Nucleic Acid Biochemistry — MCQs

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?

Q147Easy

Deficiency of purine nucleoside phosphorylase causes which of the following?

Q148Easy

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

Q149Easy

Nucleic acids absorb UV light maximally at which wavelength?

Q150Easy

Which arm of tRNA binds it to the ribosomal surface?

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 141: Gout is a disorder of-

A. Purine metabolism (Correct Answer)
B. Pyrimidine metabolism
C. Ketone metabolism
D. Protein metabolism

Explanation: **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.

Question 142: Pyrimidine overproduction results in all of the following except:

A. Hyperuricemia (Correct Answer)
B. Reye syndrome
C. Orotic aciduria
D. Megaloblastic anemia

Explanation: **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.

Question 143: Which statement is true about the structure of DNA?

A. Purines are adenine and guanine, and pyrimidines are uracil and cytosine.
B. Watson and Crick discovered the DNA structure in 1973.
C. It has a deoxyribose-phosphate backbone with bases stacked inside. (Correct Answer)
D. It mainly consists of a left-handed helix.

Explanation: ### Explanation **Correct Option: C** DNA (Deoxyribonucleic acid) is a double-stranded helical molecule. The structural framework consists of a **deoxyribose-phosphate backbone** formed by 3'–5' phosphodiester bonds. This backbone is hydrophilic and located on the exterior, while the hydrophobic nitrogenous bases are **stacked inside** the helix, perpendicular to the axis. This arrangement minimizes the contact of hydrophobic bases with water, contributing to the stability of the double helix. **Analysis of Incorrect Options:** * **Option A:** While Adenine (A) and Guanine (G) are purines, the pyrimidines in DNA are **Cytosine (C) and Thymine (T)**. Uracil (U) is found exclusively in RNA, replacing Thymine. * **Option B:** James Watson and Francis Crick proposed the double-helix model in **1953** (not 1973), based on Rosalind Franklin’s X-ray diffraction data. They were awarded the Nobel Prize in 1962. * **Option D:** The most common physiological form of DNA is **B-DNA**, which is a **right-handed helix**. Z-DNA is the only major form that is a left-handed helix, but it is not the "main" form. **High-Yield Clinical Pearls for NEET-PG:** * **Chargaff’s Rule:** In double-stranded DNA, A=T and G≡C; therefore, Purines = Pyrimidines. * **Bonding:** A and T are linked by **2 hydrogen bonds**, while G and C are linked by **3 hydrogen bonds**. Higher G-C content increases the melting temperature (Tm) of DNA. * **DNA Forms:** * **B-DNA:** Right-handed, 10 base pairs (bp) per turn (Standard). * **A-DNA:** Right-handed, 11 bp per turn (Dehydrated). * **Z-DNA:** Left-handed, 12 bp per turn (Zig-zag backbone).

Question 144: Which of the following statements is NOT true regarding hypoxanthine-guanine phosphoribosyltransferase?

A. Predominantly present in the liver.
B. Involved in purine metabolism.
C. Requires phosphoribosyl pyrophosphate (PRPP) as a substrate. (Correct Answer)
D. Its deficiency leads to Lesch-Nyhan disease.

Explanation: ### Explanation **Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)** is a key enzyme in the **Purine Salvage Pathway**. It catalyzes the conversion of free purine bases (Hypoxanthine and Guanine) into their respective nucleotides (IMP and GMP). **Why Option C is the "Correct" (False) Statement:** The question asks for the statement that is **NOT** true. However, based on biochemical principles, **Option C is actually a true statement.** HGPRT absolutely requires **Phosphoribosyl pyrophosphate (PRPP)** as a substrate to donate the ribose-5-phosphate unit to the nitrogenous base. *Note for NEET-PG:* In many competitive exams, if an option describes a fundamental requirement of the enzyme (like PRPP for HGPRT), it is factually correct. If this was the designated "incorrect" answer in a source, it may be due to a typographical error in the question stem or options. **Option A** is often the intended "false" statement in similar MCQ banks because, while HGPRT is present in the liver, its **highest activity** and most critical physiological role are in the **Brain (Basal Ganglia)** and **Erythrocytes**, which lack the de novo synthesis pathway. **Analysis of Other Options:** * **Option B:** True. HGPRT is the central enzyme of the purine salvage pathway, recycling bases to prevent their degradation to uric acid. * **Option D:** True. A complete deficiency of HGPRT leads to **Lesch-Nyhan Syndrome**, characterized by hyperuricemia, intellectual disability, and self-mutilation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Lesch-Nyhan Syndrome:** X-linked recessive inheritance. Look for "orange sand" (urite crystals) in the diaper of infants. 2. **Biochemical Consequence:** HGPRT deficiency leads to an accumulation of PRPP, which over-activates *PRPP Glutamyl Amidotransferase*, further increasing *de novo* purine synthesis and worsening hyperuricemia. 3. **Kelley-Seegmiller Syndrome:** Partial deficiency of HGPRT, presenting primarily with gouty arthritis without severe neurological features.

Question 145: Triple bonds are found between which base pairs?

A. Adenine–Thymine
B. Cytosine–Guanine (Correct Answer)
C. Adenine–Guanine
D. Cytosine–Thymine

Explanation: ### 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.

Question 146: 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?

A. HGPRTase (Correct Answer)
B. Adenosine deaminase
C. APRTase
D. Acid maltase

Explanation: ***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.

Question 147: Deficiency of purine nucleoside phosphorylase causes which of the following?

A. Complement deficiency
B. Cellular immunodeficiency (Correct Answer)
C. Humoral immunodeficiency
D. Combined immunodeficiency

Explanation: ### **Explanation** **Purine Nucleoside Phosphorylase (PNP)** is a key enzyme in the purine salvage pathway. It is responsible for converting **Inosine to Hypoxanthine** and **Guanosine to Guanine**. #### **Why Cellular Immunodeficiency is Correct** A deficiency in PNP leads to the accumulation of its substrates, specifically **deoxyguanosine (dG)**. Intracellularly, dG is phosphorylated into **dGTP**. High levels of dGTP are toxic to rapidly dividing cells, particularly **T-lymphocytes**. * dGTP inhibits **ribonucleotide reductase**, the enzyme required for DNA synthesis. * T-cells are more sensitive to this metabolic toxicity than B-cells, leading to a profound decrease in T-cell numbers and function (**Cellular Immunodeficiency**). #### **Why Other Options are Incorrect** * **Complement deficiency:** This involves proteins of the innate immune system (e.g., C3, C4) and is typically caused by genetic mutations in the complement cascade, not purine metabolism. * **Humoral immunodeficiency:** This refers to B-cell/antibody deficiency. In PNP deficiency, B-cell function and immunoglobulin levels are usually **normal or only mildly affected**. * **Combined immunodeficiency (SCID):** While PNP deficiency is often grouped with SCID, it is classically distinguished by its **selective** impact on T-cells. **Adenosine Deaminase (ADA) deficiency** is the classic cause of Severe Combined Immunodeficiency (SCID), affecting both T-cells and B-cells. #### **High-Yield Clinical Pearls for NEET-PG** 1. **PNP vs. ADA:** * **ADA Deficiency:** Affects T-cells AND B-cells (**SCID**). * **PNP Deficiency:** Affects primarily T-cells (**Cellular Immunodeficiency**). 2. **Clinical Presentation:** Patients present with recurrent viral, fungal, and protozoal infections starting in infancy, often accompanied by **neurological symptoms** (spasticity, developmental delay) and **autoimmune disorders**. 3. **Inheritance:** PNP deficiency is **Autosomal Recessive**. 4. **Biochemical Marker:** Low serum and urinary uric acid levels (due to the block in the purine breakdown pathway).

Question 148: The melting temperature of DNA is directly proportional to which of the following?

A. The percentage of Guanine-Cytosine (GC) base pairs (Correct Answer)
B. The percentage of Adenine-Thymine (AT) base pairs
C. The length of the DNA molecule
D. None of the above

Explanation: **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.

Question 149: Nucleic acids absorb UV light maximally at which wavelength?

A. 260 nm (Correct Answer)
B. 280 nm
C. 410 nm
D. 320 nm

Explanation: ### 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.

Question 150: Which arm of tRNA binds it to the ribosomal surface?

A. DHU arm
B. Pseudouridine arm (Correct Answer)
C. Acceptor arm
D. Anticodon arm

Explanation: ### 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).

Practice by Chapter

Nucleotide Structure and Function

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DNA Structure and Replication

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RNA Structure and Types

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Transcription: RNA Synthesis

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Post-Transcriptional Modifications

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Translation: Protein Synthesis

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Genetic Code and Codon Usage

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Regulation of Gene Expression

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Mutations and DNA Repair

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Purine Metabolism and Disorders

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Pyrimidine Metabolism and Disorders

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Nucleotide Degradation and Salvage Pathways

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