Nucleic Acid Biochemistry Practice Questions

Q: Which of the following is NOT a common substrate for purine and pyrimidine synthesis?

Glycine. To master nucleotide metabolism for NEET-PG, it is essential to distinguish between the precursors used in purine versus pyrimidine rings. ### **Explanation of the Correct Answer** **Glycine** is the correct answer because it is **exclusive to purine synthesis**. In the de novo purine pathway, the entire glycine molecule is incorporated to provide carbons C4, C5, and nitrogen N7. It plays no role in the pyrimidine biosynthetic pathway. ### **Analysis of Incorrect Options** * **Glutamine (Option A):** This is a common substrate. It provides the **amide nitrogen** for both pathways: N3 and N9 in purines, and the nitrogen for the carbamoyl phosphate precursor in pyrimidines. * **Aspartate (Option C):** This is a common substrate. In purines, it provides N1. In pyrimidines, the entire aspartate molecule contributes to the majority of the ring (C4, C5, C6, and N1). * **Carbon Dioxide (Option D):** This is a common substrate. It provides C6 in the purine ring (via respiratory $CO_2$) and C2 in the pyrimidine ring (via bicarbonate). ### **High-Yield Clinical Pearls for NEET-PG** * **Purine Precursors:** "Cats Purr (Purine) on **GAG**" — **G**lycine, **A**spartate, **G**lutamine. Plus $CO_2$ and $N^{10}$-formyl tetrahydrofolate. * **Pyrimidine Precursors:** Aspartate, Glutamine, and $CO_2$. * **Rate-Limiting Enzymes:** * Purine: PRPP Glutamyl Amidotransferase. * Pyrimidine: Cytosolic Carbamoyl Phosphate Synthetase II (CPS-II). * **Leflunomide:** A drug used in Rheumatoid Arthritis that inhibits **Dihydroorotate dehydrogenase**, blocking pyrimidine synthesis.

Q: What is involved in the formation of d-TMP from d-UMP?

N5, N10-methylene tetrahydrofolate. ### Explanation The conversion of **dUMP (deoxyuridine monophosphate)** to **dTMP (deoxythymidine monophosphate)** is a critical step in DNA synthesis, catalyzed by the enzyme **Thymidylate Synthase**. **1. Why Option A is Correct:** Thymidylate synthase requires a methyl group donor to convert the uracil base into a thymine base. **N5, N10-methylene tetrahydrofolate (THF)** serves as both the **one-carbon donor** and the **reducing agent** in this reaction. During the transfer, the methylene group is reduced to a methyl group, and N5, N10-methylene THF is oxidized to **Dihydrofolate (DHF)**. This is the only reaction in folate metabolism where THF is oxidized to DHF. **2. Why Other Options are Incorrect:** * **B. Formimino folate:** This is an intermediate in the catabolism of histidine (converted to glutamate). It does not participate in nucleotide synthesis. * **C. N5 formyl folate (Leucovorin):** Also known as Folinic acid, it is used clinically to "rescue" cells from methotrexate toxicity but is not the direct co-enzyme for dTMP synthesis. * **D. Dihydrofolate:** This is the *product* of the reaction, not the substrate/co-factor required for the formation of dTMP. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Rate-Limiting Step:** This reaction is the rate-limiting step for DNA synthesis. * **5-Fluorouracil (5-FU):** A chemotherapy agent that acts as a suicide inhibitor of Thymidylate synthase. * **Methotrexate:** Inhibits **Dihydrofolate Reductase (DHFR)**, the enzyme responsible for regenerating THF from DHF. This depletes the pool of N5, N10-methylene THF, halting dTMP synthesis and causing "thymineless death" of cells. * **FIGLU Test:** High levels of Formiminoglutamate (FIGLU) in urine indicate Vitamin B9 (Folate) deficiency.

Q: Which of the following X-linked conditions presents as urolithiasis with gouty arthritis?

Lesch-Nyhan syndrome. **Explanation:** **Lesch-Nyhan Syndrome (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. Its absence leads to the accumulation of PRPP (phosphoribosyl pyrophosphate), which stimulates *de novo* purine synthesis. The excess purines are eventually broken down into **uric acid**, leading to severe hyperuricemia. * **Clinical Presentation:** The triad of **hyperuricemia** (causing gouty arthritis and urolithiasis/orange sand in diapers), **neurological impairment** (choreoathetosis, spasticity), and characteristic **self-mutilating behavior** (biting lips and fingers). **Incorrect Options:** * **Holt-Oram Syndrome:** An autosomal dominant "heart-hand" syndrome characterized by radial ray defects and cardiac septal defects (ASD/VSD). It has no relation to purine metabolism. * **SCID (Severe Combined Immunodeficiency):** While one form is caused by **Adenosine Deaminase (ADA) deficiency** (also a purine enzyme), it presents with profound lymphopenia and recurrent infections, not gout or self-mutilation. * **Cystic Fibrosis:** An autosomal recessive disorder involving the CFTR gene, affecting chloride transport. It presents with thick mucus, bronchiectasis, and pancreatic insufficiency. **High-Yield NEET-PG Pearls:** * **Mnemonic for HGPRT deficiency:** **H**yperuricemia, **G**out, **P**issed off (self-mutilation), **R**etardation (intellectual disability), **D**ys**T**onia. * **Treatment:** Allopurinol or Febuxostat (Xanthine Oxidase inhibitors) are used to manage hyperuricemia, but they do not improve neurological symptoms. * **Diagnosis:** Suggested by high serum uric acid and confirmed by low HGPRT enzyme activity in erythrocytes or fibroblasts.

Q: A nucleic acid was analyzed and found to contain 32% adenine, 18% guanine, 17% cytosine, and 33% thymine. What type of nucleic acid must this be?

Double-stranded DNA. ### Explanation **1. Why the Correct Answer is Right (Chargaff’s Rule)** The correct answer is **Double-stranded DNA (dsDNA)**. This determination is based on **Chargaff’s Rule**, which states that in a double-stranded DNA molecule, the molar ratio of purines to pyrimidines is 1:1. Specifically: * **Adenine (A) = Thymine (T)** * **Guanine (G) = Cytosine (C)** In the given data: * A (32%) is approximately equal to T (33%). * G (18%) is approximately equal to C (17%). The presence of **Thymine** confirms it is DNA (not RNA), and the near-equal proportions of complementary bases confirm it is **double-stranded**, where base-pairing occurs via hydrogen bonds. **2. Why the Incorrect Options are Wrong** * **A & C (RNA Options):** These are incorrect because the analysis shows the presence of **Thymine**. RNA contains **Uracil** instead of Thymine. * **B (Single-stranded DNA):** In ssDNA, there is no complementary strand to mandate base-pairing. Therefore, the ratios of A:T and G:C are usually unequal and random. Since the provided percentages show A≈T and G≈C, the molecule must be double-stranded. **3. NEET-PG High-Yield Pearls** * **Chargaff’s Rule Applicability:** It applies **only** to double-stranded DNA/RNA. It does **not** apply to single-stranded viruses (e.g., $\phi$X174) or mitochondrial DNA during certain replication phases. * **Base Pairing:** A-T pairs have **2 hydrogen bonds**, while G-C pairs have **3 hydrogen bonds**. * **Melting Temperature ($T_m$):** DNA with a higher G-C content has a higher $T_m$ because triple hydrogen bonds require more energy to denature. * **Formula:** Purines (A+G) = Pyrimidines (T+C). In this question: $32+18 = 50\%$ and $33+17 = 50\%$.

Q: What are nucleosomes composed of?

DNA and Histones. **Explanation:** A **nucleosome** is the fundamental structural unit of chromatin in eukaryotic cells, often described as "beads on a string." It consists of a segment of **DNA** (approximately 147 base pairs) wrapped 1.67 times around a protein core called the **Histone Octamer**. This octamer is composed of two copies each of four core histones: **H2A, H2B, H3, and H4**. * **Why Option B is correct:** DNA is negatively charged (due to phosphate groups) and histones are positively charged (rich in basic amino acids like **Lysine and Arginine**). This electrostatic attraction allows the DNA to wrap tightly around the histone core, facilitating the packaging of long DNA strands into the nucleus. * **Why Options A, C, and D are incorrect:** **RNA** is not a structural component of the nucleosome. While RNA is involved in transcription and translation, it does not form the "bead" structure of chromatin. **NEET-PG High-Yield Pearls:** 1. **Linker DNA:** The DNA between two nucleosomes is called linker DNA, which is associated with **Histone H1**. H1 is not part of the core octamer; it helps stabilize the 30nm chromatin fiber. 2. **Amino Acid Composition:** Histones are exceptionally rich in **Arginine and Lysine**, which is a frequent exam question. 3. **Acetylation vs. Methylation:** Histone **acetylation** (by HATs) decreases the positive charge, leading to relaxed chromatin (Euchromatin) and active transcription. **Deacetylation** (by HDACs) leads to condensed chromatin (Heterochromatin) and gene silencing.

Q: Which of the following is considered an abnormal base found in tRNA?

Dihydrouracil. ### Explanation **Correct Option: A. Dihydrouracil** Transfer RNA (tRNA) is unique among nucleic acids because it undergoes extensive post-transcriptional modifications. While it is synthesized using the four standard bases (A, U, G, C), specific enzymes modify these bases to create "abnormal" or "modified" bases. **Dihydrouracil** is formed by the saturation of the double bond in uracil. It is a hallmark of the **D-loop** (Dihydrouridine loop) of tRNA, which is essential for recognition by the enzyme aminoacyl-tRNA synthetase. Other common modified bases in tRNA include Pseudouridine (in the TψC loop) and Inosine (in the anticodon loop). **Analysis of Incorrect Options:** * **B. Orotic Acid:** This is a normal metabolic intermediate in the **de novo pyrimidine synthesis pathway**. It is the precursor to UMP but is not incorporated into the final structure of mature tRNA. * **C. Methyl xanthine:** These are a group of alkaloids (like caffeine, theobromine, and theophylline) that act as phosphodiesterase inhibitors. They are not components of nucleic acids. * **D. Cystine:** This is a sulfur-containing **amino acid** formed by the oxidation of two cysteine residues. It is related to protein structure (disulfide bonds), not nucleic acid composition. **High-Yield Clinical Pearls for NEET-PG:** * **The "Cloverleaf" vs. "L-shape":** tRNA has a 2D cloverleaf secondary structure but a 3D **L-shaped** tertiary structure. * **The 3' End:** All tRNA molecules end in the sequence **5'-CCA-3'**. The amino acid attaches to the 3'-hydroxyl group of the Adenosine. * **Pseudouridine (ψ):** Known as the "5th ribonucleoside," it is found in the TψC loop, which helps in binding the tRNA to the ribosome. * **Inosine:** Found in the wobble position (1st base of anticodon), allowing a single tRNA to recognize multiple codons.

Q: What is the common end product of catabolism of all pyrimidines?

Urea. ### Explanation **Correct Answer: C. Urea** The catabolism of pyrimidines (Cytosine, Uracil, and Thymine) differs significantly from purine catabolism. While purines are excreted as uric acid, the pyrimidine ring is completely cleaved into highly soluble products. * **Cytosine and Uracil** are broken down into **$\beta$-alanine**, $NH_3$, and $CO_2$. * **Thymine** is broken down into **$\beta$-aminoisobutyrate**, $NH_3$, and $CO_2$. The ammonia ($NH_3$) generated from the deamination and ring-opening of all three pyrimidines enters the **Urea Cycle** in the liver. Therefore, the nitrogenous waste from the complete catabolism of all pyrimidines is ultimately excreted as **Urea**. --- ### Why other options are incorrect: * **A. Beta-alanine:** This is an intermediate product specifically of Uracil and Cytosine catabolism, but not Thymine (which produces $\beta$-aminoisobutyrate). It is not the final common nitrogenous excretory product. * **B. Uric acid:** This is the end product of **Purine** (Adenine and Guanine) catabolism in humans. * **D. Xanthine:** This is an intermediate in the purine degradation pathway, converted into uric acid by the enzyme Xanthine Oxidase. --- ### High-Yield Clinical Pearls for NEET-PG: 1. **Solubility:** Unlike purine end-products (uric acid), pyrimidine catabolites are highly water-soluble; thus, pyrimidine overproduction does not lead to conditions like Gout. 2. **$\beta$-aminoisobutyrate:** Increased urinary excretion of this metabolite is a diagnostic marker for high cell turnover (e.g., leukemia or post-radiation therapy), as it is unique to Thymine (DNA) degradation. 3. **Rate-limiting enzyme:** Dihydropyrimidine dehydrogenase (DPD) is the rate-limiting enzyme in pyrimidine catabolism. DPD deficiency can lead to severe toxicity in patients treated with the chemotherapy drug **5-Fluorouracil (5-FU)**.

Question 1

Which of the following statements about the guanine ring is FALSE?

Accepted Answer

Aspartate acts as the amino group donor for forming guanine.

Answer

It contains 9 atoms.

Answer

The carbonyl group at C6 acts as a hydrogen bond acceptor.

Answer

It forms three hydrogen bonds with cytosine.

Question 2

Which of the following is NOT a feature of pyrimidine metabolism?

Accepted Answer

In Reye syndrome, there is decreased cytosolic carbamoyl phosphate

Answer

Requires CAD polypeptide chain

Answer

6-azauridine leads to orotic aciduria

Answer

Dihydroorotate dehydrogenase is mitochondrial

Question 3

Which of the following is NOT a common substrate for purine and pyrimidine synthesis?

Accepted Answer

Glycine

Answer

Glutamine

Answer

Aspartate

Answer

Carbon dioxide

Question 4

What is involved in the formation of d-TMP from d-UMP?

Accepted Answer

N5, N10-methylene tetrahydrofolate

Answer

Formimino folate

Answer

N5 formyl folate

Answer

Dihydrofolate

Question 5

Which of the following X-linked conditions presents as urolithiasis with gouty arthritis?

Accepted Answer

Lesch-Nyhan syndrome

Answer

Holt-Oram syndrome

Answer

SCID

Answer

Cystic fibrosis

Question 6

A nucleic acid was analyzed and found to contain 32% adenine, 18% guanine, 17% cytosine, and 33% thymine. What type of nucleic acid must this be?

Accepted Answer

Double-stranded DNA

Answer

Single-stranded RNA

Answer

Single-stranded DNA

Answer

Double-stranded RNA

Question 7

What are nucleosomes composed of?

Accepted Answer

DNA and Histones

Answer

DNA and RNA

Answer

RNA and Histones

Answer

DNA, RNA, and Histones

Question 8

Which of the following is considered an abnormal base found in tRNA?

Accepted Answer

Dihydrouracil

Answer

Orotic acid

Answer

Methyl xanthine

Answer

Cystine

Question 9

Which of the following statements regarding nucleic acids is not true?

Accepted Answer

tRNA does not contain thymine as one of its pyrimidine bases.

Answer

mRNA is synthesized from the template strand of DNA.

Answer

mRNA is synthesized from the non-coding strand of DNA.

Answer

Two strands of DNA are antiparallel in nature.

Question 10

What is the common end product of catabolism of all pyrimidines?

Accepted Answer

Urea

Answer

Beta alanine

Answer

Uric acid

Answer

Xanthine

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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