Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 71: Which of the following types of RNA contains abnormal purine bases?

A. Transfer RNA (t-RNA) (Correct Answer)
B. Messenger RNA (m-RNA)
C. Ribosomal RNA (r-RNA)
D. 16S RNA

Explanation: **Explanation** **1. Why Transfer RNA (t-RNA) is Correct:** Transfer RNA (t-RNA) is unique among RNA types because it undergoes extensive **post-transcriptional modifications**. Approximately 10–15% of the nucleotides in t-RNA are modified or "unusual." These include abnormal purine and pyrimidine bases that are essential for stabilizing the t-RNA structure and ensuring accurate codon-anticodon recognition. * **Examples of abnormal purines in t-RNA:** Inosine (formed by deamination of Adenosine), 1-methyladenosine, and N6-isopentenyladenosine. * **Other unusual bases:** Pseudouridine (Ψ), Dihydrouridine (D), and Ribothymidine (T). **2. Why the Other Options are Incorrect:** * **Messenger RNA (m-RNA):** Primarily consists of the four standard bases (A, G, C, U). While eukaryotic m-RNA has a 7-methylguanosine cap, it does not contain the high density of diverse abnormal purines characteristic of t-RNA. * **Ribosomal RNA (r-RNA) & 16S RNA:** These are structural components of ribosomes. While they do undergo some methylation, they lack the variety of complex modified purines (like Inosine) found in the "wobble position" of t-RNA. 16S RNA is specifically a component of the prokaryotic 30S ribosomal subunit. **3. NEET-PG High-Yield Clinical Pearls:** * **The "Wobble" Hypothesis:** Inosine (an abnormal purine) is often found at the first position of the t-RNA anticodon, allowing it to pair with U, C, or A in the m-RNA. * **Smallest RNA:** t-RNA is the smallest (73–93 nucleotides) and is often called "Soluble RNA" (sRNA). * **Structure:** The secondary structure is a **Cloverleaf**, while the tertiary structure is **L-shaped**. * **DHU Loop:** Contains Dihydrouridine; it is the site for recognition by the enzyme aminoacyl-tRNA synthetase. * **TψC Loop:** Contains Pseudouridine; it is involved in binding the t-RNA to the ribosomal surface.

Question 72: What is involved in the formation of d-TMP from d-UMP?

A. N5, N10-methylene tetrahydrofolate (Correct Answer)
B. Formimino folate
C. N5 formyl folate
D. Dihydrofolate

Explanation: **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:** In this reaction, **N5, N10-methylene tetrahydrofolate (THF)** acts as both a **one-carbon donor** and a **reducing agent**. It transfers a methyl group to the C5 position of the pyrimidine ring of dUMP. During this process, 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 Incorrect Options are Wrong:** * **B. Formimino folate:** This is an intermediate in the catabolism of Histidine (FIGLU to Glutamate). It does not participate in nucleotide synthesis. * **C. N5 formyl folate:** Also known as **Folinic Acid (Leucovorin)**. While used clinically to "rescue" cells from methotrexate toxicity, it is not the direct co-factor for dTMP synthesis. * **D. Dihydrofolate:** This is the **product** of the thymidylate synthase reaction, not the substrate/co-factor required for the methylation of dUMP. **Clinical Pearls & High-Yield Facts:** * **5-Fluorouracil (5-FU):** A suicide inhibitor of Thymidylate Synthase; it stops dTMP production ("Thymineless death"). * **Methotrexate:** Inhibits **Dihydrofolate Reductase (DHFR)**, preventing the regeneration of THF from DHF, thereby indirectly inhibiting dTMP synthesis. * **Rate-Limiting Step:** This reaction is the rate-limiting step for DNA synthesis, making it a primary target for several chemotherapeutic agents.

Question 73: Which of the following statements about the guanine ring is FALSE?

A. It contains 9 atoms.
B. Aspartate acts as the amino group donor for forming guanine. (Correct Answer)
C. The carbonyl group at C6 acts as a hydrogen bond acceptor.
D. It forms three hydrogen bonds with cytosine.

Explanation: **Explanation:** The synthesis of purine nucleotides (Adenine and Guanine) involves the assembly of a bicyclic ring on a ribose-5-phosphate backbone. While both purines share a common precursor, **Inosine Monophosphate (IMP)**, their pathways diverge during the final amination steps. **Why Option B is False (The Correct Answer):** In the conversion of IMP to **Guanosine Monophosphate (GMP)**, the amino group donor is **Glutamine**. The process involves two steps: first, IMP is oxidized to Xanthosine Monophosphate (XMP) by IMP dehydrogenase; second, XMP is aminated by GMP synthase using Glutamine. * **Contrast:** **Aspartate** acts as the amino group donor for the synthesis of **Adenosine Monophosphate (AMP)** from IMP. **Analysis of Other Options:** * **Option A:** Purines (Guanine and Adenine) are dicyclic structures consisting of a pyrimidine ring fused to an imidazole ring, totaling **9 atoms** (5 carbons and 4 nitrogens). * **Option C:** In the DNA double helix, the **C6 carbonyl oxygen** of guanine serves as a **hydrogen bond acceptor**, pairing with the amino group of cytosine. * **Option D:** Guanine pairs with Cytosine via **three hydrogen bonds** (C6=O to N4-NH2; N1-H to N3; N2-NH2 to C2=O). This makes G-C rich DNA more stable (higher melting temperature) than A-T rich DNA. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Purine Synthesis:** Glutamine-PRPP amidotransferase. * **Mycophenolate Mofetil:** An immunosuppressant that inhibits **IMP dehydrogenase**, selectively depriving T and B cells of GMP. * **Sources of Purine Atoms:** "CO2 (C6), Aspartate (N1), Glycine (C4, C5, N7), Glutamine (N3, N9), and THF (C2, C8)."

Question 74: Which of the following is NOT a feature of pyrimidine metabolism?

A. Requires CAD polypeptide chain
B. 6-azauridine leads to orotic aciduria
C. Dihydroorotate dehydrogenase is mitochondrial
D. In Reye syndrome, there is decreased cytosolic carbamoyl phosphate (Correct Answer)

Explanation: **Explanation:** The correct answer is **D** because in **Reye Syndrome**, there is actually an **increase** (not a decrease) in cytosolic carbamoyl phosphate. Reye syndrome involves mitochondrial damage, leading to the failure of the urea cycle. When Ornithine Transcarbamoylase (OTC) is overwhelmed or dysfunctional, mitochondrial **Carbamoyl Phosphate (CP)** leaks into the cytosol. There, it enters the pyrimidine synthesis pathway, leading to the overproduction of orotic acid (secondary orotic aciduria). **Analysis of Incorrect Options:** * **Option A:** Pyrimidine synthesis begins with the **CAD protein**, a single trifunctional polypeptide in the cytosol containing **C**arbamoyl phosphate synthetase II, **A**spartate transcarbamoylase, and **D**ihydroorotase. * **Option B:** **6-azauridine** is a competitive inhibitor of Orotidylate decarboxylase. Blocking this enzyme prevents the conversion of OMP to UMP, resulting in the excretion of orotic acid in urine. * **Option C:** **Dihydroorotate dehydrogenase** is the only enzyme of the pyrimidine biosynthetic pathway located on the inner **mitochondrial membrane**; all others are cytosolic. **Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** CPS-II (inhibited by UTP, activated by PRPP). * **Leflunomide:** An immunosuppressant used in Rheumatoid Arthritis that inhibits Dihydroorotate dehydrogenase. * **Orotic Aciduria Type I:** Due to deficiency of UMP Synthase; presents with megaloblastic anemia that does not respond to B12/Folate and failure to thrive. Treatment is oral **Uridine**. * **Differentiating Orotic Aciduria:** If hyperammonemia is present, suspect a Urea Cycle defect (OTC deficiency); if ammonia is normal, suspect UMP Synthase deficiency.

Question 75: Which metabolic process is common to both humans and bacteria?

A. Purine synthesis (Correct Answer)
B. Nitrogen fixation
C. Mucolipid formation
D. Nonoxidative photophosphorylation

Explanation: **Explanation:** **1. Why Purine Synthesis is Correct:** Purine synthesis (both *de novo* and salvage pathways) is a fundamental metabolic process essential for the survival of all living organisms, including humans and bacteria. Purines (Adenine and Guanine) are the building blocks of DNA, RNA, and energy carriers like ATP and GTP. The *de novo* pathway, which builds the purine ring from simple precursors like amino acids (Glycine, Aspartate, Glutamine), CO₂, and folic acid derivatives, is highly conserved across species. **2. Why Other Options are Incorrect:** * **Nitrogen Fixation:** This is the conversion of atmospheric N₂ into ammonia. It is performed exclusively by certain prokaryotes (e.g., *Rhizobium*, *Azotobacter*) using the enzyme nitrogenase. Humans cannot fix nitrogen and must obtain it through dietary protein. * **Mucolipid Formation:** Mucolipids are complex lipids typically associated with animal cell lysosomes. While bacteria have complex cell walls (peptidoglycans/LPS), the specific synthesis of mucolipids is a eukaryotic characteristic. * **Nonoxidative Photophosphorylation:** This is a light-dependent process used by photosynthetic organisms (plants and certain bacteria like Cyanobacteria) to generate ATP. Humans are heterotrophs and rely on oxidative phosphorylation in the mitochondria. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Common Precursor:** Inosine Monophosphate (IMP) is the common precursor for both AMP and GMP in both humans and bacteria. * **Drug Target:** The reliance of bacteria on *de novo* folate synthesis for purine production is the basis for **Sulfonamides** (inhibit dihydropteroate synthase). Humans, however, lack this pathway and must absorb preformed folate from the diet. * **Rate-limiting Step:** The first committed step in purine synthesis is catalyzed by **PRPP glutamine amidotransferase**, which is inhibited by feedback from AMP and GMP.

Question 76: The Watson and Crick model describes which of the following?

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

Explanation: The Watson and Crick model (1953) specifically describes the **double-helical structure of B-DNA**. This model established that DNA consists of two antiparallel polynucleotide strands held together by hydrogen bonds between complementary nitrogenous bases (Adenine with Thymine; Guanine with Cytosine). This discovery was fundamental to understanding how genetic information is stored and replicated in living organisms. **Explanation of Options:** * **A. DNA (Correct):** The model describes the right-handed double helix, where the sugar-phosphate backbone is on the outside and the nitrogenous bases are stacked inside. * **B. mRNA (Incorrect):** Messenger RNA is typically a single-stranded linear molecule that carries genetic codes from DNA to ribosomes. * **C. rRNA (Incorrect):** Ribosomal RNA is a structural component of ribosomes. While it has complex folding, it does not follow the Watson-Crick double helix model. * **D. tRNA (Incorrect):** Transfer RNA has a characteristic **"Cloverleaf" secondary structure** (described by Robert Holley) and an **"L-shaped" tertiary structure**. **High-Yield Clinical Pearls for NEET-PG:** * **Dimensions of B-DNA:** One complete turn is **3.4 nm** (34 Å) long and contains **10 base pairs**. The distance between adjacent base pairs is **0.34 nm**. * **Chargaff’s Rule:** In DNA, the amount of Purines (A+G) always equals the amount of Pyrimidines (T+C). This rule applies to double-stranded DNA but **not** to single-stranded RNA. * **Bonding:** A=T (2 hydrogen bonds); G≡C (3 hydrogen bonds). Higher G-C content increases the melting temperature (Tm) of DNA. * **Z-DNA:** A rare, left-handed helix with a zigzag backbone, often found in regions with alternating purine-pyrimidine sequences.

Question 77: Which of the following is NOT a common substrate for purine and pyrimidine synthesis?

A. Glutamine
B. Glycine (Correct Answer)
C. Aspartate
D. Carbon dioxide

Explanation: 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.

Question 78: In humans, where is uric acid primarily formed?

A. Liver
B. Gastrointestinal mucosa (Correct Answer)
C. Kidney
D. Joints

Explanation: **Explanation:** The formation of uric acid is the final step in the catabolism of purine nucleotides (Adenine and Guanine). This process is catalyzed by the enzyme **Xanthine Oxidase (XO)**, which converts hypoxanthine to xanthine and xanthine to uric acid. **Why Gastrointestinal (GI) Mucosa is the correct answer:** While many tissues possess the enzymes for purine metabolism, **Xanthine Oxidase activity is highest in the GI mucosa and the liver.** In humans, the GI mucosa is considered a primary site for the final conversion of dietary and endogenous purines into uric acid before they enter the systemic circulation or are excreted. This high concentration of XO in the gut serves as a metabolic barrier for ingested purines. **Analysis of Incorrect Options:** * **A. Liver:** The liver is indeed a major site of uric acid production; however, in the context of standardized medical examinations (like NEET-PG) focusing on specific enzyme distribution, the GI mucosa is often highlighted as the site with the highest specific activity of Xanthine Oxidase. * **C. Kidney:** The kidney is the primary organ for the **excretion** of uric acid (approx. 70%), but it is not the primary site of its formation. * **D. Joints:** Joints are the site of **deposition** of monosodium urate crystals in Gout, leading to inflammatory arthritis. They do not produce uric acid. **High-Yield Clinical Pearls for NEET-PG:** * **Allopurinol & Febuxostat:** These are XO inhibitors used to treat Gout by decreasing uric acid production. * **Uricase Enzyme:** Humans lack the enzyme *Uricase* (present in other mammals), which converts uric acid to the more soluble **Allantoin**. This is why humans are prone to hyperuricemia. * **Von Gierke’s Disease:** Associated with hyperuricemia due to increased pentose phosphate pathway activity (leading to increased ribose-5-phosphate and purine synthesis) and decreased renal excretion.

Question 79: What is involved in the formation of d-TMP from d-UMP?

A. N5, N10-methylene tetrahydrofolate (Correct Answer)
B. Formimino folate
C. N5 formyl folate
D. Dihydrofolate

Explanation: ### 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.

Question 80: Which of the following X-linked conditions presents as urolithiasis with gouty arthritis?

A. Holt-Oram syndrome
B. Lesch-Nyhan syndrome (Correct Answer)
C. SCID
D. Cystic fibrosis

Explanation: **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.

Practice by Chapter

Nucleotide Structure and Function

Practice Questions

DNA Structure and Replication

Practice Questions

RNA Structure and Types

Practice Questions

Transcription: RNA Synthesis

Practice Questions

Post-Transcriptional Modifications

Practice Questions

Translation: Protein Synthesis

Practice Questions

Genetic Code and Codon Usage

Practice Questions

Regulation of Gene Expression

Practice Questions

Mutations and DNA Repair

Practice Questions

Purine Metabolism and Disorders

Practice Questions

Pyrimidine Metabolism and Disorders

Practice Questions

Nucleotide Degradation and Salvage Pathways

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free