Nucleic Acid Biochemistry Practice Questions

Q: Which enzyme catalyzes the production of orotidylic acid?

Orotate phosphoribosyl transferase. **Explanation:** The production of **Orotidylic acid (OMP)** is a crucial step in the **de novo synthesis of pyrimidines**. 1. **Why Option D is Correct:** Orotidylic acid (Orotidine 5'-monophosphate) is formed by the transfer of a phosphoribosyl moiety from **PRPP** (5-phosphoribosyl-1-pyrophosphate) to **Orotate**. This reaction is catalyzed by **Orotate phosphoribosyl transferase (OPRTase)**. In humans, this enzyme is part of a bifunctional protein called **UMP Synthase**, which also contains OMP decarboxylase activity (converting OMP to UMP). 2. **Why Other Options are Incorrect:** * **A. Aspartyl transcarbamoylase:** Catalyzes the first committed step of the pathway, combining Carbamoyl phosphate and Aspartate to form N-carbamoyl aspartate. * **B. Dihydroorotase:** Catalyzes the ring closure of N-carbamoyl aspartate to form L-Dihydroorotate. * **C. Dihydroorotate dehydrogenase:** An enzyme located on the inner mitochondrial membrane that oxidizes Dihydroorotate to form Orotate. **Clinical Pearls & High-Yield Facts:** * **Hereditary Orotic Aciduria:** Caused by a deficiency in the bifunctional enzyme **UMP Synthase**. It presents with megaloblastic anemia (refractory to B12/Folate), orotic acid crystals in urine, and growth retardation. Treatment involves **Uridine** supplementation. * **Leflunomide:** An immunosuppressant used in Rheumatoid Arthritis that inhibits **Dihydroorotate dehydrogenase**, thereby blocking pyrimidine synthesis in T-cells. * **Rate-limiting step:** In humans, the rate-limiting enzyme for pyrimidine synthesis is **Carbamoyl Phosphate Synthetase II (CPS-II)**, located in the cytosol.

Q: Gout is a disorder of:

Purine Metabolism. **Explanation:** **1. Why Purine Metabolism is Correct:** Gout is a metabolic disorder characterized by **hyperuricemia** (elevated levels of uric acid in the blood). In humans, **Uric Acid** is the final end-product of **Purine catabolism** (Adenine and Guanine). When there is either overproduction of uric acid (due to enzyme defects like PRPP synthetase overactivity or HGPRT deficiency in Lesch-Nyhan syndrome) or decreased renal excretion, uric acid levels rise. This leads to the deposition of **Monosodium Urate (MSU) crystals** in joints and soft tissues, causing acute inflammatory arthritis. **2. Why Other Options are Incorrect:** * **Pyrimidine Metabolism:** The end-products of pyrimidine catabolism (Cytosine, Thymine, Uracil) are highly water-soluble compounds like **β-alanine and β-aminoisobutyrate**, which are easily excreted in urine and do not cause gout. * **Ketone Metabolism:** Disorders here lead to ketoacidosis (e.g., Diabetic Ketoacidosis). While high ketone levels can compete with uric acid for excretion in the kidneys (potentially triggering a gout flare), gout is fundamentally a primary disorder of purines. * **Protein Metabolism:** While purines are found in protein-rich foods, "protein metabolism" generally refers to amino acid breakdown and urea cycle disorders. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Identification of **negatively birefringent, needle-shaped crystals** under polarized light microscopy. * **Key Enzyme:** **Xanthine Oxidase** converts hypoxanthine to xanthine and xanthine to uric acid. It is the target of **Allopurinol** (a suicide inhibitor). * **Von Gierke’s Disease (G6PD deficiency):** A common biochemical link where lactic acidosis inhibits uric acid excretion, leading to secondary gout. * **Drug-induced Gout:** Thiazide and Loop diuretics are notorious for increasing serum uric acid levels.

Q: Which of the following best represents commonalities in the de novo synthesis pathways of IMP and UMP?

Both require PRPP, Glutamine, and Aspartic Acid.. This question tests your understanding of the overlapping precursors required for **Purine (IMP)** and **Pyrimidine (UMP)** nucleotide synthesis. ### **Explanation of the Correct Answer (A)** Both pathways share three fundamental building blocks: 1. **PRPP (5-Phosphoribosyl-1-pyrophosphate):** Acts as the ribose sugar donor for both. In purine synthesis, PRPP is the starting scaffold; in pyrimidine synthesis, it is added after the ring is formed. 2. **Glutamine:** Serves as the nitrogen donor for both. It provides N3 and N9 for purines and N3 for pyrimidines (via carbamoyl phosphate). 3. **Aspartic Acid:** Contributes atoms to both rings. It provides N1, C2, and C3 of the pyrimidine ring and N1 of the purine ring. ### **Analysis of Incorrect Options** * **Option B:** While correct, it is incomplete. Glutamine is a vital shared precursor that must be included for the "best" representation. * **Option C:** **Folate derivatives** (N10-formyl THF) are exclusive to **Purine** synthesis (providing C2 and C8). Pyrimidine synthesis (UMP) does not require folate; only the conversion of dUMP to dTMP (catalyzed by Thymidylate Synthase) requires folate. * **Option D:** **Glycine** is a precursor **only for Purines** (providing C4, C5, and N7). It is not involved in pyrimidine synthesis. ### **High-Yield Clinical Pearls for NEET-PG** * **The "Common" Precursors:** Remember the mnemonic **"GAP"** (Glutamine, Aspartic Acid, PRPP) for both pathways. * **Rate-Limiting Steps:** * Purine: Glutamine-PRPP amidotransferase. * Pyrimidine: Cytosolic Carbamoyl Phosphate Synthetase II (CPS-II). * **Leflunomide:** An immunosuppressant that inhibits **Dihydroorotate dehydrogenase**, blocking de novo pyrimidine synthesis. * **Methotrexate:** Inhibits Dihydrofolate Reductase (DHFR), affecting both purine synthesis and dTMP production, thus halting DNA replication.

Q: PRPP glutamyl amidotransferase is increased to increase purine synthesis in which of the following?

Liver. **Explanation:** The synthesis of purine nucleotides occurs via two pathways: the **De Novo pathway** and the **Salvage pathway**. **1. Why Liver is the Correct Answer:** The **Liver** is the primary site for the de novo synthesis of purine nucleotides. The rate-limiting and committed step of this pathway is catalyzed by the enzyme **PRPP glutamyl amidotransferase** (Amidophosphoribosyltransferase). This enzyme converts PRPP (5-phosphoribosyl-1-pyrophosphate) to 5-phosphoribosylamine using glutamine. In the liver, this enzyme is highly active and regulated by feedback inhibition (AMP/GMP) and substrate availability (PRPP), allowing the liver to export nucleosides and bases to other tissues. **2. Why Other Options are Incorrect:** * **RBCs, Brain, and Polymorphs:** These tissues lack the complete enzymatic machinery required for the de novo synthesis of purines. Specifically, they have low or negligible levels of PRPP glutamyl amidotransferase. Instead, these tissues rely almost exclusively on the **Salvage Pathway** (using enzymes like HGPRT and APRT) to meet their purine requirements by utilizing preformed bases (hypoxanthine, guanine, adenine) transported from the liver. **Clinical Pearls & High-Yield Facts:** * **Rate-limiting step:** PRPP glutamyl amidotransferase is the key regulatory enzyme for de novo purine synthesis. * **Lesch-Nyhan Syndrome:** Caused by a deficiency of **HGPRT** (Salvage pathway). This leads to an accumulation of PRPP, which over-activates PRPP glutamyl amidotransferase, resulting in excessive de novo purine production and severe hyperuricemia. * **Inhibitor:** The drug **6-Mercaptopurine** acts as a competitive inhibitor of PRPP glutamyl amidotransferase, halting DNA synthesis in rapidly dividing cells.

Q: What is the cause of increased uric acid production in tumors?

Increased purine degradation. **Explanation:** The correct answer is **A. Increased purine degradation.** **1. Why it is correct:** Tumors are characterized by rapid cell turnover and high rates of cell death (lysis). When tumor cells break down—either spontaneously or due to chemotherapy (Tumor Lysis Syndrome)—large amounts of nucleic acids are released. The catabolism of **purines** (Adenine and Guanine) follows a specific pathway: they are converted into Xanthine and subsequently oxidized by the enzyme **Xanthine Oxidase** into **Uric Acid**. Because uric acid is the final metabolic end-product of purine metabolism in humans, massive cell turnover directly leads to hyperuricemia. **2. Why the other options are incorrect:** * **B & D (Pyrimidine degradation/synthesis):** Unlike purines, the end-products of pyrimidine catabolism (β-alanine and β-aminoisobutyrate) are highly water-soluble and do not form uric acid. Therefore, pyrimidine turnover does not contribute to gout or hyperuricemia. * **C (De novo purine synthesis):** While rapidly dividing tumor cells do increase de novo synthesis to create new DNA, the *excess* uric acid seen clinically in cancer patients is primarily a result of the **breakdown** of existing cellular bulk rather than the synthetic phase. **Clinical Pearls for NEET-PG:** * **Tumor Lysis Syndrome (TLS):** A metabolic emergency characterized by Hyperuricemia, Hyperkalemia, Hyperphosphatemia, and **Hypocalcemia**. * **Management:** Allopurinol (Xanthine Oxidase inhibitor) or Rasburicase (recombinant Urate Oxidase) is used to manage high uric acid levels. * **Key Enzyme:** Xanthine Oxidase is the rate-limiting step in uric acid formation and a frequent target for pharmacology questions.

Q: Allopurinol prevents the conversion of which of the following substances?

Hypoxanthine to xanthine. ### Explanation **Correct Option: A. Hypoxanthine to xanthine** Allopurinol is a structural analog of hypoxanthine and acts as a potent **suicide inhibitor** of the enzyme **Xanthine Oxidase (XO)**. In the purine degradation pathway, Xanthine Oxidase catalyzes two sequential steps: 1. **Hypoxanthine → Xanthine** 2. **Xanthine → Uric Acid** By inhibiting XO, allopurinol prevents the formation of both xanthine and uric acid, leading to an increase in more soluble precursors (hypoxanthine and xanthine) which are easily excreted by the kidneys. **Analysis of Incorrect Options:** * **B. Xanthine to hypoxanthine:** This is the reverse reaction. Purine catabolism is an irreversible pathway; xanthine does not convert back to hypoxanthine. * **C. Hypoxanthine to I.M.P:** This reaction is catalyzed by **HGPRT** (Hypoxanthine-Guanine Phosphoribosyltransferase) as part of the **Purine Salvage Pathway**. Allopurinol actually *promotes* this step because, when XO is inhibited, hypoxanthine levels rise and are diverted toward IMP synthesis, further reducing de novo purine production. * **D. All of the above:** Incorrect as only Option A describes the inhibited catabolic step. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Allopurinol is converted by XO into **Alloxanthine (Oxypurinol)**, which binds tightly to the enzyme's active site (suicide inhibition). * **Drug Interaction:** Since **6-Mercaptopurine** and **Azathioprine** are metabolized by Xanthine Oxidase, their dosage must be reduced by 75% if co-administered with Allopurinol to avoid toxicity. * **Lesch-Nyhan Syndrome:** Allopurinol is used to treat hyperuricemia in these patients but does not improve neurological symptoms.

Q: What is a product of purine metabolism?

uric acid. **Explanation:** The correct answer is **D. uric acid**. **1. Why Uric Acid is Correct:** In humans, **uric acid** is the final metabolic breakdown product of purine nucleotides (Adenine and Guanine). The pathway involves the conversion of adenosine and guanosine into **Xanthine**, which is then oxidized by the enzyme **Xanthine Oxidase** to form uric acid. Because humans lack the enzyme *uricase*, we cannot further break down uric acid into allantoin, making it the terminal excretory product excreted via urine. **2. Why Other Options are Incorrect:** * **A. Beta-alanine:** This is a breakdown product of **pyrimidine** catabolism (specifically Uracil and Cytosine). * **B & C. Ammonia and Carbon Dioxide:** While these are produced during various metabolic cycles (like the Urea cycle or TCA cycle), they are the end products of **pyrimidine** catabolism (specifically Thymine breaks down into $\beta$-aminoisobutyrate, $CO_2$, and $NH_3$). In purine metabolism, the carbon skeleton remains largely intact as the purine ring is not cleaved into $CO_2$ and $NH_3$ in humans. **3. High-Yield Clinical Pearls for NEET-PG:** * **Gout:** Caused by hyperuricemia (elevated uric acid), leading to the deposition of **monosodium urate crystals** in joints (Tophi). * **Lesch-Nyhan Syndrome:** A deficiency of **HGPRT** (salvage pathway) leads to excessive de novo purine synthesis and massive overproduction of uric acid, characterized by self-mutilation and mental retardation. * **Pharmacology Link:** **Allopurinol** and **Febuxostat** treat gout by inhibiting **Xanthine Oxidase**, thereby reducing uric acid production. * **Von Gierke’s Disease:** Often presents with hyperuricemia due to increased pentose phosphate pathway activity, leading to increased PRPP and purine synthesis.

Q: Increased uric acid level in plasma is due to which of the following mechanisms?

Decreased purine reutilization. **Explanation:** The correct answer is **D. Decreased purine reutilization.** **Understanding the Mechanism:** Uric acid is the final metabolic breakdown product of purine nucleotides (Adenine and Guanine) in humans. Purines are managed by the body through two pathways: the **De Novo Synthesis** pathway (creating new purines) and the **Salvage Pathway** (reutilizing existing purines). The Salvage Pathway, primarily mediated by the enzyme **HGPRT** (Hypoxanthine-Guanine Phosphoribosyltransferase), recycles free purine bases back into nucleotides (IMP and GMP). When purine reutilization is decreased (as seen in **Lesch-Nyhan Syndrome**), two things happen: 1. Free purine bases are not recycled and are instead diverted to the degradation pathway, forming uric acid. 2. The lack of salvaged nucleotides leads to a loss of feedback inhibition on the *PRPP Amidotransferase* enzyme, significantly accelerating *de novo* purine synthesis, which further increases the purine load for degradation. **Why other options are incorrect:** * **A & B:** Increased utilization or decreased synthesis of purines would lead to a *reduction* in the pool of purines available for breakdown, thereby lowering uric acid levels. * **C:** Uric acid is the end product of purine degradation. Therefore, *increased* (not decreased) degradation leads to hyperuricemia. **High-Yield Clinical Pearls for NEET-PG:** * **Lesch-Nyhan Syndrome:** An X-linked recessive deficiency of **HGPRT**. Characterized by hyperuricemia, gout, intellectual disability, and distinctive **self-mutilation**. * **Von Gierke’s Disease:** Can cause hyperuricemia due to increased G6P entering the Pentose Phosphate Pathway, raising PRPP levels and driving purine synthesis. * **Drug Link:** **Allopurinol** treats hyperuricemia by inhibiting **Xanthine Oxidase**, the enzyme responsible for the final steps of uric acid production.

Q: To which nitrogen atom of the pyrimidine ring does the first carbon of the pentose sugar of a nucleic acid attach?

N-1 of pyrimidine. **Explanation:** In nucleic acid biochemistry, the formation of a **nucleoside** involves a covalent bond between the pentose sugar (ribose or deoxyribose) and a nitrogenous base. This specific linkage is known as a **β-N-glycosidic bond**. **Why Option B is correct:** The bond always forms between the **C-1 (anomeric carbon)** of the pentose sugar and a specific nitrogen atom of the base. For **pyrimidines** (Cytosine, Thymine, and Uracil), which are single-ring structures, the attachment occurs at the **N-1 position**. **Why other options are incorrect:** * **Option A (N-9 of pyrimidine):** This is incorrect because pyrimidines only have two nitrogen atoms (at positions 1 and 3). There is no N-9 in a pyrimidine ring. * **Option C (N-1 of purine):** This is incorrect. In **purines** (Adenine and Guanine), which are double-ring structures, the sugar attaches to the **N-9 position**, not N-1. **High-Yield Facts for NEET-PG:** * **Purines (A, G):** Attach via **N-9** to C-1 of sugar. (Mnemonic: "Pure As Gold" - Purines are larger, so they use the larger number, 9). * **Pyrimidines (C, T, U):** Attach via **N-1** to C-1 of sugar. * **Bond Type:** It is a **β-configuration** bond because the base lies above the plane of the sugar ring. * **Clinical Relevance:** Several antiviral and anticancer drugs (e.g., 5-Fluorouracil, Zidovudine) are **nucleoside analogues** that work by modifying these rings or the glycosidic linkage to inhibit DNA/RNA synthesis.

Q: Base unstacking of DNA results in which of the following phenomena?

Hyperchromicity. **Explanation:** **Correct Option: B. Hyperchromicity** Hyperchromicity refers to the increase in the absorption of ultraviolet (UV) light (at 260 nm) by DNA when it is denatured. In its native double-helical state, the nitrogenous bases are "stacked" tightly within the hydrophobic core, which limits their ability to absorb light. When DNA is heated or exposed to extreme pH, the hydrogen bonds break (denaturation), leading to **base unstacking**. This exposes the conjugated double bonds of the purines and pyrimidines, significantly increasing their UV absorbance. **Incorrect Options:** * **A. Hypochromicity:** This is the opposite effect; it refers to the *decrease* in light absorption. This occurs when single-stranded DNA renatures (anneals) back into a double helix, as the bases become restacked. * **C. Electrophoresis:** This is a laboratory technique used to separate DNA fragments based on their size and charge (DNA is negatively charged due to its phosphate backbone) using an electric field. It is not a direct optical consequence of base unstacking. * **D. Linear dichromicity:** This relates to the differential absorption of polarized light based on the orientation of molecules. While DNA exhibits this property, it is not the standard term used to describe the change in absorbance during denaturation. **High-Yield Clinical Pearls for NEET-PG:** * **Melting Temperature (Tm):** The temperature at which 50% of DNA is denatured. A higher **G-C content** increases the Tm because G-C pairs have three hydrogen bonds, whereas A-T pairs have only two. * **Absorbance Peak:** DNA and RNA absorb maximally at **260 nm**, while proteins absorb at **280 nm**. The 260/280 ratio is used to assess DNA purity. * **Reversibility:** Denaturation is reversible (renaturation/annealing), a principle fundamental to **PCR (Polymerase Chain Reaction)** and **Southern Blotting**.

Question 1

Which enzyme catalyzes the production of orotidylic acid?

Accepted Answer

Orotate phosphoribosyl transferase

Answer

Aspartyl transcarbamoylase

Answer

Dihydroorotase

Answer

Dihydroorotate dehydrogenase

Question 2

Gout is a disorder of:

Accepted Answer

Purine Metabolism

Answer

Pyrimidine Metabolism

Answer

Ketone Metabolism

Answer

Protein metabolism

Question 3

Which of the following best represents commonalities in the de novo synthesis pathways of IMP and UMP?

Accepted Answer

Both require PRPP, Glutamine, and Aspartic Acid.

Answer

Both require PRPP and Aspartic Acid.

Answer

Both require PRPP, Folate Derivatives, and Glutamine.

Answer

Both require Folate Derivatives, Glycine, and Aspartic Acid.

Question 4

PRPP glutamyl amidotransferase is increased to increase purine synthesis in which of the following?

Accepted Answer

Liver

Answer

RBC

Answer

Brain

Answer

Polymorphs

Question 5

What is the cause of increased uric acid production in tumors?

Accepted Answer

Increased purine degradation

Answer

Increased pyrimidine degradation

Answer

Denovo purine synthesis

Answer

Increased pyrimidine synthesis

Question 6

Allopurinol prevents the conversion of which of the following substances?

Accepted Answer

Hypoxanthine to xanthine

Answer

Xanthine to hypoxanthine

Answer

Hypoxanthine to I.M.P

Answer

All of the above

Question 7

What is a product of purine metabolism?

Accepted Answer

uric acid

Answer

beta-alanine

Answer

ammonia

Answer

carbon dioxide

Question 8

Increased uric acid level in plasma is due to which of the following mechanisms?

Accepted Answer

Decreased purine reutilization

Answer

Increased purine utilization

Answer

Decreased purine synthesis

Answer

Decreased purine degradation

Question 9

To which nitrogen atom of the pyrimidine ring does the first carbon of the pentose sugar of a nucleic acid attach?

Accepted Answer

N-1 of pyrimidine

Answer

N-9 of pyrimidine

Answer

N-1 of purine

Answer

None of the above

Question 10

Base unstacking of DNA results in which of the following phenomena?

Accepted Answer

Hyperchromicity

Answer

Hypochromicity

Answer

Electrophoresis

Answer

Linear dichromicity

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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