Nucleic Acid Biochemistry Practice Questions

Q: What is the end product of pyrimidine catabolism?

Carbon dioxide (CO2) and water (H2O). **Explanation:** The catabolism of pyrimidines (Cytosine, Uracil, and Thymine) differs significantly from purine catabolism. While purines are degraded into insoluble uric acid, pyrimidines are broken down into highly soluble products that are easily excreted or utilized by the body. **1. Why Option B is Correct:** The pyrimidine ring is cleaved to produce **CO₂ and H₂O** as the ultimate metabolic end products. Specifically: * **Cytosine and Uracil** are degraded into **β-alanine**, which is further converted into CO₂, H₂O, and NH₃. * **Thymine** is degraded into **β-aminoisobutyrate**, which eventually breaks down into CO₂, H₂O, and NH₃. Because these products are simple, non-toxic molecules, pyrimidine catabolism does not lead to clinical disorders like gout. **2. Why Other Options are Incorrect:** * **Option A:** While ammonia (NH₃) is produced during the deamination steps of pyrimidine breakdown, it is an intermediate that is quickly diverted to the **Urea Cycle** for detoxification. It is not considered the final "end product" of the carbon skeleton. * **Option C:** Although both are produced, standard biochemical nomenclature identifies the final respiratory/metabolic disposal of the carbon skeleton as CO₂ and H₂O. **High-Yield Clinical Pearls for NEET-PG:** * **β-aminoisobutyrate:** The excretion of this metabolite in urine is a specific marker for high DNA turnover (e.g., leukemia or post-radiation therapy). * **Solubility:** Unlike purine end-products (uric acid), pyrimidine end-products are **highly water-soluble**. * **Rate-limiting enzyme:** Dihydropyrimidine dehydrogenase (DPD) is the key enzyme in pyrimidine catabolism. A deficiency in DPD can lead to severe toxicity when treating patients with the chemotherapy drug **5-Fluorouracil (5-FU)**.

Q: Which of the following compounds is an analogue of hypoxanthine?

Allopurinol. **Explanation:** **Allopurinol** is a structural analogue of **hypoxanthine**, a naturally occurring purine base. This structural similarity is the basis for its clinical use in treating gout. Allopurinol acts as a **suicide inhibitor** of the enzyme **Xanthine Oxidase**. Under normal conditions, xanthine oxidase converts hypoxanthine to xanthine and xanthine to uric acid. Allopurinol competes for the active site and is converted into oxypurinol (alloxanthine), which binds irreversibly to the enzyme, thereby reducing uric acid production and increasing levels of more soluble precursors (hypoxanthine and xanthine). **Analysis of Incorrect Options:** * **Arabinoside C (Cytarabine):** This is a pyrimidine analogue (specifically a cytosine analogue) used primarily as a chemotherapy agent in leukemias. It interferes with DNA synthesis. * **Ribose phosphate:** This is a phosphorylated five-carbon sugar that serves as a structural component of nucleotides, not a nitrogenous base analogue. * **5-phosphoribosylpyrophosphate (PRPP):** This is an activated form of ribose-5-phosphate. It is a key substrate in both the *de novo* and salvage pathways of purine and pyrimidine synthesis, but it is not a base analogue. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Allopurinol is a classic example of **competitive inhibition** (initially) and **suicide inhibition** (by its metabolite oxypurinol). * **Drug Interaction:** Because xanthine oxidase also metabolizes **6-Mercaptopurine (6-MP)** and **Azathioprine**, co-administration with Allopurinol leads to toxic levels of these drugs. The dose of 6-MP must be reduced by 75%. * **Lesch-Nyhan Syndrome:** Allopurinol is used to manage hyperuricemia in these patients, though it does not improve neurological symptoms.

Q: At Tm or melting temperature, what percentage of double-stranded DNA (dsDNA) is denatured?

50%. ### Explanation **Core Concept: DNA Denaturation and Tm** The **Melting Temperature (Tm)** is defined as the temperature at which **50% of the double-stranded DNA (dsDNA) is denatured** into single-stranded DNA (ssDNA). At this specific point, the DNA exists in a state of equilibrium where half of the helical structure has "unzipped" due to the breakage of hydrogen bonds between complementary base pairs. **Why Option B is Correct:** The Tm represents the midpoint of the hyperchromic shift. When DNA denatures, its absorbance of UV light at 260 nm increases (Hyperchromicity). The Tm is the temperature corresponding to the midpoint between the minimum absorbance (fully double-stranded) and maximum absorbance (fully single-stranded). **Analysis of Incorrect Options:** * **Option A (25%):** At this stage, the temperature is below the Tm; the DNA remains predominantly double-stranded. * **Option C (75%):** This occurs at temperatures slightly above the Tm as the DNA continues to transition toward a fully denatured state. * **Option D (100%):** This represents complete denaturation. This occurs well above the Tm, where all hydrogen bonds are broken, and the DNA exists entirely as single strands. **High-Yield Facts for NEET-PG:** 1. **G-C Content:** Tm is directly proportional to the G-C content of DNA. G-C pairs have **3 hydrogen bonds**, making them more thermally stable than A-T pairs (which have 2). 2. **Ionic Strength:** Increasing salt concentration (e.g., $Na^+$) increases Tm because cations neutralize the negatively charged phosphate backbone, reducing repulsion between strands. 3. **Hyperchromic Effect:** Denatured ssDNA absorbs **more** UV light at 260 nm than dsDNA due to the exposure of nitrogenous bases. 4. **Formamide/Urea:** These chemical denaturants **lower** the Tm by disrupting hydrogen bonding.

Question 1

Nucleotides serve all of the following roles, EXCEPT:

Accepted Answer

Structural component of membrane

Answer

Monomeric units of nucleic acids

Answer

Mediators in cellular signalling

Answer

Source of energy

Question 2

What is the end product of pyrimidine catabolism?

Accepted Answer

Carbon dioxide (CO2) and water (H2O)

Answer

Ammonia (NH3)

Answer

Both ammonia and carbon dioxide/water

Answer

None of the above

Question 3

Which of the following compounds is an analogue of hypoxanthine?

Accepted Answer

Allopurinol

Answer

Arabinoside C

Answer

Ribose phosphate

Answer

5-phosphoribosylpyrophosphate (PRPP)

Question 4

At Tm or melting temperature, what percentage of double-stranded DNA (dsDNA) is denatured?

Accepted Answer

50%

Answer

25%

Answer

75%

Answer

100%

Question 5

Which of the following is a feature of pyrimidine metabolism?

Accepted Answer

Reye syndrome decreases cytosolic carbamoyl phosphate

Answer

Requires a CAD polypeptide chain

Answer

6-azauridine causes orotic aciduria

Answer

DHOA-dehydrogenase is mitochondrial

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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