurs primarily in the liver. The purine skeleton is built atom-by-atom onto a ribose-5-phosphate base. **Why Glycine is Correct:** Glycine is a crucial precursor in de novo purine synthesis. It contributes three specific atoms to the purine nucleus: **C4, C5, and N7**. It is the only amino acid that is incorporated into the ring in its entirety. **Analysis of Incorrect Options:** * **Ornithine:** This is an intermediate in the Urea cycle. It is not involved in the synthesis of nucleic acids. * **Alanine:** While a common glucogenic amino acid, it does not contribute atoms to the purine or pyrimidine rings. * **Threonine:** This is an essential amino acid but plays no role in the structural formation of nitrogenous bases. **High-Yield NEET-PG Pearls:** To master purine synthesis questions, remember the "Sources of Purine Atoms" mnemonic: 1. **Glycine:** C4, C5, N7 (The entire molecule). 2. **Aspartate:** N1. 3. **Glutamine (Amide N):** N3 and N9. 4. **Tetrahydrofolate (N10-formyl THF):** C2 and C8. 5. **CO₂ (Respiratory):** C6. **Clinical Correlation:** The rate-limiting step of this pathway is catalyzed by **PRPP glutamyl amidotransferase**. Drugs like **Methotrexate** and **6-Mercaptopurine** interfere with purine synthesis, making them effective as anticancer and immunosuppressive agents. Conversely, the **Salvage Pathway** (using HGPRT) is vital for the brain; its deficiency leads to **Lesch-Nyhan Syndrome**.

Question 124: An 8-month-old child presents with a tendency to compulsively bite his fingers. The mother reports noticing yellow-orange crystals in his diapers. A genetic defect in which of the following pathways should be suspected?

A. Aromatic amino acid metabolism
B. Branched chain amino acid metabolism
C. Purine metabolism (Correct Answer)
D. Pyrimidine metabolism

Explanation: **Explanation:** The clinical presentation of self-mutilation (compulsive finger biting) and "orange-sand" crystals in the diaper is a classic description of **Lesch-Nyhan Syndrome**. This is an X-linked recessive disorder caused by a complete deficiency of the enzyme **Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT)**. **1. Why Purine Metabolism is Correct:** HGPRT is a key enzyme in the **Purine Salvage Pathway**. It normally converts hypoxanthine to IMP and guanine to GMP. When HGPRT is deficient, these purine bases cannot be salvaged and are instead degraded into **Uric Acid**. The resulting hyperuricemia leads to the formation of sodium urate crystals (orange crystals in diapers), gouty arthritis, and severe neurological symptoms, including intellectual disability and characteristic self-mutilating behavior. **2. Why Incorrect Options are Wrong:** * **Aromatic amino acid metabolism:** Defects here lead to conditions like Phenylketonuria (mousy odor) or Alkaptonuria (darkening of urine), not self-mutilation or urate crystals. * **Branched-chain amino acid metabolism:** Defects lead to Maple Syrup Urine Disease (MSUD), characterized by a burnt-sugar urine odor and neurological deterioration, but not uric acid stones. * **Pyrimidine metabolism:** Defects (like Orotic Aciduria) cause megaloblastic anemia and growth retardation, but do not present with hyperuricemia or self-mutilation. **Clinical Pearls for NEET-PG:** * **Mnemonic for HGPRT deficiency:** **H**yperuricemia, **G**out, **P**issed off (aggression/self-mutilation), **R**etardation, **T**one (dystonia). * **Diagnostic finding:** Elevated serum uric acid and elevated **PRPP (Phosphoribosyl pyrophosphate)** levels (due to decreased utilization in salvage). * **Treatment:** Allopurinol (manages uric acid but does not fix neurological symptoms).

Question 125: What is the end product of purine metabolism in non-primate mammals?

A. Uric acid
B. Ammonia
C. Urea
D. Allantoin (Correct Answer)

Explanation: **Explanation:** The metabolism of purine nucleotides (Adenine and Guanine) follows a specific catabolic pathway. In humans and higher primates, the final product is **Uric acid**. However, in most other mammals (non-primates), the enzyme **Urate oxidase (Uricase)** further oxidizes uric acid into a more soluble compound called **Allantoin**. 1. **Why Allantoin is correct:** Non-primate mammals possess the enzyme **Uricase**, which converts the relatively insoluble uric acid into **allantoin**. Allantoin is highly water-soluble and easily excreted by the kidneys. Humans lack this enzyme due to a functional mutation in the urate oxidase gene during evolution. 2. **Why other options are incorrect:** * **Uric acid:** This is the end product in **humans, higher primates, birds, and reptiles**. In humans, the absence of uricase leads to higher serum urate levels, which can predispose to Gout. * **Ammonia & Urea:** These are the primary end products of **protein (amino acid) metabolism** and the nitrogen cycle, not the specific catabolic pathway of the purine ring. **High-Yield Clinical Pearls for NEET-PG:** * **Rasburicase/Pegloticase:** These are recombinant forms of the enzyme **Urate oxidase** used clinically to treat severe gout or Tumor Lysis Syndrome by converting uric acid to allantoin. * **Xanthine Oxidase:** This is the key regulatory enzyme that converts Hypoxanthine to Xanthine and Xanthine to Uric acid. It is inhibited by **Allopurinol** and **Febuxostat**. * **Solubility:** Uric acid is poorly soluble at acidic pH; Allantoin is 10-100 times more soluble than uric acid.

Question 126: RNA is present in which of the following locations?

A. Cytoplasm
B. Nucleus
C. Ribosome
D. All of the above (Correct Answer)

Explanation: **Explanation:** RNA (Ribonucleic Acid) is a versatile molecule involved in protein synthesis and gene regulation, distributed across various cellular compartments depending on its specific type and function. 1. **Nucleus:** This is the site of **transcription**. DNA is used as a template to synthesize mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA). Specifically, the **nucleolus** is the dense region within the nucleus where rRNA is synthesized and ribosomal subunits are assembled. 2. **Cytoplasm:** Once processed, mRNA and tRNA are exported from the nucleus into the cytoplasm. Here, they participate in **translation**, the process of decoding genetic information into proteins. 3. **Ribosomes:** Ribosomes themselves are "ribonucleoprotein" complexes. They are composed of approximately 60% **rRNA** and 40% protein. rRNA provides the structural framework and the catalytic activity (peptidyl transferase) required for peptide bond formation. Since RNA is actively synthesized in the nucleus, functions within the cytoplasm, and forms the core structure of ribosomes, **"All of the above"** is the correct answer. **High-Yield NEET-PG Pearls:** * **Most Abundant RNA:** rRNA (~80% of total cellular RNA). * **Smallest RNA:** tRNA (often called "Soluble RNA" or "Adapter molecule"). * **Least Stable/Shortest Half-life:** mRNA (rapidly degraded after translation). * **Catalytic RNA:** Known as **Ribozymes** (e.g., Peptidyl transferase, SnRNAs in splicing). * **Mitochondria:** Also contain their own specific RNA and ribosomes (mitoribosomes), which is another potential location for RNA.

Question 127: In DNA structure, what is the maximum number of hydrogen bonds observed between a purine-pyrimidine base pair?

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

Explanation: **Explanation:** In DNA structure, base pairing follows **Chargaff’s Rule**, where a purine always pairs with a pyrimidine to maintain a constant distance between the sugar-phosphate backbones. The stability of this pairing is determined by the number of hydrogen bonds formed. **1. Why Guanine-Cytosine is correct:** The **Guanine-Cytosine (G-C)** pair forms **three hydrogen bonds** (between the carbonyl group, amino group, and ring nitrogen). This is the maximum number of hydrogen bonds observed between any base pair in DNA. Because of this third bond, G-C rich DNA sequences have higher thermal stability and a higher **melting temperature (Tm)** compared to A-T rich sequences. **2. Why the other options are incorrect:** * **Adenine-Thymine (A-T):** This pair forms only **two hydrogen bonds**. While it follows the purine-pyrimidine rule, it is less stable than the G-C pair. * **Adenine-Guanine (A-G):** This is a purine-purine pairing. It does not occur in standard B-DNA because it would cause a "bulge" in the double helix due to the large size of both bases. * **Cytosine-Thymine (C-T):** This is a pyrimidine-pyrimidine pairing. It does not occur in standard DNA as it would result in a "gap" or narrowing of the helix. **NEET-PG High-Yield Pearls:** * **Melting Temperature (Tm):** Directly proportional to the G-C content. High G-C content is often found in **promoter regions** (CpG islands). * **TATA Box:** Found in promoters; it is A-T rich because the two hydrogen bonds are easier to "melt" or unzip during the initiation of transcription. * **Bonding Sites:** In G-C pairs, bonds occur at positions (O6-N4, N1-N3, and N2-O2).

Question 128: Which of the following is NOT provided by glycine in purine synthesis?

A. Carbon-4
B. Carbon-5
C. Nitrogen-4 (Correct Answer)
D. Nitrogen-7

Explanation: In purine ring synthesis, the entire **glycine** molecule is incorporated into the structure, contributing three specific atoms. The purine ring is a double-ring structure (a six-membered pyrimidine ring fused to a five-membered imidazole ring) built atom-by-atom on a ribose-5-phosphate backbone. ### Why Nitrogen-4 is the Correct Answer **Nitrogen-4 (N4)** is not a component of the purine ring. The nitrogen atoms in the purine nucleus are located at positions **1, 3, 7, and 9**. Therefore, N4 does not exist in the purine skeleton. Even if the question implied the nitrogen at position 3 or 9, those are provided by **Glutamine**, while the nitrogen at position 1 is provided by **Aspartate**. ### Analysis of Incorrect Options Glycine is unique because it is the only amino acid that contributes its entire carbon-nitrogen skeleton to the purine ring: * **Carbon-4 (C4):** Provided by the carboxyl carbon of glycine. * **Carbon-5 (C5):** Provided by the alpha-carbon of glycine. * **Nitrogen-7 (N7):** Provided by the amino group of glycine. ### High-Yield NEET-PG Clinical Pearls To master purine synthesis for the exam, remember the "Mnemonic for Sources": 1. **Glycine:** C4, C5, and N7 (The "Whole Molecule" donor). 2. **Aspartate:** N1. 3. **Glutamine (Amide N):** N3 and N9. 4. **Tetrahydrofolate (N10-formyl THF):** C2 and C8. 5. **CO₂ (Respiratory):** C6. **Clinical Correlation:** The rate-limiting step of purine synthesis is catalyzed by **PRPP glutamyl amidotransferase**. Drugs like **Methotrexate** and **Trimethoprim** inhibit folate metabolism, indirectly blocking the contribution of C2 and C8, thereby halting DNA synthesis in rapidly dividing cells.

Question 129: A nucleoside is composed of which of the following?

A. A pyrimidine base
B. A purine base
C. A sugar moiety
D. A pyrimidine base, a purine base, and a sugar moiety (Correct Answer)

Explanation: **Explanation:** The fundamental building blocks of nucleic acids (DNA and RNA) are organized into a hierarchy of complexity: **Nitrogenous Base → Nucleoside → Nucleotide.** A **Nucleoside** is formed when a nitrogenous base (either a Purine or a Pyrimidine) is linked to a five-carbon sugar (Ribose or Deoxyribose) via a **β-N-glycosidic bond**. Therefore, the correct answer encompasses the components of a nucleoside: a base (purine or pyrimidine) and a sugar moiety. * **Why Option D is correct:** It correctly identifies that a nucleoside consists of a nitrogenous base (which can be either a purine like Adenine/Guanine or a pyrimidine like Cytosine/Thymine/Uracil) covalently bonded to a pentose sugar. * **Why Options A, B, and C are incorrect:** These options are incomplete. A base alone (A or B) or a sugar alone (C) does not constitute a nucleoside; they must be chemically bonded to form the unit. **High-Yield NEET-PG Clinical Pearls:** 1. **Nucleoside vs. Nucleotide:** A Nucleotide = Nucleoside + Phosphate group. The phosphate is usually attached to the 5' carbon of the sugar via an ester bond. 2. **The Bond:** In purines, the sugar attaches at the **N-9** position; in pyrimidines, it attaches at the **N-1** position. 3. **Pharmacology Link:** Many antiviral (e.g., Acyclovir, Zidovudine) and anticancer (e.g., 5-Fluorouracil) drugs are **Nucleoside Analogs**. They lack a phosphate group, allowing them to enter cells easily before being phosphorylated into active nucleotides that inhibit DNA synthesis. 4. **Synthetic Importance:** In the "Salvage Pathway," the enzyme **HGPRT** converts free bases back into nucleotides, a process deficient in **Lesch-Nyhan Syndrome**.

Question 130: Which of the following statements is FALSE regarding gout?

A. It is due to increased metabolism of pyrimidines. (Correct Answer)
B. It is due to increased metabolism of purines.
C. Uric acid levels may not be elevated in all cases.
D. It has a predilection for the great toe.

Explanation: ### Explanation **1. Why Option A is the correct (False) statement:** Gout is a disorder of **purine metabolism**, not pyrimidine metabolism. The end product of purine (Adenine and Guanine) catabolism in humans is **uric acid**. In contrast, the end products of pyrimidine (Cytosine, Thymine, and Uracil) catabolism are highly soluble molecules like **$\beta$-alanine** and **$\beta$-aminoisobutyrate**, which are easily excreted in urine and do not cause crystal deposition or gout. **2. Analysis of other options:** * **Option B:** This is true. Hyperuricemia, the precursor to gout, results from either the overproduction of purines (e.g., Lesch-Nyhan syndrome, PRPP synthetase overactivity) or decreased renal excretion of uric acid. * **Option C:** This is true. While hyperuricemia is the hallmark, serum uric acid levels can be **normal or even low during an acute attack** of gout because the urate crystals have shifted from the blood into the joint space. * **Option D:** This is true. The first metatarsophalangeal joint (great toe) is the most common site of the initial attack, a clinical condition known as **Podagra**. **3. NEET-PG High-Yield Pearls:** * **Gold Standard Diagnosis:** Identification of **negatively birefringent, needle-shaped** monosodium urate crystals under polarized light microscopy. * **Enzyme Defects:** Gout is associated with **Von Gierke’s disease** (due to increased G6P entering the HMP shunt, increasing ribose-5-phosphate and PRPP). * **Drug of Choice:** **NSAIDs** are first-line for acute attacks; **Allopurinol** (a xanthine oxidase inhibitor) is used for chronic management but should never be started during an acute flare. * **Dietary triggers:** High-protein diets (red meat), seafood, and alcohol (especially beer) exacerbate gout.

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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