Nucleic Acid Biochemistry Practice Questions

Q: Gout is a disorder of-

Purine metabolism. **Explanation:** **Gout** is a clinical syndrome characterized by hyperuricemia (elevated serum uric acid levels), which leads to the deposition of monosodium urate crystals in joints and soft tissues. **1. Why Purine Metabolism is Correct:** Uric acid is the final metabolic breakdown product of **purine nucleotides** (Adenine and Guanine) in humans. The pathway involves the conversion of purines to Xanthine, which is then oxidized to Uric Acid by the enzyme **Xanthine Oxidase**. Gout occurs due to either the overproduction of uric acid (e.g., PRPP synthetase overactivity, Lesch-Nyhan syndrome) or, more commonly, decreased renal excretion of uric acid. **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 and do not cause gout. * **Ketone Metabolism:** Disorders here lead to ketoacidosis or hypoglycemia, not urate deposition. However, note that ketones can compete with uric acid for excretion in the kidneys, potentially triggering a gout flare. * **Protein Metabolism:** While purines are found in nucleoproteins, "protein metabolism" generally refers to amino acid breakdown, which primarily results in **urea** formation via the urea cycle. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard Diagnosis:** Identification of **negatively birefringent, needle-shaped crystals** under polarized light microscopy. * **Drug of Choice:** **Allopurinol** (a suicide inhibitor of Xanthine Oxidase) for chronic gout; **NSAIDs** or Colchicine for acute attacks. * **Von Gierke’s Disease (G6PD deficiency):** A common biochemical association where lactic acidosis inhibits uric acid excretion, leading to secondary gout. * **Lesch-Nyhan Syndrome:** Deficiency of **HGPRT** (salvage pathway) leads to massive purine overproduction and self-mutilation.

Q: Pyrimidine overproduction results in all of the following except:

Hyperuricemia. **Explanation:** The correct answer is **Hyperuricemia** because the catabolism of pyrimidines (Cytosine, Uracil, Thymine) results in highly water-soluble end products: **$\beta$-alanine** and **$\beta$-aminoisobutyrate**. Unlike purine catabolism, which produces insoluble uric acid leading to hyperuricemia and gout, pyrimidine breakdown does not contribute to uric acid levels. **Analysis of Options:** * **Hyperuricemia (A):** This is a hallmark of **purine** overproduction or impaired excretion. Pyrimidine metabolism is independent of the xanthine oxidase pathway that produces uric acid. * **Reye Syndrome (B):** In Reye syndrome, mitochondrial damage leads to hyperammonemia. This causes an accumulation of Carbamoyl Phosphate in the mitochondria, which leaks into the cytosol and enters the pyrimidine synthesis pathway, causing secondary overproduction of orotic acid. * **Orotic Aciduria (C):** This is a direct result of pyrimidine pathway derangement. Hereditary Orotic Aciduria (Type I) occurs due to a deficiency of **UMPS (Uridine Monophosphate Synthase)**, leading to a massive buildup and excretion of orotic acid. * **Megaloblastic Anemia (D):** In Hereditary Orotic Aciduria, the inability to synthesize pyrimidines leads to a lack of TTP and dCTP required for DNA synthesis in RBC precursors. This results in megaloblastic anemia that is **refractory** to Vitamin B12 and Folate. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme of Pyrimidine synthesis:** Carbamoyl Phosphate Synthetase II (CPS-II), located in the cytosol (inhibited by UTP). * **Differentiating Orotic Aciduria:** If Orotic aciduria is accompanied by **hyperammonemia**, suspect **OTC (Ornithine Transcarbamylase) deficiency** (Urea cycle defect). If ammonia levels are normal, suspect **Hereditary Orotic Aciduria**. * **Treatment:** Hereditary Orotic Aciduria is treated with **Oral Uridine**, which bypasses the metabolic block.

Q: Triple bonds are found between which base pairs?

Cytosine–Guanine. ### Explanation **Core Concept: Watson-Crick Base Pairing** In the DNA double helix, nitrogenous bases pair specifically via hydrogen bonds. The stability of the DNA structure depends on these bonds. * **Cytosine (C) and Guanine (G)** form **three (triple) hydrogen bonds**. This is due to the specific arrangement of hydrogen bond donors (amino and imino groups) and acceptors (carbonyl and ring nitrogens) on their respective rings. * **Adenine (A) and Thymine (T)** form only **two (double) hydrogen bonds**. **Analysis of Options:** * **Option B (Correct):** Cytosine and Guanine share three hydrogen bonds. Because of this extra bond, G-C rich DNA sequences are more stable and require higher temperatures to denature (higher Melting Temperature, $T_m$). * **Option A (Incorrect):** Adenine and Thymine pair via two hydrogen bonds. In RNA, Adenine pairs with Uracil, also via two bonds. * **Options C & D (Incorrect):** These represent Purine-Purine (A-G) or Pyrimidine-Pyrimidine (C-T) pairings. Under normal physiological conditions, these do not occur because they would distort the uniform width of the DNA helix. **NEET-PG High-Yield Pearls:** 1. **Melting Temperature ($T_m$):** The $T_m$ of DNA is directly proportional to the G-C content. Higher G-C content = Higher $T_m$. 2. **Chargaff’s Rule:** In double-stranded DNA, the amount of $A=T$ and $G=C$; therefore, Total Purines = Total Pyrimidines. 3. **Bond Strength:** The C-G bond is stronger than the A-T bond. This is why "TATA boxes" (A-T rich regions) are found in promoter sequences—they are easier for the cell to "unzip" to initiate transcription. 4. **Clinical Correlation:** Drugs like **Dactinomycin** (Actinomycin D) exert their effect by intercalating specifically into the minor groove of DNA at G-C rich cytotoxic sites, inhibiting RNA synthesis.

Q: A 3-year-old male child presents with severe self-mutilation behavior (repetitive biting of his lips and fingers causing scarring), choreoathetosis, spasticity, and intellectual disability. Laboratory investigations reveal elevated serum uric acid levels. Which of the following enzymes is likely to be deficient in this child?

HGPRTase. ***HGPRTase*** - **HGPRTase deficiency** causes **Lesch-Nyhan syndrome**, characterized by **self-mutilation behavior**, **hyperuricemia**, and **neurological abnormalities** in male children. - The deficiency leads to accumulation of **hypoxanthine** and **guanine**, causing excessive **uric acid production** and developmental delays. *Adenosine deaminase* - **ADA deficiency** causes **severe combined immunodeficiency (SCID)**, presenting with recurrent infections and immune system failure. - Does not cause **self-mutilation** or **hyperuricemia**, but rather **lymphopenia** and **opportunistic infections**. *APRTase* - **APRTase deficiency** leads to **2,8-dihydroxyadenine nephropathy** with kidney stone formation and renal dysfunction. - Does not present with **neurological symptoms** or **behavioral abnormalities** like self-mutilation. *Acid maltase* - **Acid maltase deficiency** causes **Pompe disease**, a glycogen storage disorder affecting muscles and heart. - Presents with **muscle weakness**, **cardiomegaly**, and **hepatomegaly**, not hyperuricemia or self-mutilation.

Q: The melting temperature of DNA is directly proportional to which of the following?

The percentage of Guanine-Cytosine (GC) base pairs. **Explanation:** The **Melting Temperature (Tm)** is defined as the temperature at which 50% of double-stranded DNA denatures into single strands. This process involves breaking the hydrogen bonds between complementary base pairs. **Why Option A is correct:** The stability of the DNA helix is primarily determined by the hydrogen bonding between bases. **Guanine (G) and Cytosine (C)** are held together by **three hydrogen bonds**, whereas Adenine (A) and Thymine (T) are held by only two. Because triple bonds require more thermal energy to break than double bonds, DNA with a higher **GC content** has a higher melting temperature. Therefore, Tm is directly proportional to the percentage of GC pairs. **Why other options are incorrect:** * **Option B:** Increasing AT base pairs decreases the overall stability of the helix because they only possess two hydrogen bonds. Thus, Tm is inversely proportional to AT content. * **Option C:** While the total energy required to denature a very long molecule is higher, the *temperature* at which it melts (Tm) is fundamentally a function of the base composition (GC ratio) and ionic strength, rather than the absolute length of the polymer in a clinical/biochemical context. **High-Yield Facts for NEET-PG:** * **Hyperchromicity:** Denatured (single-stranded) DNA absorbs more UV light at **260 nm** than double-stranded DNA. This increase in absorbance is used to measure Tm. * **Ionic Strength:** Tm also increases with higher salt concentrations (e.g., $Na^+$) because cations neutralize the negative charges on the phosphate backbone, reducing repulsion between strands. * **Formamide:** This chemical lowers the Tm by disrupting hydrogen bonds, often used in labs to denature DNA at lower temperatures.

Q: Nucleic acids absorb UV light maximally at which wavelength?

260 nm. ### Explanation **1. Why 260 nm is Correct:** Nucleic acids (DNA and RNA) contain nitrogenous bases (purines and pyrimidines) which possess **conjugated double bond systems**. These aromatic rings undergo resonance, allowing them to absorb ultraviolet (UV) light. The maximum absorption (λmax) occurs at **260 nm**. This property is routinely used in laboratories for the **quantitation** of DNA and RNA; the concentration of a sample can be calculated based on its absorbance at this specific wavelength using the Beer-Lambert Law. **2. Analysis of Incorrect Options:** * **280 nm (Option B):** This is the peak absorption wavelength for **proteins**. It is due to the presence of aromatic amino acids, primarily **Tryptophan** and Tyrosine. The ratio of absorbance at 260/280 nm is a critical high-yield index used to assess the purity of nucleic acid samples (a ratio of ~1.8 for DNA and ~2.0 for RNA indicates high purity). * **410 nm (Option C):** This falls within the visible light spectrum (violet). This wavelength is associated with the **Soret peak** of heme-containing proteins like hemoglobin. * **320 nm (Option D):** This wavelength is often used as a background correction in spectrophotometry because neither nucleic acids nor proteins absorb significantly here. **3. High-Yield Clinical Pearls for NEET-PG:** * **Hyperchromicity:** When DNA is denatured (double strand to single strand), its absorbance at 260 nm **increases**. This is known as the Hyperchromic effect. * **Tm (Melting Temperature):** The temperature at which 50% of DNA is denatured. DNA with higher **G-C content** has a higher Tm because G-C pairs have three hydrogen bonds compared to two in A-T pairs. * **Purity Check:** If the 260/280 ratio is <1.8, it suggests protein contamination in the DNA sample.

Q: Which arm of tRNA binds it to the ribosomal surface?

Pseudouridine arm. ### Explanation The correct answer is **B. Pseudouridine arm**. Transfer RNA (tRNA) has a characteristic "cloverleaf" secondary structure with four major arms, each serving a distinct functional role during translation. **1. Why the Pseudouridine arm is correct:** The **TψC arm** (where ψ represents pseudouridine) is named for the presence of the sequence Ribothymidine-Pseudouridine-Cytidine. This arm is responsible for **binding the tRNA molecule to the ribosomal surface**, specifically to the 5S rRNA of the large ribosomal subunit (60S in eukaryotes, 50S in prokaryotes). This interaction ensures the tRNA is correctly positioned within the A, P, or E sites during protein synthesis. **2. Why the other options are incorrect:** * **DHU arm (A):** Contains Dihydrouridine. Its primary role is **recognition by the specific Aminoacyl-tRNA synthetase** enzyme, ensuring the correct amino acid is attached to the tRNA. * **Acceptor arm (C):** This is the 3' end of the tRNA (ending in the sequence **CCA**). It is the site where the **amino acid is covalently attached** via an ester bond. * **Anticodon arm (D):** Contains the three-nucleotide anticodon sequence that **recognizes and base-pairs with the complementary codon** on the mRNA strand. ### High-Yield NEET-PG Pearls: * **Smallest RNA:** tRNA is the smallest (75–90 nucleotides; 4S) and is also known as "Soluble RNA." * **Post-transcriptional modifications:** tRNA is rich in unusual bases (Pseudouridine, Dihydrouridine, Methylguanosine). * **The "Adapter Molecule":** Termed by Francis Crick because it bridges the gap between mRNA codons and amino acids. * **Variable Arm:** The length of this arm determines the classification of tRNA into Class 1 (short) or Class 2 (long).

Question 1

Gout is a disorder of-

Accepted Answer

Purine metabolism

Answer

Pyrimidine metabolism

Answer

Ketone metabolism

Answer

Protein metabolism

Question 2

Pyrimidine overproduction results in all of the following except:

Accepted Answer

Hyperuricemia

Answer

Reye syndrome

Answer

Orotic aciduria

Answer

Megaloblastic anemia

Question 3

Which statement is true about the structure of DNA?

Accepted Answer

It has a deoxyribose-phosphate backbone with bases stacked inside.

Answer

Purines are adenine and guanine, and pyrimidines are uracil and cytosine.

Answer

Watson and Crick discovered the DNA structure in 1973.

Answer

It mainly consists of a left-handed helix.

Question 4

Which of the following statements is NOT true regarding hypoxanthine-guanine phosphoribosyltransferase?

Accepted Answer

Requires phosphoribosyl pyrophosphate (PRPP) as a substrate.

Answer

Predominantly present in the liver.

Answer

Involved in purine metabolism.

Answer

Its deficiency leads to Lesch-Nyhan disease.

Question 5

Triple bonds are found between which base pairs?

Accepted Answer

Cytosine–Guanine

Answer

Adenine–Thymine

Answer

Adenine–Guanine

Answer

Cytosine–Thymine

Question 6

A 3-year-old male child presents with severe self-mutilation behavior (repetitive biting of his lips and fingers causing scarring), choreoathetosis, spasticity, and intellectual disability. Laboratory investigations reveal elevated serum uric acid levels. Which of the following enzymes is likely to be deficient in this child?

Accepted Answer

HGPRTase

Answer

Adenosine deaminase

Answer

APRTase

Answer

Acid maltase

Question 7

Deficiency of purine nucleoside phosphorylase causes which of the following?

Accepted Answer

Cellular immunodeficiency

Answer

Complement deficiency

Answer

Humoral immunodeficiency

Answer

Combined immunodeficiency

Question 8

The melting temperature of DNA is directly proportional to which of the following?

Accepted Answer

The percentage of Guanine-Cytosine (GC) base pairs

Answer

The percentage of Adenine-Thymine (AT) base pairs

Answer

The length of the DNA molecule

Answer

None of the above

Question 9

Nucleic acids absorb UV light maximally at which wavelength?

Accepted Answer

260 nm

Answer

280 nm

Answer

410 nm

Answer

320 nm

Question 10

Which arm of tRNA binds it to the ribosomal surface?

Accepted Answer

Pseudouridine arm

Answer

DHU arm

Answer

Acceptor arm

Answer

Anticodon arm

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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