Nucleic Acid Biochemistry Practice Questions

Q: Uric acid is converted to allantoin in:

Catabolism of purines. **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.

Q: Where does the synthesis of rRNA take place?

Nucleolus. **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.

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

All of the above. **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.

Q: Orotic aciduria is due to deficiency of which enzyme?

Decarboxylase. **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.

Q: PRPP participates in purine biosynthesis in which step?

Serves as a scaffold for assembly of the purine ring. **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.

Q: Which of the following is not a pyrimidine?

Adenine. **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.

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

tRNA. **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.

Question 1

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?

Accepted Answer

Hypoxanthine-guanine phosphoribosyltransferase (HGPrtase)

Answer

Adenosine deaminase

Answer

Alkaline phosphatase (APase)

Answer

Acid maltase

Question 2

Uric acid is converted to allantoin in:

Accepted Answer

Catabolism of purines

Answer

Catabolism of pyrimidines

Answer

Synthesis of pyrimidines

Answer

Synthesis of purines

Question 3

Where does the synthesis of rRNA take place?

Accepted Answer

Nucleolus

Answer

Cytosol

Answer

Nucleus

Answer

Mitochondria

Question 4

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?

Accepted Answer

Serum Uric Acid Levels

Answer

Serum Lead Levels

Answer

Serum Alkaline Phosphatase Levels

Answer

Serum Lactate Dehydrogenase Levels

Question 5

At physiological pH, what is the charge of DNA?

Accepted Answer

All of the above

Answer

Acidic

Answer

Negatively charged

Answer

Amphipathic

Question 6

Orotic aciduria is due to deficiency of which enzyme?

Accepted Answer

Decarboxylase

Answer

Isomerase

Answer

Tyrosinase

Answer

Homogentisate oxidase

Question 7

Azaserine inhibits which of the following enzymes?

Accepted Answer

Formyl glycinamide ribonucleotide amidotransferase

Answer

Glycinamide ribonucleotide synthetase

Answer

Glycinamide ribonucleotide transformylase

Answer

Inosine monophosphate synthase

Question 8

PRPP participates in purine biosynthesis in which step?

Accepted Answer

Serves as a scaffold for assembly of the purine ring

Answer

Post assembly of the purine ring

Answer

Both

Answer

None

Question 9

Which of the following is not a pyrimidine?

Accepted Answer

Adenine

Answer

Uracil

Answer

Thymine

Answer

Cytosine

Question 10

Which type of RNA has the highest percentage of modified bases?

Accepted Answer

tRNA

Answer

mRNA

Answer

rRNA

Answer

snRNA

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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