Nucleic Acid Biochemistry Practice Questions

Q: Which is the most abundant type of RNA molecule in a cell?

Ribosomal RNA (rRNA). **Explanation:** The correct answer is **Ribosomal RNA (rRNA)**. In a typical eukaryotic cell, RNA distribution is not uniform, and rRNA constitutes the vast majority of the total cellular RNA. **1. Why rRNA is the correct answer:** Ribosomal RNA (rRNA) accounts for approximately **80%** of the total RNA in a cell. It is the structural and catalytic component of ribosomes, the "protein factories" of the cell. Its high abundance is necessary to support the massive scale of protein synthesis required for cellular function and maintenance. **2. Why other options are incorrect:** * **Transfer RNA (tRNA):** Comprises about **15%** of total RNA. While numerous, they are smaller molecules (75–95 nucleotides) and serve as adapters that carry amino acids to the ribosome. * **Messenger RNA (mRNA):** Despite its critical role in carrying genetic code, it is the most heterogeneous but least stable, accounting for only **2–5%** of total RNA. * **Micro RNA (miRNA):** These are regulatory non-coding RNAs present in trace amounts, involved in post-transcriptional gene silencing. **High-Yield NEET-PG Clinical Pearls:** * **Abundance Mnemonic:** **R > T > M** (rRNA > tRNA > mRNA). * **Size Mnemonic:** **R > T > M** (rRNA is the largest/longest, mRNA is the most variable in size). * **Synthesis Sites:** rRNA is synthesized in the **nucleolus** (except 5S rRNA), while mRNA and tRNA are synthesized in the nucleoplasm. * **RNA Polymerases:** Remember **1, 2, 3** for **R, M, T**: * Pol I: rRNA * Pol II: mRNA (and miRNA) * Pol III: tRNA (and 5S rRNA)

Q: A 60-year-old man experienced a sudden onset of severe pain in his right great toe, a condition he has had previously. He admits to forgetting his maintenance medication for over two weeks. His maintenance medication most likely blocks which one of the following reactions?

Hypoxanthine to xanthine. The clinical presentation of sudden, severe pain in the first metatarsophalangeal joint (podagra) in an older male is classic for **Acute Gouty Arthritis** [1]. The patient’s history of recurrence and non-compliance with maintenance medication points toward **Allopurinol** or **Febuxostat** as the drug in question [2]. **1. Why the Correct Answer is Right:** Maintenance therapy for gout aims to lower serum uric acid levels to prevent future attacks. Allopurinol and Febuxostat are **Xanthine Oxidase inhibitors** [2]. In the purine degradation pathway, the enzyme Xanthine Oxidase catalyzes two critical steps [3]: * **Hypoxanthine → Xanthine** * **Xanthine → Uric Acid** By blocking these reactions, the production of poorly soluble uric acid is reduced, preventing the deposition of monosodium urate crystals in joints [3]. **2. Why the Other Options are Wrong:** * **Option A (IMP to GMP):** This step is catalyzed by IMP dehydrogenase. Inhibitors of this enzyme include **Mycophenolate mofetil**, used as an immunosuppressant, not for gout. * **Option B (Adenosine to inosine):** This is catalyzed by **Adenosine Deaminase (ADA)**. A deficiency in this enzyme leads to Severe Combined Immunodeficiency (SCID). * **Option C (dUMP to dTMP):** This is catalyzed by Thymidylate Synthase. Inhibitors like **5-Fluorouracil (5-FU)** are used in cancer chemotherapy. **3. NEET-PG High-Yield Pearls:** * **Acute Management:** NSAIDs (first-line), Colchicine, or Glucocorticoids. * **Chronic Management:** Xanthine Oxidase inhibitors (Allopurinol) [2]. * **Important Contraindication:** Never start Allopurinol during an acute attack, as rapid fluctuations in serum urate levels can worsen or prolong the inflammation. * **Drug Interaction:** Allopurinol increases the toxicity of **Azathioprine** and **6-Mercaptopurine** because these drugs are metabolized by Xanthine Oxidase.

Q: An RNA molecule is extracted from a culture of Gram-negative bacteria. This extract contained dihydrouracil, pseudouridine, and thymidine residues. Which of the following is the most likely composition of the 3' end of this molecule?

CCA. **Explanation:** The presence of modified bases such as **dihydrouracil (D), pseudouridine (ψ), and ribothymidine (T)** is the classic biochemical hallmark of **Transfer RNA (tRNA)**. These bases are formed post-transcriptionally and are essential for the structural integrity and function of the tRNA molecule (e.g., the TψC loop and the D-loop). All mature tRNA molecules, whether in prokaryotes or eukaryotes, end with the sequence **5’-CCA-3’** at their **3’ terminus**. This is known as the **acceptor stem**. The amino acid is covalently attached to the 3’-hydroxyl group of the adenosine (A) residue by the enzyme aminoacyl-tRNA synthetase. **Analysis of Options:** * **B (CCA): Correct.** This is the universal attachment site for amino acids at the 3' end of tRNA. * **A (AUG): Incorrect.** This is the universal **start codon** found on mRNA, which codes for Methionine. * **C (UAG): Incorrect.** This is the **Amber stop codon**, one of the three codons that signal the termination of translation on mRNA. * **D (PolyA): Incorrect.** Poly-A tails are added to the 3' end of **eukaryotic mRNA** for stability; they are not found in tRNA or typically in bacterial mRNA. **High-Yield NEET-PG Pearls:** * **Post-transcriptional modification:** In prokaryotes, the CCA sequence may be encoded genetically, but in eukaryotes, it is added post-transcriptionally by the enzyme **tRNA nucleotidyltransferase**. * **Charging:** The process of adding an amino acid to the CCA end is called "charging" or aminoacylation. * **Cloverleaf vs. L-shape:** tRNA has a 2D "cloverleaf" secondary structure but a 3D "L-shaped" tertiary structure.

Q: Which type of RNA contains the anticodon arm?

tRNA. **Explanation:** **tRNA (Transfer RNA)** is the correct answer because it acts as the "adapter molecule" during protein synthesis. Its primary function is to translate the genetic code from mRNA into a specific sequence of amino acids. The **anticodon arm** is a critical structural feature of tRNA; it contains a triplet of nucleotides (the anticodon) that is complementary to the codon found on the mRNA strand. This base-pairing ensures that the correct amino acid (attached to the 3' CCA end) is incorporated into the growing polypeptide chain. **Why other options are incorrect:** * **mRNA (Messenger RNA):** Contains the **codons**, not the anticodons. It serves as the template that carries genetic information from DNA to the ribosome. * **rRNA (Ribosomal RNA):** Forms the structural and catalytic core of the ribosome (e.g., the 28S rRNA in eukaryotes acts as a peptidyl transferase ribozyme). It does not contain an anticodon arm. * **snRNA (Small Nuclear RNA):** Involved in **splicing** of pre-mRNA within the nucleus as part of the spliceosome complex (e.g., U1, U2, U4, U5, U6). **High-Yield Facts for NEET-PG:** * **Cloverleaf Model:** The secondary structure of tRNA is a cloverleaf, while the tertiary structure is **L-shaped**. * **Wobble Hypothesis:** Occurs at the 3rd base of the mRNA codon and the 1st base of the tRNA anticodon, allowing one tRNA to recognize multiple codons. * **Unusual Bases:** tRNA is rich in modified bases like pseudouridine (in the TψC arm) and dihydrouridine (in the D-arm). * **Charging:** The enzyme **Aminoacyl-tRNA synthetase** is responsible for attaching the correct amino acid to the tRNA, a process requiring ATP.

Q: An 8-month-old child presents with a tendency to compulsively bite his fingers. The mother reports noticing yellow-orange crystals in his diapers. A genetic defect in which of the following pathways should be suspected?

Purine metabolism. **Explanation:** The clinical presentation of self-mutilation (compulsive finger biting) and "orange-sand" crystals in the diaper is a classic description of **Lesch-Nyhan Syndrome**. This is an X-linked recessive disorder caused by a complete deficiency of the enzyme **Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT)**. **1. Why Purine Metabolism is Correct:** HGPRT is a key enzyme in the **Purine Salvage Pathway**. It normally converts hypoxanthine to IMP and guanine to GMP. When HGPRT is deficient, these purine bases cannot be salvaged and are instead degraded into **Uric Acid**. The resulting hyperuricemia leads to the formation of sodium urate crystals (orange crystals in diapers), gouty arthritis, and severe neurological symptoms, including intellectual disability and characteristic self-mutilating behavior. **2. Why Incorrect Options are Wrong:** * **Aromatic amino acid metabolism:** Defects here lead to conditions like Phenylketonuria (mousy odor) or Alkaptonuria (darkening of urine), not self-mutilation or urate crystals. * **Branched-chain amino acid metabolism:** Defects lead to Maple Syrup Urine Disease (MSUD), characterized by a burnt-sugar urine odor and neurological deterioration, but not uric acid stones. * **Pyrimidine metabolism:** Defects (like Orotic Aciduria) cause megaloblastic anemia and growth retardation, but do not present with hyperuricemia or self-mutilation. **Clinical Pearls for NEET-PG:** * **Mnemonic for HGPRT deficiency:** **H**yperuricemia, **G**out, **P**issed off (aggression/self-mutilation), **R**etardation, **T**one (dystonia). * **Diagnostic finding:** Elevated serum uric acid and elevated **PRPP (Phosphoribosyl pyrophosphate)** levels (due to decreased utilization in salvage). * **Treatment:** Allopurinol (manages uric acid but does not fix neurological symptoms).

Q: What is the bond between the strands in the given diagram?

Hydrogen bond. ***Hydrogen bond*** - The **two strands** of the DNA double helix are held together by **hydrogen bonds** between complementary base pairs (**A-T** forms 2 H-bonds, **G-C** forms 3 H-bonds). - These **non-covalent interactions** allow for **strand separation** during processes like replication and transcription while maintaining structural stability. *Phosphodiester bond* - This bond connects the **sugar-phosphate backbone** within a single DNA strand, not between strands. - It forms the **covalent linkage** between the 3' carbon of one sugar and the 5' phosphate of the next nucleotide. *Covalent bond* - While **phosphodiester** and **glycosidic bonds** are types of covalent bonds, they don't directly connect the two DNA strands. - The inter-strand connections are specifically **weak hydrogen bonds**, not strong covalent bonds. *Glycosidic bond* - This bond connects the **nitrogenous base** to the **sugar (deoxyribose)** within each nucleotide. - It's an **intra-nucleotide bond**, not an inter-strand connection in the DNA double helix.

Q: In DNA structure, what is the maximum number of hydrogen bonds observed between a purine-pyrimidine base pair?

Guanine-Cytosine. **Explanation:** In DNA structure, base pairing follows **Chargaff’s Rule**, where a purine always pairs with a pyrimidine to maintain a constant distance between the sugar-phosphate backbones. The stability of this pairing is determined by the number of hydrogen bonds formed. **1. Why Guanine-Cytosine is correct:** The **Guanine-Cytosine (G-C)** pair forms **three hydrogen bonds** (between the carbonyl group, amino group, and ring nitrogen). This is the maximum number of hydrogen bonds observed between any base pair in DNA. Because of this third bond, G-C rich DNA sequences have higher thermal stability and a higher **melting temperature (Tm)** compared to A-T rich sequences. **2. Why the other options are incorrect:** * **Adenine-Thymine (A-T):** This pair forms only **two hydrogen bonds**. While it follows the purine-pyrimidine rule, it is less stable than the G-C pair. * **Adenine-Guanine (A-G):** This is a purine-purine pairing. It does not occur in standard B-DNA because it would cause a "bulge" in the double helix due to the large size of both bases. * **Cytosine-Thymine (C-T):** This is a pyrimidine-pyrimidine pairing. It does not occur in standard DNA as it would result in a "gap" or narrowing of the helix. **NEET-PG High-Yield Pearls:** * **Melting Temperature (Tm):** Directly proportional to the G-C content. High G-C content is often found in **promoter regions** (CpG islands). * **TATA Box:** Found in promoters; it is A-T rich because the two hydrogen bonds are easier to "melt" or unzip during the initiation of transcription. * **Bonding Sites:** In G-C pairs, bonds occur at positions (O6-N4, N1-N3, and N2-O2).

Q: Which of the following is NOT provided by glycine in purine synthesis?

Nitrogen-4. In purine ring synthesis, the entire **glycine** molecule is incorporated into the structure, contributing three specific atoms. The purine ring is a double-ring structure (a six-membered pyrimidine ring fused to a five-membered imidazole ring) built atom-by-atom on a ribose-5-phosphate backbone. ### Why Nitrogen-4 is the Correct Answer **Nitrogen-4 (N4)** is not a component of the purine ring. The nitrogen atoms in the purine nucleus are located at positions **1, 3, 7, and 9**. Therefore, N4 does not exist in the purine skeleton. Even if the question implied the nitrogen at position 3 or 9, those are provided by **Glutamine**, while the nitrogen at position 1 is provided by **Aspartate**. ### Analysis of Incorrect Options Glycine is unique because it is the only amino acid that contributes its entire carbon-nitrogen skeleton to the purine ring: * **Carbon-4 (C4):** Provided by the carboxyl carbon of glycine. * **Carbon-5 (C5):** Provided by the alpha-carbon of glycine. * **Nitrogen-7 (N7):** Provided by the amino group of glycine. ### High-Yield NEET-PG Clinical Pearls To master purine synthesis for the exam, remember the "Mnemonic for Sources": 1. **Glycine:** C4, C5, and N7 (The "Whole Molecule" donor). 2. **Aspartate:** N1. 3. **Glutamine (Amide N):** N3 and N9. 4. **Tetrahydrofolate (N10-formyl THF):** C2 and C8. 5. **CO₂ (Respiratory):** C6. **Clinical Correlation:** The rate-limiting step of purine synthesis is catalyzed by **PRPP glutamyl amidotransferase**. Drugs like **Methotrexate** and **Trimethoprim** inhibit folate metabolism, indirectly blocking the contribution of C2 and C8, thereby halting DNA synthesis in rapidly dividing cells.

Question 1

Which is the most abundant type of RNA molecule in a cell?

Accepted Answer

Ribosomal RNA (rRNA)

Answer

Transfer RNA (tRNA)

Answer

Messenger RNA (mRNA)

Answer

Micro RNA (miRNA)

Question 2

A 60-year-old man experienced a sudden onset of severe pain in his right great toe, a condition he has had previously. He admits to forgetting his maintenance medication for over two weeks. His maintenance medication most likely blocks which one of the following reactions?

Accepted Answer

Hypoxanthine to xanthine

Answer

IMP to GMP

Answer

Adenosine to inosine

Answer

dUMP to dTMP

Question 3

An RNA molecule is extracted from a culture of Gram-negative bacteria. This extract contained dihydrouracil, pseudouridine, and thymidine residues. Which of the following is the most likely composition of the 3' end of this molecule?

Accepted Answer

CCA

Answer

AUG

Answer

UAG

Answer

PolyA

Question 4

Chargaff's rule states that in DNA:

Accepted Answer

The total amount of purines equals the total amount of pyrimidines.

Answer

The ratio of purines to pyrimidines is 2.

Answer

The amount of adenine (A) equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C).

Answer

All of the above.

Question 5

Which type of RNA contains the anticodon arm?

Accepted Answer

tRNA

Answer

mRNA

Answer

rRNA

Answer

snRNA

Question 6

An 8-month-old child presents with a tendency to compulsively bite his fingers. The mother reports noticing yellow-orange crystals in his diapers. A genetic defect in which of the following pathways should be suspected?

Accepted Answer

Purine metabolism

Answer

Aromatic amino acid metabolism

Answer

Branched chain amino acid metabolism

Answer

Pyrimidine metabolism

Question 7

What is the bond between the strands in the given diagram?

Accepted Answer

Hydrogen bond

Answer

Phosphodiester bond

Answer

Covalent bond

Answer

Glycosidic bond

Question 8

In DNA structure, what is the maximum number of hydrogen bonds observed between a purine-pyrimidine base pair?

Accepted Answer

Guanine-Cytosine

Answer

Adenine-Thymine

Answer

Adenine-Guanine

Answer

Cytosine-Thymine

Question 9

Which of the following is NOT provided by glycine in purine synthesis?

Accepted Answer

Nitrogen-4

Answer

Carbon-4

Answer

Carbon-5

Answer

Nitrogen-7

Question 10

Identify the bonds labeled X, Y, and Z in the structure of DNA.

Accepted Answer

X-Hydrogen bond, Y-beta N-glycosidic bond, Z-3' 5' phosphodiester bond

Answer

X-Hydrogen bond, Y-beta O-glycosidic bond, Z-2' 5' phosphodiester bond

Answer

X-Hydrogen bond, Y-beta N-glycosidic bond, Z-2' 3' phosphodiester bond

Answer

X-Hydrogen bond, Y-phosphodiester bond, Z-3' 5' beta N-glycosidic bond

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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