Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 31: What is the total number of ATP molecules required for de novo purine synthesis?

A. 2
B. 5 (Correct Answer)
C. 6
D. 4

Explanation: **Explanation:** The synthesis of the first purine nucleotide, **Inosine Monophosphate (IMP)**, is an energy-intensive process occurring in the cytosol. To reach the final product (IMP) from the starting material (Ribose-5-Phosphate), a total of **5 ATP molecules** are consumed. 1. **Step 1:** Ribose-5-Phosphate to PRPP (1 ATP → AMP; equivalent to **2 high-energy phosphate bonds**). 2. **Step 5:** Formation of Glycinamide ribonucleotide (GAR) (**1 ATP**). 3. **Step 7:** Formation of Formylglycinamidine ribonucleotide (FGAM) (**1 ATP**). 4. **Step 8:** Closure of the imidazole ring to form AIR (**1 ATP**). 5. **Step 10:** Formation of Succinylaminoimidazole carboxamide ribonucleotide (SAICAR) (**1 ATP**). While 6 high-energy phosphate bonds are broken, the question refers to the number of distinct enzymatic steps requiring ATP, which is 5. **Analysis of Incorrect Options:** * **Option A (2):** This significantly underestimates the complexity of the 11-step pathway. * **Option C (6):** This is a common point of confusion. While 6 high-energy phosphate bonds are consumed (because the first step produces AMP), the number of ATP molecules utilized as substrates is 5. * **Option D (4):** This does not account for all the phosphorylation and ring-closure steps required for the purine skeleton. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting step:** Glutamine-PRPP amidotransferase (Step 2). * **Key Donors:** Glycine (entire molecule), Aspartate (Nitrogen), Glutamine (Nitrogen), and **Tetrahydrofolate (THF)** (Carbon atoms C2 and C8). * **Clinical Correlation:** Methotrexate and Sulfonamides inhibit purine synthesis by interfering with folate metabolism, making them effective as anti-cancer and anti-microbial agents respectively.

Question 32: Which of the following does not contribute to the pyrimidine ring synthesis?

A. Aspartate
B. Glutamine
C. Tetrahydrofolate (THF) (Correct Answer)
D. Bicarbonate

Explanation: ### Explanation The synthesis of the pyrimidine ring is a de novo process that constructs a six-membered heterocyclic ring. Unlike purine synthesis, which is built upon a ribose-5-phosphate foundation, the pyrimidine ring is synthesized first and then attached to the ribose unit. **Why Tetrahydrofolate (THF) is the Correct Answer:** Tetrahydrofolate (THF) derivatives (specifically N10-formyl THF) are essential donors of carbon atoms in **purine** synthesis (C2 and C8). In **pyrimidine** synthesis, THF is **not** involved in the formation of the ring itself. Its only role in pyrimidine metabolism occurs *after* the ring is formed, during the conversion of dUMP to dTMP by thymidylate synthase, where N5,N10-methylene THF acts as a methyl donor. **Analysis of Incorrect Options:** * **Aspartate (A):** Provides the majority of the pyrimidine ring, contributing three carbons (C4, C5, C6) and one nitrogen (N1). * **Glutamine (B):** The amide group of glutamine provides the nitrogen at the N3 position. This reaction is catalyzed by **Carbamoyl Phosphate Synthetase II (CPS-II)**, the rate-limiting enzyme of the pathway. * **Bicarbonate (D):** Provides the carbon atom at the C2 position of the ring. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** CPS-II (located in the cytosol), inhibited by UTP and activated by PRPP. * **Leflunomide:** An immunosuppressant used in Rheumatoid Arthritis that inhibits **Dihydroorotate dehydrogenase**, blocking de novo pyrimidine synthesis. * **Orotic Aciduria:** Caused by a deficiency of UMP Synthase. It presents with megaloblastic anemia (refractory to B12/Folate) and growth retardation. Treatment is oral **Uridine** supplementation. * **Mnemonic for Pyrimidine atoms:** **"CAD"** — **C**arbamoyl phosphate (from Glutamine + CO2) and **As**partate.

Question 33: What protein forms the core of a nucleosome?

A. DNA
B. RNA
C. Histones (Correct Answer)
D. None of the above

Explanation: **Explanation:** The **nucleosome** is the fundamental repeating unit of eukaryotic chromatin, often described as "beads on a string." **1. Why Histones are correct:** The core of a nucleosome consists of an **octamer of histone proteins**. This octamer contains two copies each of four highly conserved histones: **H2A, H2B, H3, and H4**. Approximately 146 base pairs of DNA wrap around this protein core 1.75 times. Histones are rich in basic amino acids like **Lysine and Arginine**, giving them a positive charge. This allows them to bind tightly to the negatively charged phosphate backbone of DNA. **2. Why other options are incorrect:** * **DNA:** DNA is the genetic material that wraps *around* the core; it does not form the core itself. * **RNA:** RNA is involved in transcription and translation (mRNA, tRNA, rRNA) but is not a structural component of the nucleosome core. **3. NEET-PG High-Yield Facts:** * **Linker Histone:** **H1** is known as the "linker histone." It binds to the DNA between nucleosome beads and helps stabilize the 30-nm chromatin fiber. It is **not** part of the octamer core. * **Charge Interaction:** The interaction between histones and DNA is ionic, not covalent. * **Acetylation:** Histone acetylation (by HATs) neutralizes the positive charge on lysine, weakening the DNA-histone bond and leading to **euchromatin** (transcriptionally active). * **Deacetylation:** Histone deacetylation (by HDACs) restores the positive charge, leading to **heterochromatin** (transcriptionally silent).

Question 34: Which of the following nitrogenous bases is not found in RNA?

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

Explanation: ### Explanation **Core Concept:** Nucleic acids (DNA and RNA) are composed of nitrogenous bases categorized into **Purines** (Adenine and Guanine) and **Pyrimidines** (Cytosine, Thymine, and Uracil). The fundamental biochemical distinction between DNA and RNA lies in their sugar moiety and one specific pyrimidine base. While DNA uses **Thymine (5-methyluracil)**, RNA replaces it with **Uracil**. **Why Thymine is the Correct Answer:** Thymine is exclusive to DNA. Chemically, Thymine is simply Uracil with a methyl group at the C5 position. This methylation provides DNA with greater photochemical stability, which is essential for preserving the long-term integrity of the genetic blueprint. In RNA, which is generally short-lived and involved in protein synthesis, Uracil is used instead because it is energetically "cheaper" for the cell to produce. **Analysis of Incorrect Options:** * **A. Adenine & C. Guanine:** These are purines. Both are universal bases found in both DNA and RNA. * **D. Cytosine:** This is a pyrimidine found in both DNA and RNA. It pairs with Guanine via three hydrogen bonds. **High-Yield Clinical Pearls for NEET-PG:** * **The "Uracil Exception":** While Uracil is the hallmark of RNA, it can occasionally appear in DNA due to the spontaneous **deamination of Cytosine**. The enzyme *Uracil DNA Glycosylase* removes these errors to prevent mutations. * **5-Fluorouracil (5-FU):** A common chemotherapy agent that acts as a pyrimidine analog. It inhibits *Thymidylate Synthase*, blocking the conversion of dUMP to dTMP (Thymine synthesis), thereby inhibiting DNA replication. * **Base Pairing:** In RNA, Adenine pairs with Uracil (A=U) via two hydrogen bonds, whereas in DNA, Adenine pairs with Thymine (A=T).

Question 35: beta-hydroxybutyric aciduria is associated with deficiency of which enzyme?

A. Orotidylic acid decarboxylase
B. Orotate phosphoribosyl transferase
C. PRPP Synthase
D. Dihydropyrimidine dehydrogenase (Correct Answer)

Explanation: **Explanation:** The correct answer is **Dihydropyrimidine dehydrogenase (DPD)**. **1. Why Dihydropyrimidine dehydrogenase is correct:** Dihydropyrimidine dehydrogenase (DPD) is the **rate-limiting enzyme** in the catabolism of pyrimidines (uracil and thymine). * Thymine is normally degraded into **$\beta$-aminoisobutyrate**. * Uracil is normally degraded into **$\beta$-alanine**. When DPD is deficient, uracil and thymine cannot be processed further down their metabolic pathways. Instead, they are shunted toward alternative pathways or excreted. Specifically, a deficiency leads to the accumulation and urinary excretion of uracil, thymine, and **$\beta$-hydroxybutyric acid** (a byproduct of altered pyrimidine metabolism in this context). **2. Why the other options are incorrect:** * **Options A & B (Orotidylic acid decarboxylase & Orotate phosphoribosyl transferase):** These two enzymes are part of the bifunctional enzyme **UMP Synthase**. Their deficiency leads to **Hereditary Orotic Aciduria**, characterized by orotic acid crystals in urine and megaloblastic anemia, but not $\beta$-hydroxybutyric aciduria. * **Option C (PRPP Synthase):** Overactivity of this enzyme leads to increased purine synthesis and **Gout**. It is involved in the de novo synthesis pathway, not the catabolic pathway associated with $\beta$-hydroxybutyric aciduria. **Clinical Pearls for NEET-PG:** * **Pharmacogenetic Importance:** DPD deficiency is clinically critical because DPD is responsible for degrading the chemotherapy drug **5-Fluorouracil (5-FU)**. * Patients with DPD deficiency are at high risk for **severe, potentially fatal toxicity** (neurotoxicity and pancytopenia) if treated with standard doses of 5-FU or Capecitabine. * **Diagnostic Marker:** Elevated levels of uracil and thymine in the urine are the hallmarks of this condition.

Question 36: Phosphoribosyl pyrophosphate (PRPP) is used in:

A. Purine synthesis and Pyrimidine synthesis
B. Histidine synthesis
C. Niacin synthesis
D. All of the above (Correct Answer)

Explanation: **Explanation:** Phosphoribosyl pyrophosphate (PRPP) is a high-energy sugar phosphate that serves as the essential **activated form of ribose-5-phosphate**. It acts as the universal ribose donor in several metabolic pathways. 1. **Purine and Pyrimidine Synthesis:** PRPP is indispensable for both. In **purine synthesis** (de novo), it provides the ribose scaffold upon which the ring is built (starting with the enzyme PRPP glutamyl amidotransferase). In **pyrimidine synthesis**, PRPP is added to the completed base (orotic acid) to form OMP. It is also required for the **salvage pathways** (HGPRT and APRT enzymes). 2. **Histidine Synthesis:** PRPP reacts with ATP in the first step of histidine biosynthesis, providing the carbon atoms for the imidazole ring. 3. **Niacin (NAD+/NADP+) Synthesis:** PRPP is required for the synthesis of NAD+ from nicotinic acid (Preiss-Handler pathway) and nicotinamide, as well as the de novo synthesis from tryptophan (via quinolinate phosphoribosyltransferase). **Clinical Pearls & High-Yield Facts:** * **Rate-Limiting Step:** PRPP synthetase (which converts Ribose-5-P to PRPP) is inhibited by purine nucleotides (feedback inhibition). * **Lesch-Nyhan Syndrome:** A deficiency in HGPRT leads to an **accumulation of PRPP**, which over-activates de novo purine synthesis, resulting in extreme hyperuricemia. * **Von Gierke Disease:** Increased G6P shunts into the HMP shunt, raising Ribose-5-P and PRPP levels, which contributes to hyperuricemia in these patients. * **Orotic Aciduria:** A defect in the enzyme that utilizes PRPP in pyrimidine synthesis (UMPS) leads to megaloblastic anemia and growth retardation.

Question 37: Which one of the following bonds links two nucleotides in a nucleic acid?

A. 3'-3' phosphodiester bond
B. 3'-5' phosphodiester bond (Correct Answer)
C. 5'-5' phosphodiester bond
D. 5'-5' phosphotriester bond

Explanation: ### Explanation **1. Why the Correct Answer is Right:** In nucleic acids (DNA and RNA), nucleotides are linked together to form a linear polymer through **3'-5' phosphodiester bonds**. This bond forms when the phosphate group attached to the **5' carbon** of one pentose sugar (ribose or deoxyribose) reacts with the hydroxyl (-OH) group on the **3' carbon** of the adjacent nucleotide's sugar. This linkage creates the "sugar-phosphate backbone," providing the molecule with a distinct polarity (a 5' end and a 3' end). This directionality is crucial for processes like DNA replication and transcription, which always occur in a 5' to 3' direction. **2. Why the Other Options are Incorrect:** * **A & C (3'-3' and 5'-5' bonds):** These bonds do not occur in the standard backbone of DNA or RNA. Such linkages would disrupt the linear polarity required for genetic coding and enzyme recognition. (Note: A 5'-5' triphosphate bridge is found uniquely in the **mRNA 5' cap**). * **D (5'-5' phosphotriester bond):** A phosphotriester involves three ester bonds to a single phosphate. While these occur in some synthetic oligonucleotides or chemical intermediates, they are not the natural linkage in biological nucleic acids. **3. High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** DNA polymerase adds new nucleotides only to the **3' -OH group**; hence, DNA synthesis is always **5' → 3'**. * **Cleavage:** **Phosphodiesterases** (like snake venom or restriction enzymes) are enzymes that hydrolyze these 3'-5' bonds. * **Charge:** The phosphate group in the phosphodiester bond is acidic, giving DNA/RNA an overall **negative charge**, which is why they migrate toward the anode (+) during electrophoresis. * **Stability:** RNA is more susceptible to alkaline hydrolysis than DNA because of the additional 2' -OH group on the ribose sugar, which can attack the 3'-5' phosphodiester bond.

Question 38: Lesch-Nyhan syndrome is due to:

A. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) deficiency (complete) (Correct Answer)
B. Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) deficiency (partial)
C. Hexosaminidase deficiency (complete)
D. Hexosaminidase deficiency (partial)

Explanation: **Explanation:** **Lesch-Nyhan Syndrome (LNS)** is an X-linked recessive disorder characterized by a **complete deficiency** of the enzyme **Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)**. This enzyme is crucial for the **Purine Salvage Pathway**, where it converts hypoxanthine to IMP and guanine to GMP. When HGPRT is absent, these purines cannot be salvaged and are instead degraded into **uric acid**, leading to severe hyperuricemia. Furthermore, the lack of salvage leads to a compensatory increase in *de novo* purine synthesis (due to increased PRPP levels), further exacerbating uric acid production. **Analysis of Options:** * **Option A (Correct):** Complete deficiency of HGPRT results in the full clinical spectrum of LNS, including neurological dysfunction and self-mutilation. * **Option B (Incorrect):** A **partial deficiency** of HGPRT leads to **Kelley-Seegmiller syndrome**. While these patients suffer from gout and kidney stones due to hyperuricemia, they typically lack the severe neurological and behavioral symptoms (like self-mutilation) seen in LNS. * **Options C & D (Incorrect):** **Hexosaminidase A deficiency** is associated with **Tay-Sachs disease**, a lysosomal storage disorder, not purine metabolism. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Hyperuricemia (orange sand in diapers/uric acid stones), Intellectual disability, and **Self-mutilation** (biting lips and fingers). * **Biochemical Hallmark:** Elevated PRPP levels and decreased IMP/GMP. * **Treatment:** Allopurinol or Febuxostat (to manage uric acid), but these do not reverse neurological symptoms. * **Inheritance:** X-linked recessive (affects males).

Question 39: A nucleotide is composed of which of the following?

A. Nucleobase and sugar
B. Nucleobase and phosphate
C. Nucleobase, sugar, and phosphate (Correct Answer)
D. Sugar and phosphate

Explanation: ### Explanation The fundamental building blocks of nucleic acids (DNA and RNA) are **nucleotides**. A nucleotide is a chemical compound consisting of three distinct components: 1. **Nitrogenous Base (Nucleobase):** Either a Purine (Adenine, Guanine) or a Pyrimidine (Cytosine, Thymine, Uracil). 2. **Pentose Sugar:** Ribose (in RNA) or 2-deoxyribose (in DNA). 3. **Phosphate Group:** Attached to the 5' carbon of the sugar via an ester bond. The correct answer is **C** because it includes all three essential components required to form the monomeric unit of nucleic acids. #### Analysis of Incorrect Options: * **Option A (Nucleobase and sugar):** This describes a **Nucleoside**. A nucleoside lacks the phosphate group. (Mnemonic: Nucleo**S**ide = **S**ugar + Base). * **Option B and D:** These are incomplete chemical fragments. A phosphate group cannot bond directly to a nitrogenous base without the sugar scaffold, and a sugar-phosphate chain without a base forms the "backbone" but not a functional nucleotide. #### NEET-PG High-Yield Pearls: * **Bonding:** The base is linked to the sugar via an **N-glycosidic bond** (at N-9 of purines or N-1 of pyrimidines). * **Charge:** Nucleotides are acidic and carry a **negative charge** due to the phosphate group. * **Synthetic Analogs:** Many anti-cancer and anti-viral drugs are **nucleoside analogs** (e.g., Zidovudine, Acyclovir) which must be phosphorylated into nucleotides within the cell to become biologically active. * **Energy Currency:** Nucleotides are not just for DNA; they serve as energy carriers (ATP/GTP) and secondary messengers (cAMP).

Question 40: Which method is used to monitor DNA denaturation in vitro?

A. Centrifugation
B. UV absorption (Correct Answer)
C. Chromatography
D. HPLC

Explanation: **Explanation:** **1. Why UV Absorption is Correct:** DNA denaturation (the separation of double-stranded DNA into single strands) is monitored using the **Hyperchromic Effect**. Nitrogenous bases in DNA strongly absorb UV light at a wavelength of **260 nm**. In a double helix, these bases are "stacked" and shielded, which limits their light absorption. When DNA denatures (due to heat or alkali), the stacking interactions are disrupted, exposing the bases. This results in a significant increase (approx. 30-40%) in UV absorbance. The temperature at which 50% of the DNA is denatured is known as the **Melting Temperature (Tm)**. **2. Why Other Options are Incorrect:** * **A. Centrifugation:** This technique (e.g., Ultracentrifugation) separates molecules based on density, size, or shape (Svedberg units). While it can separate DNA from other organelles, it is not a real-time monitor for the denaturation process. * **C. Chromatography:** This is a broad term for separating mixtures (e.g., ion-exchange or size-exclusion). While it can purify DNA, it does not measure the transition from double-stranded to single-stranded states. * **D. HPLC (High-Performance Liquid Chromatography):** Used for high-resolution separation and quantification of nucleotides or drugs, but it is not the standard kinetic tool for monitoring DNA melting curves. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tm and GC Content:** The Melting Temperature (Tm) is directly proportional to the **G-C content** because G-C pairs have three hydrogen bonds, while A-T pairs have only two. * **Hyperchromicity:** An increase in absorbance upon denaturation. * **Hypochromicity:** A decrease in absorbance (occurs during DNA renaturation/annealing). * **Purity Ratio:** A pure DNA sample has an **A260/A280 ratio of ~1.8**. A lower ratio suggests protein contamination.

Practice by Chapter

Nucleotide Structure and Function

Practice Questions

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

Practice Questions

Regulation of Gene Expression

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Mutations and DNA Repair

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Purine Metabolism and Disorders

Practice Questions

Pyrimidine Metabolism and Disorders

Practice Questions

Nucleotide Degradation and Salvage Pathways

Practice Questions

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