Nucleic Acid Biochemistry Practice Questions

Q: Which of the following nucleoside and nucleotide analogues can be used as an antibiotic?

Fluorocytosine. **Explanation:** The correct answer is **Fluorocytosine (5-Fluorocytosine or 5-FC)**. **Why it is correct:** Fluorocytosine is a pyrimidine analogue used specifically as an **antifungal antibiotic**. Its selectivity stems from the fact that fungal cells possess the enzyme **cytosine deaminase**, which converts 5-FC into 5-fluorouracil (5-FU). 5-FU is then metabolized into 5-FdUMP, a potent inhibitor of **thymidylate synthase**, thereby halting DNA synthesis in the fungus. Human cells lack cytosine deaminase, making the drug relatively non-toxic to the host. **Why the other options are incorrect:** * **Fluorodeoxyuridine (5-FdU):** This is a pyrimidine analogue used primarily as a **chemotherapeutic agent** (anticancer) rather than an antibiotic. * **Aminopterin & Methotrexate:** These are **folate analogues** (antifolates) that inhibit the enzyme **dihydrofolate reductase (DHFR)**. While they interfere with nucleotide synthesis, they are used as immunosuppressants or anticancer drugs, not as antibiotics. **High-Yield Clinical Pearls for NEET-PG:** * **Synergy:** 5-FC is frequently used in combination with **Amphotericin B** for treating Cryptococcal meningitis to enhance penetration and prevent resistance. * **Side Effects:** Despite its selectivity, 5-FC can cause dose-dependent **bone marrow suppression** due to the conversion of some 5-FC to 5-FU by human intestinal microflora. * **Mechanism Landmark:** Remember that 5-FC acts by inhibiting **Thymidylate Synthase** (via its metabolite 5-FdUMP), a common target for several pyrimidine-based drugs.

Q: Which nitrogenous base is not present in DNA?

Uracil. **Explanation:** The fundamental difference between DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid) lies in their pentose sugar and their nitrogenous base composition. Both nucleic acids utilize four nitrogenous bases, but they differ in one specific pyrimidine. **Why Uracil is the Correct Answer:** Uracil is a pyrimidine base found exclusively in **RNA**. In DNA, the pyrimidine **Thymine** (5-methyluracil) is used instead. The presence of thymine in DNA is a crucial evolutionary adaptation for genetic stability; if DNA used uracil, the cell would have difficulty distinguishing between naturally occurring uracil and uracil formed by the spontaneous deamination of cytosine, leading to frequent mutations. **Analysis of Incorrect Options:** * **Adenine (A) & Guanine (G):** These are **Purines** (double-ring structures) and are present in both DNA and RNA. * **Thymine (T):** This is a **Pyrimidine** specific to DNA. It pairs with Adenine via two hydrogen bonds. **High-Yield NEET-PG Clinical Pearls:** 1. **Thymine vs. Uracil:** Thymine is chemically known as **5-methyluracil**. The methylation of uracil to thymine is catalyzed by the enzyme *thymidylate synthase*, a key target for the chemotherapy drug **5-Fluorouracil (5-FU)**. 2. **Base Pairing:** In DNA, A pairs with T (2 hydrogen bonds) and G pairs with C (3 hydrogen bonds). Higher G-C content increases the melting temperature ($T_m$) of DNA. 3. **Deamination:** Spontaneous deamination of **Cytosine** produces **Uracil**. This is a common type of DNA damage repaired by the *Base Excision Repair (BER)* pathway using the enzyme *Uracil DNA Glycosylase*.

Q: Deoxyribonucleic acid (DNA) is formed from:

Ribonucleotide diphosphate. ### Explanation The synthesis of DNA requires deoxyribonucleotides (dNTPs). The critical step in converting RNA components into DNA components is the reduction of the sugar moiety, which occurs at a specific stage of phosphorylation. **Why Ribonucleotide Diphosphate is Correct:** The enzyme **Ribonucleotide Reductase (RNR)** is responsible for converting ribonucleotides into deoxyribonucleotides. This enzyme specifically acts on **ribonucleotide diphosphates (rNDPs)** (e.g., ADP, GDP, CDP, UDP) to convert them into **deoxyribonucleotide diphosphates (dNDPs)** (e.g., dADP, dGDP, dCDP, dUDP). These dNDPs are subsequently phosphorylated by kinases into dNTPs, which serve as the actual building blocks for DNA polymerase during replication. **Analysis of Incorrect Options:** * **A. Ribonuclease:** This is an enzyme that degrades RNA into smaller components; it is not a substrate for DNA synthesis. * **B. Ribonucleotide monophosphate (rNMP):** RNR does not recognize monophosphates as substrates. * **D. Ribonucleotide triphosphate (rNTP):** While dNTPs are the final precursors added to the DNA chain, the *formation* of the "deoxy" version happens at the diphosphate level. (Note: In some prokaryotes, RNR can act on triphosphates, but in humans/eukaryotes, it is strictly diphosphates). **NEET-PG High-Yield Pearls:** * **Ribonucleotide Reductase (RNR):** This is the **rate-limiting enzyme** for DNA synthesis. It requires **Thioredoxin** (and NADPH) as a cofactor to provide reducing equivalents. * **Clinical Correlation:** The drug **Hydroxyurea** inhibits Ribonucleotide Reductase. It is used clinically in Sickle Cell Anemia (to increase HbF) and in Myeloproliferative disorders (to inhibit rapid DNA synthesis). * **Regulation:** RNR is inhibited by dATP (feedback inhibition) and activated by ATP.

Q: What is the first pyrimidine nucleotide to be synthesized?

Orotidine monophosphate (OMP). **Explanation:** In pyrimidine synthesis, the nitrogenous base ring is synthesized first and then attached to the ribose-5-phosphate sugar. This is a key distinction from purine synthesis, where the ring is built directly onto the sugar. **1. Why Orotidine monophosphate (OMP) is correct:** The synthesis begins with the formation of Carbamoyl Phosphate, which eventually forms **Orotic acid**. The enzyme **ORPT** (Orotate phosphoribosyltransferase) then transfers a ribose-5-phosphate group from PRPP to Orotic acid. This reaction produces **Orotidine monophosphate (OMP)**, which is the very first nucleotide (base + sugar + phosphate) formed in the de novo pyrimidine pathway. OMP is subsequently decarboxylated by OMP decarboxylase to form UMP. **2. Why the other options are incorrect:** * **Uridine monophosphate (UMP):** While UMP is the "parent" pyrimidine from which others are derived, it is formed *after* the decarboxylation of OMP. * **Thymidine monophosphate (TMP):** This is a downstream product formed from dUDP/dUMP via the action of Thymidylate synthase. * **Inosine monophosphate (IMP):** This is the first nucleotide formed in the **Purine** synthesis pathway, not the pyrimidine pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Bifunctional Enzyme:** In humans, ORPT and OMP decarboxylase are two domains of a single polypeptide chain called **UMP Synthase**. * **Orotic Aciduria:** A deficiency in UMP Synthase leads to Orotic Aciduria (Type I), characterized by megaloblastic anemia (unresponsive to B12/Folate) and growth retardation. It is treated with **oral Uridine** to bypass the metabolic block. * **Rate-limiting step:** The formation of Carbamoyl Phosphate by **CPS-II** (cytosolic) is the regulated step of pyrimidine synthesis.

Q: What is present on the 3' end of all functional, mature tRNAs?

The sequence 5'CCA3'. **Explanation:** **1. Why Option C is correct:** All functional, mature transfer RNA (tRNA) molecules possess a specific trinucleotide sequence at their 3' terminus: **5'-CCA-3'**. This sequence is added post-transcriptionally by the enzyme **tRNA nucleotidyltransferase**. The 3' hydroxyl group of the terminal **Adenosine (A)** serves as the attachment site for a specific amino acid, forming an aminoacyl-tRNA. This process, catalyzed by aminoacyl-tRNA synthetase, is essential for translating the genetic code into a polypeptide chain. **2. Why other options are incorrect:** * **Option A (Cloverleaf loop):** This refers to the secondary structure of tRNA, which includes the D-loop, T$\psi$C loop, and anticodon loop. It is the overall shape, not a specific sequence at the 3' end. * **Option B (Anticodon):** The anticodon is located in the **anticodon loop**, situated at the opposite end of the tRNA molecule from the 3' acceptor stem. It base-pairs with the mRNA codon. * **Option D (Codon):** A codon is a three-nucleotide sequence found on **mRNA**, not tRNA. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Acceptor Stem:** The 3' end is part of the "Acceptor Stem." Remember the mnemonic: **"CCA: Can Carry Amino acids."** * **Post-transcriptional Modification:** In eukaryotes, the CCA tail is not coded by the DNA but is added after transcription. * **Charging:** The process of attaching an amino acid to the 3' CCA end is called "charging" or "loading" of tRNA. * **T$\psi$C Loop:** Contains pseudouridine and ribothymidine; it is responsible for binding the tRNA to the **ribosomal surface** (70S or 80S). * **D-Loop:** Contains Dihydrouridine; it is involved in recognition by the specific **Aminoacyl-tRNA synthetase**.

Q: Which amino acid does NOT contribute to the biosynthesis of purine ribonucleotides?

Histidine. To master purine biosynthesis for NEET-PG, it is essential to memorize the specific contributors to the purine ring (Inosine Monophosphate - IMP). ### **Explanation** The purine ring is a bicyclic structure synthesized "de novo" by adding atoms to a ribose-5-phosphate base. The atoms are derived from specific amino acids, CO₂, and folate derivatives. * **Why Histidine is the Correct Answer:** While Histidine is a basic amino acid, it **does not** contribute any atoms to the purine ring. Interestingly, the relationship is actually the reverse: **ATP (a purine) is a precursor for the biosynthesis of Histidine** in plants and microorganisms. In humans, Histidine is an essential amino acid and must be obtained through diet. * **Why the Other Options are Incorrect:** * **Glycine (Option D):** This is the most significant contributor. It provides the entire C4, C5, and N7 backbone of the purine ring. * **Aspartate (Option A):** It provides the **Nitrogen at position 1 (N1)**. It also plays a role in the conversion of IMP to AMP. * **Glutamine/Glutamate (Option C):** Glutamine acts as the nitrogen donor for **N3 and N9** positions. (Note: While the question lists Glutamate, in biochemical pathways, Glutamine is the direct donor, often interconverted with glutamate). ### **High-Yield Clinical Pearls for NEET-PG** 1. **Mnemonic for Purine Sources:** **"CO2, Gly, Asp, Gln, and Formyl-THF."** * **Glycine:** Entire C4-C5-N7. * **Aspartate:** N1. * **Glutamine:** N3, N9. * **THFs (N10-formyl THF):** C2 and C8. * **CO₂:** C6. 2. **Rate-limiting step:** The conversion of PRPP to 5-phosphoribosylamine by **PRPP glutamyl amidotransferase**. 3. **Drug Link:** **Methotrexate** inhibits dihydrofolate reductase, depleting the folate pool and thus inhibiting C2 and C8 synthesis in purines.

Q: Which of the following types of RNA contains abnormal purine bases?

Transfer RNA (t-RNA). **Explanation** **1. Why Transfer RNA (t-RNA) is Correct:** Transfer RNA (t-RNA) is unique among RNA types because it undergoes extensive **post-transcriptional modifications**. Approximately 10–15% of the nucleotides in t-RNA are modified or "unusual." These include abnormal purine and pyrimidine bases that are essential for stabilizing the t-RNA structure and ensuring accurate codon-anticodon recognition. * **Examples of abnormal purines in t-RNA:** Inosine (formed by deamination of Adenosine), 1-methyladenosine, and N6-isopentenyladenosine. * **Other unusual bases:** Pseudouridine (Ψ), Dihydrouridine (D), and Ribothymidine (T). **2. Why the Other Options are Incorrect:** * **Messenger RNA (m-RNA):** Primarily consists of the four standard bases (A, G, C, U). While eukaryotic m-RNA has a 7-methylguanosine cap, it does not contain the high density of diverse abnormal purines characteristic of t-RNA. * **Ribosomal RNA (r-RNA) & 16S RNA:** These are structural components of ribosomes. While they do undergo some methylation, they lack the variety of complex modified purines (like Inosine) found in the "wobble position" of t-RNA. 16S RNA is specifically a component of the prokaryotic 30S ribosomal subunit. **3. NEET-PG High-Yield Clinical Pearls:** * **The "Wobble" Hypothesis:** Inosine (an abnormal purine) is often found at the first position of the t-RNA anticodon, allowing it to pair with U, C, or A in the m-RNA. * **Smallest RNA:** t-RNA is the smallest (73–93 nucleotides) and is often called "Soluble RNA" (sRNA). * **Structure:** The secondary structure is a **Cloverleaf**, while the tertiary structure is **L-shaped**. * **DHU Loop:** Contains Dihydrouridine; it is the site for recognition by the enzyme aminoacyl-tRNA synthetase. * **TψC Loop:** Contains Pseudouridine; it is involved in binding the t-RNA to the ribosomal surface.

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

N5, N10-methylene tetrahydrofolate. **Explanation:** The conversion of **dUMP (deoxyuridine monophosphate)** to **dTMP (deoxythymidine monophosphate)** is a critical step in DNA synthesis, catalyzed by the enzyme **Thymidylate Synthase**. **1. Why Option A is Correct:** In this reaction, **N5, N10-methylene tetrahydrofolate (THF)** acts as both a **one-carbon donor** and a **reducing agent**. It transfers a methyl group to the C5 position of the pyrimidine ring of dUMP. During this process, the methylene group is reduced to a methyl group, and N5, N10-methylene THF is oxidized to **Dihydrofolate (DHF)**. This is the only reaction in folate metabolism where THF is oxidized to DHF. **2. Why Incorrect Options are Wrong:** * **B. Formimino folate:** This is an intermediate in the catabolism of Histidine (FIGLU to Glutamate). It does not participate in nucleotide synthesis. * **C. N5 formyl folate:** Also known as **Folinic Acid (Leucovorin)**. While used clinically to "rescue" cells from methotrexate toxicity, it is not the direct co-factor for dTMP synthesis. * **D. Dihydrofolate:** This is the **product** of the thymidylate synthase reaction, not the substrate/co-factor required for the methylation of dUMP. **Clinical Pearls & High-Yield Facts:** * **5-Fluorouracil (5-FU):** A suicide inhibitor of Thymidylate Synthase; it stops dTMP production ("Thymineless death"). * **Methotrexate:** Inhibits **Dihydrofolate Reductase (DHFR)**, preventing the regeneration of THF from DHF, thereby indirectly inhibiting dTMP synthesis. * **Rate-Limiting Step:** This reaction is the rate-limiting step for DNA synthesis, making it a primary target for several chemotherapeutic agents.

Question 1

All of the following are true about Lesch-Nyhan syndrome, EXCEPT:

Accepted Answer

Febuxostat is routinely used for treatment

Answer

Affects almost solely males

Answer

Excessive synthesis of purines is a feature

Answer

Characterized by gouty arthritis

Question 2

Which of the following nucleoside and nucleotide analogues can be used as an antibiotic?

Accepted Answer

Fluorocytosine

Answer

Fluorodeoxyuridine

Answer

Aminopterin

Answer

Methotrexate

Question 3

Which nitrogenous base is not present in DNA?

Accepted Answer

Uracil

Answer

Adenine

Answer

Guanine

Answer

Thymine

Question 4

Deoxyribonucleic acid (DNA) is formed from:

Accepted Answer

Ribonucleotide diphosphate

Answer

Ribonuclease

Answer

Ribonucleotide monophosphate

Answer

Ribonucleotide triphosphate

Question 5

What is the first pyrimidine nucleotide to be synthesized?

Accepted Answer

Orotidine monophosphate (OMP)

Answer

Thymidine monophosphate (TMP)

Answer

Inosine monophosphate (IMP)

Answer

Uridine monophosphate (UMP)

Question 6

A 4-year-old male with mental retardation, self-mutilation, and hyperuricemia is likely to have a deficiency of an enzyme involved in which of the following metabolic pathways?

Accepted Answer

Recycling of guanine and hypoxanthine

Answer

Conversion of homogentisic acid to methylacetoacetate

Answer

Degradation of galactocerebroside

Answer

Breakdown of branched-chain amino acids

Question 7

What is present on the 3' end of all functional, mature tRNAs?

Accepted Answer

The sequence 5'CCA3'

Answer

The cloverleaf loop

Answer

The anticodon

Answer

The codon

Question 8

Which amino acid does NOT contribute to the biosynthesis of purine ribonucleotides?

Accepted Answer

Histidine

Answer

Aspartate

Answer

Glutamate

Answer

Glycine

Question 9

Which of the following types of RNA contains abnormal purine bases?

Accepted Answer

Transfer RNA (t-RNA)

Answer

Messenger RNA (m-RNA)

Answer

Ribosomal RNA (r-RNA)

Answer

16S RNA

Question 10

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

Accepted Answer

N5, N10-methylene tetrahydrofolate

Answer

Formimino folate

Answer

N5 formyl folate

Answer

Dihydrofolate

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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