Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 101: Triplex DNA is due to which of the following mechanisms?

A. Hoogsteen pairing (Correct Answer)
B. Palindromic sequences
C. Large number of guanosine repeats
D. Polypyrimidine tracts

Explanation: ### **Explanation** **1. Why Hoogsteen Pairing is Correct:** Standard DNA exists as the B-form double helix held together by **Watson-Crick base pairing** (A-T and G-C). However, **Triplex DNA (H-DNA)** forms when a third strand winds around a standard double helix. This third strand occupies the **major groove** of the duplex and forms unconventional hydrogen bonds known as **Hoogsteen base pairs**. In this arrangement, the N7 position of the purine (acting as an acceptor) and the C6 amino group participate in bonding, allowing a single purine to pair with two pyrimidines simultaneously. **2. Analysis of Incorrect Options:** * **B. Palindromic sequences:** These are sequences that read the same forward and backward on opposite strands. They are the recognition sites for restriction endonucleases and typically lead to the formation of **Hairpins** or **Cruciform structures**, not triplexes. * **C. Large number of guanosine repeats:** While G-rich sequences are involved in non-canonical structures, they specifically lead to the formation of **G-quadruplexes** (four-stranded DNA), commonly found in telomeres. * **D. Polypyrimidine tracts:** While triplex DNA often forms at mirror-repeat sequences containing polypyrimidine/polypurine tracts, the *mechanism* of the triple-bond formation itself is Hoogsteen pairing. **3. NEET-PG High-Yield Pearls:** * **H-DNA:** Another name for Triplex DNA; it often forms in vivo during replication, recombination, or transcription. * **Clinical Relevance:** Triplex-forming oligonucleotides (TFOs) are being researched for **gene therapy** to knock out specific gene expressions by blocking transcription. * **Z-DNA:** A left-handed helix with a zigzag sugar-phosphate backbone, often found in alternating purine-pyrimidine tracts (e.g., GCGCGC). * **G-Quadruplex:** Associated with **Telomeres** and the enzyme Telomerase; stabilized by Monovalent cations (like $K^+$).

Question 102: Salvage pathways of purine nucleotide synthesis are used by all except?

A. Brain
B. Liver (Correct Answer)
C. RBC
D. Leukocytes

Explanation: **Explanation:** Purine nucleotide synthesis occurs via two pathways: **De novo synthesis** (building the ring from scratch) and the **Salvage pathway** (recycling preformed bases). The **Liver** is the primary site for *de novo* purine synthesis. It possesses a full complement of enzymes required to synthesize purines from precursors like glycine, glutamine, and aspartate. Because the liver is metabolically "self-sufficient" and acts as the main exporter of purine bases to other tissues, it does not rely on the salvage pathway for its primary needs. **Analysis of Options:** * **Brain (A):** The brain has low levels of *de novo* synthesis enzymes (specifically PRPP amidotransferase). It relies heavily on the salvage pathway to maintain its nucleotide pool. * **RBCs (C):** Mature erythrocytes lack mitochondria and the complex enzymatic machinery required for the energy-intensive *de novo* pathway. They depend entirely on the salvage of hypoxanthine and guanine via the enzyme HGPRT. * **Leukocytes (D):** Similar to RBCs and the brain, peripheral leukocytes have limited *de novo* capacity and utilize salvage pathways to meet their metabolic demands. **High-Yield Clinical Pearls for NEET-PG:** * **Lesch-Nyhan Syndrome:** Caused by a deficiency of **HGPRT** (the key salvage enzyme). It leads to an accumulation of PRPP, which over-activates the *de novo* pathway, resulting in extreme hyperuricemia and self-mutilation. * **Von Gierke’s Disease:** Associated with hyperuricemia because increased G6P shunts into the Pentose Phosphate Pathway, increasing Ribose-5-Phosphate and stimulating *de novo* purine synthesis. * **Key Enzyme:** **PRPP Amidotransferase** is the rate-limiting step of *de novo* synthesis, while **HGPRT** is the hallmark of the salvage pathway.

Question 103: What is the end product of purine metabolism?

A. Creatinine
B. Uric acid (Correct Answer)
C. Xanthine
D. Phosphates

Explanation: **Explanation:** The correct answer is **Uric acid**. In humans and higher primates, the final step of purine catabolism involves the oxidation of hypoxanthine and guanine into **Xanthine**, which is then converted into **Uric acid** by the enzyme **Xanthine Oxidase**. Unlike many other mammals, humans lack the enzyme *Urate Oxidase (Uricase)*, which would otherwise convert uric acid into the more soluble allantoin; hence, uric acid remains the terminal metabolic product. **Analysis of Options:** * **A. Creatinine:** This is the breakdown product of creatine phosphate in muscle tissue, not nucleic acids. It is a marker of renal function. * **C. Xanthine:** While xanthine is an intermediate in the purine degradation pathway, it is further oxidized into uric acid. It only becomes an "end product" in rare pathological conditions like *Xanthinuria* (deficiency of xanthine oxidase). * **D. Phosphates:** These are inorganic components released during the initial breakdown of nucleotides into nucleosides, but they are not the organic end product of the nitrogenous base metabolism. **NEET-PG High-Yield Pearls:** 1. **Rate-limiting enzyme of Purine Synthesis:** PRPP Glutamyl Amidotransferase. 2. **Clinical Correlation:** Hyperuricemia (serum uric acid >7 mg/dL) can lead to **Gout**, where monosodium urate crystals deposit in joints. 3. **Pharmacology Link:** **Allopurinol** and **Febuxostat** treat gout by inhibiting Xanthine Oxidase, reducing uric acid production. 4. **Lesch-Nyhan Syndrome:** A deficiency in the salvage enzyme HGPRT leads to excessive de novo purine synthesis and massive overproduction of uric acid, presenting with self-mutilation and hyperuricemia.

Question 104: Which amino acid is a precursor for purine nucleotide synthesis?

A. Serine
B. Glycine (Correct Answer)
C. Alanine
D. Asparagine

Explanation: ### Explanation Purine nucleotides (Adenine and Guanine) are synthesized through a complex pathway where the purine ring is built atom-by-atom onto a ribose-5-phosphate base. **Why Glycine is Correct:** Glycine is a fundamental building block of the purine ring. It contributes three specific components: **C4, C5, and N7**. In the second step of *de novo* purine synthesis, glycine reacts with phosphoribosylamine (catalyzed by GAR synthetase) to form glycinamide ribonucleotide (GAR). It is the only amino acid that is incorporated into the ring structure in its entirety. **Analysis of Incorrect Options:** * **A. Serine:** While not a direct structural precursor, serine contributes indirectly by providing the one-carbon units (via the folate pool) required for C2 and C8 of the purine ring. * **C. Alanine:** Alanine does not participate in the synthesis of nitrogenous bases. It is primarily involved in glucose-alanine cycling and protein synthesis. * **D. Asparagine:** Asparagine is not a precursor. However, **Aspartate** (its acidic counterpart) is essential, providing the **N1** nitrogen atom of the purine ring. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Purine Ring Sources:** *"Pure As Gold"* (Purines: Adenine, Guanine). Sources: **Glycine** (C4, C5, N7), **Aspartate** (N1), **Glutamine** (N3, N9), **CO2** (C6), and **Tetrahydrofolate** (C2, C8). * **Rate-limiting enzyme:** Glutamine-PRPP amidotransferase. * **Inhibitor Drug:** **Methotrexate** inhibits dihydrofolate reductase, depleting the folate pool needed for C2 and C8 synthesis, thereby halting purine production and cell division.

Question 105: What type of bond is seen between the phosphate group and the 5' carbon of the ribose sugar within a nucleotide?

A. Phosphodiester bond
B. Ester bond (Correct Answer)
C. Beta N-glycosidic bond
D. Acid anhydride bond

Explanation: ### Explanation **Correct Answer: B. Ester bond** The fundamental structure of a **nucleotide** consists of a nitrogenous base, a pentose sugar, and a phosphate group. The bond between the phosphate group and the 5' hydroxyl (-OH) group of the ribose/deoxyribose sugar is formed via a dehydration reaction between an alcohol and an acid, specifically termed a **phosphoester bond**. Since the question asks for the bond *within* a single nucleotide, it is a simple ester linkage. **Analysis of Incorrect Options:** * **A. Phosphodiester bond:** This bond connects **two adjacent nucleotides** in a DNA or RNA strand. It involves two ester bonds: one connecting the 5' carbon of one sugar to the phosphate, and another connecting that same phosphate to the 3' carbon of the *next* sugar. * **C. Beta N-glycosidic bond:** This bond connects the **nitrogenous base** (N1 of pyrimidines or N9 of purines) to the **1' carbon** of the pentose sugar. * **D. Acid anhydride bond:** These are high-energy bonds found **between phosphate groups** in nucleoside triphosphates (like the alpha-beta and beta-gamma bonds in ATP). **High-Yield NEET-PG Pearls:** * **Nucleoside vs. Nucleotide:** Nucleoside = Base + Sugar; Nucleotide = Base + Sugar + Phosphate. * **Charge:** DNA is acidic and negatively charged due to the phosphate groups in the phosphodiester backbone. * **Directionality:** DNA synthesis always occurs in the **5' to 3' direction** because DNA polymerase adds new nucleotides to the free 3' -OH group. * **Chargaff’s Rule:** In double-stranded DNA, A+G (purines) = T+C (pyrimidines).

Question 106: What is involved in the formation of d-TMP from d-UMP?

A. N5, N10-methylene tetrahydrofolate (Correct Answer)
B. Formiminotetrahydrofolate
C. N5-formyltetrahydrofolate
D. Dihydrofolate

Explanation: ### Explanation The conversion of **dUMP (deoxyuridine monophosphate)** to **dTMP (deoxythymidine monophosphate)** is a critical step in DNA synthesis, catalyzed by the enzyme **Thidmidylate Synthase**. **1. Why Option A is Correct:** The synthesis of dTMP involves the addition of a methyl group to the C-5 position of the uracil ring. **N5, N10-methylene tetrahydrofolate (CH₂-THF)** acts as both the **one-carbon donor** and the **reducing agent** in this reaction. During this process, the methylene group is reduced to a methyl group, and the THF is oxidized to **Dihydrofolate (DHF)**. This is the only reaction in folate metabolism where THF is oxidized to DHF. **2. Analysis of Incorrect Options:** * **B. Formiminotetrahydrofolate:** This is an intermediate in the catabolism of **Histidine** (FIGLU to glutamate). It does not participate in pyrimidine synthesis. * **C. N5-formyltetrahydrofolate:** Also known as **Folinic acid (Leucovorin)**. While used clinically to "rescue" cells from methotrexate toxicity, it is not the direct substrate for dTMP synthesis. * **D. Dihydrofolate:** This is the **product** of the reaction, not the carbon donor. DHF must be converted back to THF by *Dihydrofolate Reductase (DHFR)* to continue the cycle. **3. NEET-PG High-Yield Pearls:** * **5-Fluorouracil (5-FU):** A suicide inhibitor of Thymidylate Synthase; it acts as a "thymineless death" agent. * **Methotrexate:** Inhibits **Dihydrofolate Reductase (DHFR)**, preventing the regeneration of THF from DHF, thereby halting dTMP synthesis. * **Rate-limiting step:** This reaction is a key target for many chemotherapeutic agents because dTMP is unique to DNA (not found in RNA).

Question 107: Which of the following is an example of an unusual base?

A. Dihydrouracil (Correct Answer)
B. Adenine
C. Guanine
D. Uracil

Explanation: **Explanation:** In nucleic acid biochemistry, bases are categorized into **major (standard) bases** and **unusual (modified) bases**. **Why Dihydrouracil is the correct answer:** Dihydrouracil is a modified pyrimidine base formed by the enzymatic reduction of uracil. It is considered an "unusual" base because it is not one of the five standard bases used in the primary genetic code. It is most famously found in **tRNA (Transfer RNA)**, specifically in the **D-loop** (Dihydrouridine loop), where it plays a crucial role in stabilizing the tRNA's tertiary structure and facilitating recognition by aminoacyl-tRNA synthetase. **Why the other options are incorrect:** * **Adenine (B) and Guanine (C):** These are the standard **Purine** bases found in both DNA and RNA. * **Uracil (D):** This is a standard **Pyrimidine** base found in RNA. While it is replaced by Thymine in DNA, it is a primary, major base in the transcriptome. **High-Yield NEET-PG Pearls:** * **tRNA Structure:** tRNA is the nucleic acid most enriched with unusual bases. Besides Dihydrouracil, other high-yield examples include **Pseudouridine** (found in the TψC loop) and **Inosine** (often found in the wobble position of the anticodon). * **Post-transcriptional Modification:** Unusual bases are not incorporated directly during transcription; they are formed via enzymatic modification of standard bases *after* the polynucleotide chain is synthesized. * **Methylated Bases:** In DNA, **5-methylcytosine** is a common modified base involved in gene silencing (epigenetics).

Question 108: Modified nucleotides are seen in which type of RNA molecule?

A. tRNA (Correct Answer)
B. rRNA
C. hnRNA
D. mRNA

Explanation: **Explanation:** **tRNA (Transfer RNA)** is the correct answer because it contains the highest concentration of modified bases among all RNA types (approximately 10–15% of its nucleotides). These modifications occur post-transcriptionally and are essential for the molecule's stability, L-shaped folding, and accurate codon-anticodon recognition. Common examples of modified bases in tRNA include **Pseudouridine (ψ)**, **Dihydrouridine (D)**, **Inosine (I)**, and **Ribothymidine (T)**. These give rise to the characteristic structural loops like the TψC loop and the D-loop. **Why other options are incorrect:** * **rRNA (Ribosomal RNA):** While rRNA does undergo some modifications (like methylation), the extent and variety of modified bases are significantly lower than in tRNA. * **mRNA (Messenger RNA):** In eukaryotes, mRNA undergoes modifications like 5' capping (7-methylguanosine) and 3' polyadenylation, but the internal sequence consists primarily of standard A, U, G, and C nucleotides. * **hnRNA (Heterogeneous nuclear RNA):** This is the primary transcript (pre-mRNA). While it undergoes processing to become mRNA, it does not feature the characteristic structural modified bases found in tRNA. **High-Yield NEET-PG Clinical Pearls:** 1. **Pseudouridine:** Known as the "fifth nucleotide" in RNA; it is a C-glycoside isomer of uridine. 2. **Inosine:** Found in the "Wobble position" (1st anticodon base), allowing a single tRNA to recognize multiple codons. 3. **Dihydrouridine:** Formed by the saturation of the double bond in uracil; it increases the flexibility of the RNA backbone. 4. **Enzyme:** Post-transcriptional modifications are carried out by specific enzymes like **pseudouridine synthase** and **methyltransferases**.

Question 109: What is the sugar found in RNA?

A. Ribose (Correct Answer)
B. Deoxyribose
C. Ribulose
D. Erythrose

Explanation: **Explanation:** **1. Why Ribose is Correct:** RNA (Ribonucleic Acid) is a polymer of nucleotides. Each nucleotide consists of a nitrogenous base, a phosphate group, and a **pentose sugar** (5-carbon sugar). In RNA, this sugar is **D-ribose**. Structurally, ribose is a "normal" sugar with a hydroxyl (-OH) group present on the 2' carbon atom. This 2'-OH group makes RNA more chemically reactive and less stable than DNA, which is why RNA is typically used for short-term genetic signaling (like mRNA) rather than long-term storage. **2. Why Other Options are Incorrect:** * **Deoxyribose:** This is the sugar found in **DNA**. It lacks an oxygen atom at the 2' position (hence "deoxy"). This absence of the 2'-OH group increases the stability of the DNA molecule. * **Ribulose:** This is a **ketopentose** (a 5-carbon sugar with a ketone group). It is an intermediate in the Pentose Phosphate Pathway (HMP Shunt) but is not a structural component of nucleic acids. * **Erythrose:** This is a **tetrose** (4-carbon sugar). It is involved in the HMP shunt and serves as a precursor for the synthesis of aromatic amino acids in some organisms, but it is not found in RNA. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Sugar Chemistry:** Both Ribose and Deoxyribose exist in the **β-D-furanose** form when incorporated into nucleic acids. * **The 2'-OH Significance:** The presence of the 2'-OH in ribose allows RNA to undergo **alkaline hydrolysis**, a process where RNA is degraded in basic solutions. DNA is resistant to this because it lacks the 2'-OH group. * **HMP Shunt Connection:** The ribose-5-phosphate required for nucleotide synthesis is generated via the **oxidative and non-oxidative phases** of the Pentose Phosphate Pathway. * **Drug Link:** Several antiviral and anticancer drugs (nucleoside analogs) work by modifying these sugar moieties to inhibit nucleic acid synthesis.

Question 110: What type of sugar is present in DNA?

A. Ribose sugar
B. Deoxyribose sugar (Correct Answer)
C. Both
D. None

Explanation: **Explanation:** The fundamental difference between DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) lies in the structure of their pentose sugar. **Why Deoxyribose is correct:** DNA contains **2'-deoxyribose**, a five-carbon (pentose) sugar. The prefix "deoxy" indicates that the hydroxyl group (-OH) normally found at the **C2 position** of the ribose ring is replaced by a hydrogen atom (-H). This structural modification is crucial because the absence of the 2'-OH group makes DNA chemically more stable and less susceptible to alkaline hydrolysis than RNA, fitting its role as the primary long-term storage molecule for genetic information. **Why other options are incorrect:** * **Ribose sugar:** This is found exclusively in **RNA**. The presence of the hydroxyl group at the C2 position makes RNA more reactive and structurally flexible, which is essential for its various catalytic and protein-synthesis roles. * **Both/None:** These are incorrect because the sugar-phosphate backbone of a specific nucleic acid type is uniform; DNA strictly utilizes deoxyribose to maintain its double-helical stability. **High-Yield Clinical Pearls for NEET-PG:** * **Numbering:** In a nucleotide, the sugar carbons are numbered with a "prime" (1' to 5') to distinguish them from the nitrogenous base atoms. * **Bonding:** The phosphate group attaches to the **5'-hydroxyl** and the **3'-hydroxyl** of the sugars to form the phosphodiester backbone. * **Synthesis:** The enzyme **Ribonucleotide Reductase** converts ribonucleotides to deoxyribonucleotides, a key target for the chemotherapy drug **Hydroxyurea**. * **Rule of Thumb:** DNA = **D**eoxyribose + **T**hymine; RNA = **R**ibose + **U**racil.

Practice by Chapter

Nucleotide Structure and Function

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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

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