Nucleic Acid Biochemistry — MCQs
On this page
Nucleotides serve all of the following roles, EXCEPT:
What is the end product of pyrimidine catabolism?
Which of the following compounds is an analogue of hypoxanthine?
Which of the following is the donor of ADP-ribose for ADP-ribosylation reactions?
Which of the following correctly states the base pairing rules described by Watson and Crick?
At Tm or melting temperature, what percentage of double-stranded DNA (dsDNA) is denatured?
Which of the following is a feature of pyrimidine metabolism?
A 47-year-old male patient presents with painful arthritis in the right big toe and uric acid renal stones. He has been taking allopurinol for his condition. What biochemical defect would likely be found in this patient?
Arrange the enzymes involved in purine catabolism in the correct sequence.
What is the end product of pyrimidine metabolism?
Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs
Question 1: Nucleotides serve all of the following roles, EXCEPT:
- A. Monomeric units of nucleic acids
- B. Mediators in cellular signalling
- C. Source of energy
- D. Structural component of membrane (Correct Answer)
Explanation: ### Explanation **Nucleotides** are versatile molecules consisting of a nitrogenous base, a pentose sugar, and one or more phosphate groups. While they are central to genetic and metabolic processes, they do **not** serve as structural components of cell membranes. Cell membranes are primarily composed of phospholipids, cholesterol, and proteins. #### Why Option D is Correct: **Structural component of membrane:** Nucleotides are highly hydrophilic (polar) due to their phosphate groups and sugar moieties. This makes them unsuitable for forming the hydrophobic core of the lipid bilayer. Membranes require amphipathic molecules like phospholipids; nucleotides lack the long-chain fatty acids necessary for membrane integrity. #### Why Other Options are Incorrect: * **A. Monomeric units of nucleic acids:** This is the primary role of nucleotides. DNA and RNA are polymers formed by phosphodiester bonds between deoxyribonucleotides and ribonucleotides, respectively. * **B. Mediators in cellular signaling:** Nucleotides act as crucial secondary messengers. Examples include **cAMP** (cyclic AMP) and **cGMP**, which relay signals from hormones and neurotransmitters. Additionally, ATP and adenosine act as extracellular signaling molecules (purinergic signaling). * **C. Source of energy:** **ATP** (Adenosine triphosphate) is the "universal energy currency" of the cell. Other nucleotides like GTP are also used in specific pathways (e.g., protein synthesis and gluconeogenesis). --- ### High-Yield Clinical Pearls for NEET-PG: * **Synthetic Analogues:** Many chemotherapeutic drugs are nucleotide/nucleoside analogues (e.g., **5-Fluorouracil, Methotrexate**) that inhibit DNA synthesis. * **Activated Intermediates:** Nucleotides serve as carriers for biosynthetic precursors, such as **UDP-Glucose** (glycogen synthesis) and **CDP-Choline** (phospholipid synthesis). * **Coenzyme Components:** Many essential coenzymes like **NAD+, FAD, and Coenzyme A** contain an adenine nucleotide moiety as part of their structure.
Question 2: What is the end product of pyrimidine catabolism?
- A. Ammonia (NH3)
- B. Carbon dioxide (CO2) and water (H2O) (Correct Answer)
- C. Both ammonia and carbon dioxide/water
- D. None of the above
Explanation: **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)**.
Question 3: Which of the following compounds is an analogue of hypoxanthine?
- A. Arabinoside C
- B. Allopurinol (Correct Answer)
- C. Ribose phosphate
- D. 5-phosphoribosylpyrophosphate (PRPP)
Explanation: **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.
Question 4: Which of the following is the donor of ADP-ribose for ADP-ribosylation reactions?
- A. Adenosine triphosphate
- B. Adenosine monophosphate
- C. Nicotinamide adenine dinucleotide (Correct Answer)
- D. Flavin mononucleotide
Explanation: **Explanation:** **Nicotinamide Adenine Dinucleotide (NAD+)** is the correct answer because it serves as the universal substrate for ADP-ribosylation. In this post-translational modification, the enzyme (ADP-ribosyltransferase) cleaves the glycosidic bond between nicotinamide and the ribose of NAD+, transferring the **ADP-ribose** moiety to a target protein and releasing free nicotinamide. **Analysis of Options:** * **Adenosine triphosphate (ATP):** While ATP is the primary energy currency and a phosphate donor in phosphorylation, it does not contain the specific "ADP-ribose" structure required for this reaction. * **Adenosine monophosphate (AMP):** AMP is a nucleotide but lacks the additional ribose and phosphate groups necessary to act as an ADP-ribose donor. * **Flavin mononucleotide (FMN):** FMN is a coenzyme derived from Riboflavin (Vitamin B2) involved in redox reactions; it does not participate in ADP-ribosylation. **Clinical Pearls & High-Yield Facts for NEET-PG:** 1. **Bacterial Toxins:** Several potent toxins utilize this mechanism to cause disease. * **Diphtheria toxin** and **Pseudomonas Exotoxin A** ADP-ribosylate **EF-2** (Elongation Factor 2), inhibiting protein synthesis. * **Cholera toxin** ADP-ribosylates the **Gs protein**, leading to permanent activation of adenylate cyclase and massive watery diarrhea. * **Pertussis toxin** ADP-ribosylates the **Gi protein**. 2. **DNA Repair:** The enzyme **PARP (Poly ADP-ribose polymerase)** uses NAD+ to synthesize ADP-ribose polymers on proteins involved in DNA repair. PARP inhibitors are now significant in oncology (e.g., Olaparib for BRCA-mutant cancers). 3. **Sirtuins:** These are NAD+-dependent deacetylases that link metabolic status to gene expression via ADP-ribosylation and deacetylation.
Question 5: Which of the following correctly states the base pairing rules described by Watson and Crick?
- A. Adenine pairs with Thymine, and Cytosine pairs with Guanine (A-T, C-G) (Correct Answer)
- B. Adenine pairs with Guanine, and Thymine pairs with Cytosine (A-G, T-C)
- C. Adenine pairs with Cytosine, and Thymine pairs with Guanine (A-C, T-G)
- D. Adenine pairs with Guanine, and Thymine pairs with Uracil (A-G, T-U)
Explanation: **Explanation:** The Watson-Crick model of DNA structure is based on the principle of **complementary base pairing**. In a double-stranded DNA molecule, a large double-ringed **Purine** always pairs with a smaller single-ringed **Pyrimidine**. Specifically, **Adenine (A)** pairs with **Thymine (T)** via two hydrogen bonds, and **Guanine (G)** pairs with **Cytosine (C)** via three hydrogen bonds. This specific pairing ensures a constant distance between the two sugar-phosphate backbones, maintaining the uniform 20 Å diameter of the DNA helix. **Analysis of Incorrect Options:** * **Option B & D:** These suggest Purine-Purine (A-G) or Pyrimidine-Pyrimidine (T-C) pairings. Such pairings would distort the DNA helix, making it too wide or too narrow, respectively, and are chemically unstable due to mismatched hydrogen bond donors and acceptors. * **Option C:** While A-C and T-G involve one purine and one pyrimidine, their chemical structures do not allow for stable hydrogen bonding in the standard B-DNA configuration. **High-Yield NEET-PG Pearls:** 1. **Chargaff’s Rule:** In any double-stranded DNA, the amount of A equals T, and G equals C. Therefore, the ratio of Purines to Pyrimidines is always **1:1** (A+G = T+C). 2. **Bond Stability:** G-C pairs have **three hydrogen bonds**, whereas A-T pairs have **two**. Consequently, DNA with high G-C content has a higher melting temperature (Tm). 3. **RNA Exception:** In RNA, Thymine is replaced by **Uracil (U)**; therefore, Adenine pairs with Uracil (A-U). 4. **Clinical Correlation:** Drugs like **5-Fluorouracil** and **Methotrexate** target nucleotide synthesis, disrupting these base-pairing requirements to inhibit cancer cell replication.
Question 6: At Tm or melting temperature, what percentage of double-stranded DNA (dsDNA) is denatured?
- A. 25%
- B. 50% (Correct Answer)
- C. 75%
- D. 100%
Explanation: ### 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 7: Which of the following is a feature of pyrimidine metabolism?
- A. Requires a CAD polypeptide chain
- B. 6-azauridine causes orotic aciduria
- C. DHOA-dehydrogenase is mitochondrial
- D. Reye syndrome decreases cytosolic carbamoyl phosphate (Correct Answer)
Explanation: **Explanation:** The correct answer is **D**, though it requires a nuanced understanding of the Urea Cycle and Pyrimidine synthesis crosstalk. In **Reye Syndrome**, mitochondrial damage occurs, leading to a failure of the Urea Cycle. Specifically, the deficiency of Ornithine Transcarbamoylase (OTC) or general mitochondrial dysfunction causes an accumulation of **Carbamoyl Phosphate (CP)**. This excess CP leaks from the mitochondria into the **cytosol**, where it enters the pyrimidine biosynthetic pathway, leading to increased orotic acid levels. *Note: While the question marks "decreases" as correct, standard pathophysiology indicates Reye syndrome typically **increases** cytosolic CP due to mitochondrial leakage.* **Analysis of Options:** * **A. Requires a CAD polypeptide chain:** This is a **correct** feature of pyrimidine synthesis. CAD is a trifunctional cytosolic enzyme (Carbamoyl phosphate synthetase II, Aspartate transcarbamoylase, and Dihydroorotase). * **B. 6-azauridine causes orotic aciduria:** This is also a **correct** feature. 6-azauridine is converted to 6-azauridylate, which inhibits OMP decarboxylase, leading to orotic aciduria. * **C. DHOA-dehydrogenase is mitochondrial:** This is a **correct** feature. Dihydroorotate dehydrogenase is the only enzyme of pyrimidine synthesis located in the inner mitochondrial membrane; all others are cytosolic. **NEET-PG High-Yield Pearls:** 1. **Rate-limiting step:** CPS-II (inhibited by UTP, activated by PRPP). 2. **Leflunomide:** Inhibits Dihydroorotate dehydrogenase (used in Rheumatoid Arthritis). 3. **Orotic Aciduria Type I:** Deficiency of UMP Synthase (treat with Uridine). 4. **Differentiating Orotic Aciduria:** If accompanied by **hyperammonemia**, it suggests a Urea Cycle defect (OTC deficiency); if ammonia is normal, it is a primary pyrimidine synthesis defect. *(Note: In many competitive exams, if multiple options are technically true, the "best" answer often relates to the specific clinical correlation the examiner is testing, though options A, B, and C are standard biochemical facts.)*
Question 8: A 47-year-old male patient presents with painful arthritis in the right big toe and uric acid renal stones. He has been taking allopurinol for his condition. What biochemical defect would likely be found in this patient?
- A. A defect in urea synthesis
- B. An abnormality of the purine degradation pathway (Correct Answer)
- C. An inability to synthesize non-essential amino acids
- D. Defective topoisomerases
Explanation: **Explanation:** The clinical presentation of painful arthritis in the first metatarsophalangeal joint (podagra) and uric acid nephrolithiasis is classic for **Gout**. The use of **Allopurinol**, a xanthine oxidase inhibitor, further confirms the diagnosis of hyperuricemia. **1. Why the Correct Answer is Right:** Gout is a disorder of **purine metabolism**. In humans, the final degradation product of purine nucleotides (Adenine and Guanine) is **uric acid**. Hyperuricemia occurs due to either the overproduction of purines or, more commonly, the underexcretion of uric acid. When serum levels exceed solubility limits, monosodium urate crystals precipitate in joints (causing gouty arthritis) and the renal collecting system (causing stones). Therefore, the underlying defect lies in the **purine degradation pathway**. **2. Why Incorrect Options are Wrong:** * **A & C:** Urea synthesis and non-essential amino acid synthesis are related to **protein and nitrogen metabolism**. Defects in the urea cycle lead to hyperammonemia, not hyperuricemia. * **D:** Topoisomerases are enzymes involved in DNA replication and transcription by relieving torsional strain. While they interact with nucleic acids, they are not involved in the metabolic degradation that leads to gout. **High-Yield Clinical Pearls for NEET-PG:** * **Allopurinol Mechanism:** It is a suicide inhibitor of **Xanthine Oxidase**, reducing the conversion of hypoxanthine and xanthine into uric acid. * **Lesch-Nyhan Syndrome:** An X-linked deficiency of **HGPRT** (salvage pathway) that leads to excessive purine synthesis and degradation, presenting with self-mutilation and gout. * **Von Gierke’s Disease:** Can cause secondary gout due to increased G6P entering the Pentose Phosphate Pathway, leading to increased Ribose-5-Phosphate and subsequent purine overproduction.
Question 9: Arrange the enzymes involved in purine catabolism in the correct sequence.
- A. Uricase - Adenosine Deaminase - Xanthine Oxidase - Purine Nucleoside Phosphorylase
- B. Xanthine Oxidase - Uricase - Purine Nucleoside Phosphorylase - Adenosine Deaminase
- C. Purine Nucleoside Phosphorylase - Xanthine Oxidase - Uricase - Adenosine Deaminase
- D. Adenosine Deaminase - Purine Nucleoside Phosphorylase - Xanthine Oxidase - Uricase (Correct Answer)
Explanation: ### Explanation Purine catabolism is the metabolic pathway that breaks down adenosine and guanosine nucleotides into waste products. The sequence follows a logical progression from nucleosides to free bases, and finally to uric acid (or allantoin). **1. Why Option D is Correct:** The pathway for Adenosine degradation follows this specific enzymatic sequence: * **Adenosine Deaminase (ADA):** Converts Adenosine to Inosine. * **Purine Nucleoside Phosphorylase (PNP):** Cleaves the ribose sugar from Inosine to form the free base, **Hypoxanthine**. * **Xanthine Oxidase (XO):** Oxidizes Hypoxanthine to Xanthine, and subsequently Xanthine to **Uric Acid**. * **Uricase (Urate Oxidase):** In most mammals, this enzyme converts uric acid to **Allantoin**. (Note: Humans lack functional uricase, making uric acid our end product). **2. Why Other Options are Incorrect:** * **Options A & B:** These are incorrect because **Adenosine Deaminase** must act first on the nucleoside before the base can be oxidized by Xanthine Oxidase. * **Option C:** This is incorrect because **PNP** acts on Inosine (the product of ADA). Starting with PNP bypasses the initial deamination step required for adenosine. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **ADA Deficiency:** Leads to **SCID (Severe Combined Immunodeficiency)** due to the toxic accumulation of dATP in T and B cells. It was the first disease treated with Gene Therapy. * **PNP Deficiency:** Leads to a rarer form of immunodeficiency primarily affecting **T-cells** (impairing cell-mediated immunity). * **Xanthine Oxidase:** This enzyme contains **Molybdenum**. It is the target of **Allopurinol** (suicide inhibition) used in the treatment of Gout. * **Uricase:** Humans lack this enzyme due to a mutational silencing of the gene; however, recombinant uricase (**Rasburicase**) is used clinically to treat Tumor Lysis Syndrome.
Question 10: What is the end product of pyrimidine metabolism?
- A. Urea
- B. Uric acid
- C. Beta alanine (Correct Answer)
- D. Allantoin
Explanation: ### Explanation The metabolism of pyrimidines (Cytosine, Uracil, and Thymine) differs significantly from purine metabolism. While purines are converted into insoluble uric acid, pyrimidines are catabolized into highly water-soluble products that are easily excreted or reused. **1. Why Beta-alanine is correct:** The pyrimidine ring is cleaved through a series of enzymatic steps. * **Cytosine and Uracil** are catabolized into **$\beta$-alanine**, $CO_2$, and $NH_3$. * **Thymine** is catabolized into **$\beta$-aminoisobutyrate**, $CO_2$, and $NH_3$. $\beta$-alanine is a key end product that can be further converted into Acetyl-CoA or used in the synthesis of carnosine and anserine. **2. Why other options are incorrect:** * **Option A (Urea):** While the ammonia ($NH_3$) released during pyrimidine catabolism may eventually enter the urea cycle, urea itself is the end product of **protein (amino acid) metabolism**, not the specific catabolic pathway of the pyrimidine ring. * **Option B (Uric acid):** This is the end product of **purine metabolism** (Adenine and Guanine) in humans. Excess uric acid leads to Gout. * **Option D (Allantoin):** In most mammals, uric acid is further oxidized by the enzyme *urate oxidase* to allantoin. However, **humans lack this enzyme**, making uric acid our final product. **Clinical Pearls for NEET-PG:** * **$\beta$-aminoisobutyrate** excretion in urine is a specific marker for high DNA turnover (e.g., leukemia or post-radiation therapy). * **Rate-limiting step:** The first step of pyrimidine catabolism is catalyzed by **Dihydropyrimidine dehydrogenase (DPD)**. A deficiency in this enzyme can lead to severe toxicity when treating patients with the chemotherapy drug **5-Fluorouracil (5-FU)**. * Unlike purines, pyrimidine catabolic products do not cause clinical diseases like gout because they are highly soluble.
Practice by Chapter
Nucleotide Structure and Function
Practice Questions
DNA Structure and Replication
Practice Questions
RNA Structure and Types
Practice Questions
Transcription: RNA Synthesis
Practice Questions
Post-Transcriptional Modifications
Practice Questions
Translation: Protein Synthesis
Practice Questions
Genetic Code and Codon Usage
Practice Questions
Regulation of Gene Expression
Practice Questions
Mutations and DNA Repair
Practice Questions
Purine Metabolism and Disorders
Practice Questions
Pyrimidine Metabolism and Disorders
Practice Questions
Nucleotide Degradation and Salvage Pathways
Practice Questions
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free