Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 51: Which of the following bases is not found in nucleic acids?

A. Adenine
B. Guanidine (Correct Answer)
C. Uracil
D. Cytosine

Explanation: ### Explanation **1. Why Guanidine is the Correct Answer:** The question asks for the base **not** found in nucleic acids. **Guanidine** is a nitrogenous compound (a derivative of protein metabolism found in urine), but it is not a component of DNA or RNA. It is often confused with **Guanine**, which is the actual purine base found in nucleic acids. Guanidine is chemically distinct and does not participate in base-pairing or the formation of the genetic code. **2. Analysis of Incorrect Options:** * **Adenine (A):** A purine base found in both DNA and RNA. It pairs with Thymine (in DNA) or Uracil (in RNA) via two hydrogen bonds. * **Uracil (U):** A pyrimidine base found **exclusively in RNA** (replacing Thymine). It pairs with Adenine. * **Cytosine (C):** A pyrimidine base found in both DNA and RNA. It pairs with Guanine via three hydrogen bonds. **3. High-Yield Clinical Pearls for NEET-PG:** * **Purines vs. Pyrimidines:** Remember the mnemonic **"PUR As Gold"** (Purines = Adenine, Guanine) and **"CUT the PY"** (Pyrimidines = Cytosine, Uracil, Thymine). * **Structural Difference:** Thymine is **5-methyluracil**. The presence of the methyl group in DNA (instead of Uracil) provides greater stability against spontaneous deamination. * **Base Pairing:** G-C bonds are stronger than A-T bonds because they have **three hydrogen bonds** instead of two. DNA with high G-C content has a higher melting temperature ($T_m$). * **Clinical Correlation:** **Guanethidine** (a drug) and **Metformin** (a biguanide) are pharmacological derivatives related to the guanidine structure, but they are unrelated to nucleotide synthesis.

Question 52: Dihydrouridine is found in which type of nucleic acid?

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

Explanation: ### Explanation **Correct Answer: C. tRNA** **Underlying Medical Concept:** Transfer RNA (tRNA) is unique among nucleic acids for its high content of **post-transcriptionally modified bases**. Dihydrouridine (D) is a pyrimidine derivative formed by the enzymatic reduction of uracil (saturation of the C5-C6 double bond). This modification increases the conformational flexibility of the RNA backbone because it lacks the planar structure of typical bases. Dihydrouridine is specifically located in the **D-loop** (Dihydrouridine loop) of the cloverleaf model of tRNA, which serves as a recognition site for the enzyme aminoacyl-tRNA synthetase. **Why Other Options are Incorrect:** * **A. DNA:** Primarily contains the standard bases (A, G, C, T). While epigenetic modifications like 5-methylcytosine exist, dihydrouridine is not a component of DNA. * **B. mRNA:** Primarily consists of unmodified A, G, C, and U. While it undergoes processing (5' capping and 3' polyadenylation), it does not contain the specific modified loops found in tRNA. * **D. rRNA:** While rRNA does contain some modified bases (like pseudouridine and methylated bases), dihydrouridine is the hallmark characteristic of tRNA. **High-Yield Clinical Pearls for NEET-PG:** * **Modified Bases in tRNA:** Remember the "Big Three": **D**ihydrouridine (D-loop), **P**seudouridine (TψC loop), and **I**nosine (often found in the Anticodon loop/Wobble position). * **TψC Loop:** Contains Ribothymidine, Pseudouridine, and Cytidine; it is responsible for binding the tRNA to the 50S/60S ribosomal subunit. * **CCA Sequence:** All tRNAs have a CCA sequence at the **3' end**, which is the attachment site for the amino acid (added post-transcriptionally). * **Soluble RNA:** tRNA is also known as "sRNA" because it remains in the supernatant even after centrifugation.

Question 53: Which statement is not true regarding tRNA?

A. The TpsiC arm is involved in ribosomal attachment.
B. The D arm is involved in ribosomal attachment. (Correct Answer)
C. A CCA trinucleotide sequence is attached to the acceptor arm.
D. The first nucleotide of the anticodon of tRNA is not specific.

Explanation: **Explanation:** Transfer RNA (tRNA) is a cloverleaf-shaped molecule (2D) that folds into an L-shape (3D), serving as the adapter molecule in protein synthesis. **Why Option B is the correct answer (The False Statement):** The **D arm** (Dihydroxyuridine arm) is primarily responsible for **recognition by the Aminoacyl-tRNA synthetase enzyme**, which ensures the correct amino acid is attached to the tRNA. It is **not** involved in ribosomal attachment. Ribosomal binding is the function of the TψC arm. **Analysis of other options:** * **Option A:** The **TψC arm** (Ribothymidine-Pseudouridine-Cytidine) contains the sequence necessary for binding the tRNA to the **ribosomal surface** (specifically the 5S rRNA of the large subunit). * **Option C:** All eukaryotic and prokaryotic tRNAs have a **CCA sequence** at the 3' end of the **Acceptor arm**. The amino acid attaches to the 3'-OH group of the terminal Adenosine. * **Option D:** The first nucleotide of the anticodon (5' position) is known as the **Wobble position**. Due to "Wobble base pairing," it can pair with non-standard bases on the mRNA codon (e.g., Inosine can pair with A, U, or C), making it less specific than the other two nucleotides. **High-Yield Clinical Pearls for NEET-PG:** * **Smallest RNA:** tRNA is the smallest (75–95 nucleotides; 4S). * **Post-transcriptional modifications:** tRNA contains unusual bases like Pseudouridine, Dihydrouridine, and Inosine. * **Charging:** The process of attaching an amino acid to tRNA is called "charging," catalyzed by Aminoacyl-tRNA synthetase (requires ATP). * **Initiator tRNA:** In eukaryotes, it carries Methionine; in prokaryotes, it carries N-formylmethionine (fMet).

Question 54: Modified base is seen in which of the following nucleic acids?

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

Explanation: **Explanation:** **Why tRNA is the correct answer:** Transfer RNA (tRNA) is characterized by having the highest percentage of post-transcriptionally modified bases among all nucleic acids (approximately 10–15% of its residues). These modifications are essential for stabilizing the tRNA structure (the "cloverleaf" model) and ensuring accurate codon-anticodon recognition. Common modified bases found in tRNA include: * **Pseudouridine (ψ):** Found in the TψC arm. * **Dihydrouridine (D):** Found in the DHU arm. * **Inosine:** Often found in the wobble position of the anticodon. * **Ribothymidine:** Unlike DNA, thymine is present in tRNA due to post-transcriptional methylation of uracil. **Why other options are incorrect:** * **rRNA:** While ribosomal RNA does contain some modifications (like pseudouridine and methylated bases), the frequency and variety are significantly lower than in tRNA. * **hnRNA (Heterogeneous nuclear RNA):** This is the primary transcript (pre-mRNA). While it undergoes processing (capping, tailing, splicing), it does not feature the extensive base modifications characteristic of tRNA. * **mRNA:** Eukaryotic mRNA is modified at the 5' end (7-methylguanosine cap) and the 3' end (poly-A tail), but the internal coding sequence consists almost entirely of standard A, U, G, and C bases. **High-Yield Clinical Pearls for NEET-PG:** * **The "Unusual" Base:** tRNA is the only RNA where **Thymine** is found (as Ribothymidine). * **Wobble Hypothesis:** Inosine in the tRNA anticodon allows for non-traditional base pairing, enabling one tRNA to recognize multiple codons. * **Enzyme:** Post-transcriptional modification is carried out by specific enzymes like **pseudouridine synthase** and **methyltransferases**. * **Structure:** The presence of these modified bases is vital for the L-shaped tertiary folding of tRNA.

Question 55: What are the pKa values of the primary and secondary phosphoryl groups of nucleotides?

A. 6.2 and 1.0
B. 1.0 and 6.2 (Correct Answer)
C. 6.0 and 1.0
D. 1.2 and 6.0

Explanation: **Explanation:** Nucleotides are the building blocks of nucleic acids, consisting of a nitrogenous base, a pentose sugar, and one or more phosphate groups. The phosphate groups are polyprotic acids, meaning they can release more than one proton ($H^+$), each characterized by a specific dissociation constant ($pK_a$). 1. **Primary Phosphoryl Group ($pK_a \approx 1.0$):** The first proton dissociates very readily, making it a strong acid. At physiological pH (~7.4), this group is always fully deprotonated and carries a negative charge. 2. **Secondary Phosphoryl Group ($pK_a \approx 6.2$):** The second proton dissociates less easily. Since its $pK_a$ (6.2) is relatively close to physiological pH, this group can exist in both protonated and deprotonated states, acting as a minor buffer. **Analysis of Options:** * **Option B (Correct):** Correctly identifies the highly acidic nature of the first dissociation (1.0) and the moderately acidic nature of the second (6.2). * **Option A:** Reverses the values. The primary dissociation always occurs at a lower pH than the secondary. * **Options C & D:** These provide incorrect numerical values. While 6.0 is close to 6.2, the standard biochemical value cited in major textbooks (like Harper’s Illustrated Biochemistry) for the secondary group is 6.2. **High-Yield Clinical Pearls for NEET-PG:** * **Net Charge:** Due to these low $pK_a$ values, DNA and RNA are polyanions (negatively charged) at physiological pH. This is why they migrate toward the **Anode** (positive electrode) during electrophoresis. * **Histone Interaction:** The negative charge of the phosphate backbone allows DNA to bind tightly to **Histones**, which are rich in positively charged amino acids (Lysine and Arginine). * **Energy Bonds:** The phosphoric acid anhydrides in ATP store high energy; the repulsion between the multiple negative charges (due to low $pK_a$) contributes to the instability and high energy-release potential of ATP hydrolysis.

Question 56: Nucleic acids absorb light at a wavelength of 260 nm due to which component?

A. Proteins
B. Ribose
C. Purines and Pyrimidines (Correct Answer)
D. Deoxyribose

Explanation: **Explanation:** The absorption of ultraviolet (UV) light by nucleic acids at **260 nm** is a fundamental property used in laboratory medicine for the quantification of DNA and RNA. **Why Purines and Pyrimidines are correct:** The nitrogenous bases (Purines: Adenine, Guanine; Pyrimidines: Cytosine, Thymine, Uracil) contain **conjugated double bond systems** within their heterocyclic rings. These pi-electron systems undergo electronic transitions when exposed to UV light, specifically showing maximum absorbance ($λ_{max}$) at 260 nm. This property is intrinsic to the aromatic nature of the bases themselves, not the sugar-phosphate backbone. **Why other options are incorrect:** * **Proteins (Option A):** While proteins also absorb UV light, their peak absorbance occurs at **280 nm**, primarily due to aromatic amino acids like Tryptophan and Tyrosine. The 260/280 ratio is a standard clinical tool to assess the purity of nucleic acid samples. * **Ribose and Deoxyribose (Options B & D):** These are pentose sugars. Unlike the nitrogenous bases, they lack the conjugated double bond systems required to absorb light significantly at 260 nm. They contribute to the structural backbone but not the spectrophotometric profile at this wavelength. **High-Yield NEET-PG Pearls:** 1. **Hyperchromicity:** When DNA is denatured (melted into single strands), its absorbance at 260 nm **increases** (Hyperchromic shift) because the stacked bases in the double helix are "unmasked." 2. **Purity Check:** A pure DNA sample has a **260/280 ratio of ~1.8**, while pure RNA is **~2.0**. A lower ratio indicates protein contamination. 3. **Rule of Thumb:** An $A_{260}$ of 1.0 corresponds to approximately 50 µg/mL of double-stranded DNA.

Question 57: In DNA, adenine specifically binds with which of the following?

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

Explanation: **Explanation:** The structure of DNA is governed by **Chargaff’s Rule** and the principle of **Complementary Base Pairing**. According to the Watson-Crick model, DNA consists of two antiparallel strands where a purine always pairs with a pyrimidine to maintain a constant diameter of the double helix. **Why Thymine is Correct:** Adenine (a purine) specifically pairs with **Thymine** (a pyrimidine) via **two hydrogen bonds**. This specific pairing is essential for the stability of the DNA ladder and ensures accurate replication and transcription. **Analysis of Incorrect Options:** * **Guanine:** Guanine is a purine. Purine-purine pairing (A-G) would result in a "bulge" in the DNA helix due to excessive width. Guanine specifically pairs with Cytosine via three hydrogen bonds. * **Cytosine:** Cytosine is a pyrimidine that pairs exclusively with Guanine. Adenine and Cytosine do not pair because their hydrogen-bonding donor/acceptor patterns are incompatible. * **Uracil:** While Uracil is the complementary base for Adenine, it is found **only in RNA**. In DNA, thymine (which is 5-methyluracil) is used instead to provide greater photochemical stability and to allow the cell to detect DNA damage (deamination of cytosine). **NEET-PG High-Yield Pearls:** 1. **Hydrogen Bonds:** A=T has 2 bonds; G≡C has 3 bonds. Therefore, DNA with high G-C content has a higher **Melting Temperature (Tm)**. 2. **Chargaff’s Rule:** In double-stranded DNA, the amount of A = T and G = C; thus, Total Purines (A+G) = Total Pyrimidines (T+C). 3. **Structural Difference:** Thymine is simply Uracil with a methyl group at the C5 position (**5-methyluracil**). 4. **Denaturation:** Agents like formamide and urea decrease DNA stability by disrupting these specific hydrogen bonds.

Question 58: What is the most common physiological form of DNA?

A. A-form
B. B-form (Correct Answer)
C. Z-form
D. C-form

Explanation: **Explanation:** The correct answer is **B-form**. This is the standard double-helical structure described by Watson and Crick in 1953. **1. Why B-form is correct:** B-DNA is the most stable and predominant form of DNA under physiological conditions (high hydration and low ionic strength). It is a **right-handed** helix with approximately 10.5 base pairs per turn. It features distinct major and minor grooves, which are essential for the binding of regulatory proteins and transcription factors. **2. Why other options are incorrect:** * **A-form:** This is also a right-handed helix but is shorter and wider than B-DNA. It occurs under conditions of **dehydration**. It is clinically relevant as the form adopted by **RNA-DNA hybrids** and double-stranded RNA. * **Z-form:** This is a unique **left-handed** helix with a "zigzag" sugar-phosphate backbone. It occurs in DNA sequences with alternating purines and pyrimidines (e.g., GCGCGC). It is thought to play a role in gene expression regulation and relieving torsional strain during transcription. * **C-form:** This is a right-handed helix that occurs under even lower humidity than the A-form. It is not found under physiological conditions. **High-Yield Facts for NEET-PG:** * **Directionality:** A and B forms are Right-handed; Z-form is Left-handed. * **Glycosidic Bond:** Anti-configuration in A and B forms; Syn-configuration (for purines) in Z-form. * **Base pairs per turn:** A (11), B (10.5), Z (12). * **Pitch:** B-DNA has a pitch of 3.4 nm (34 Å) per turn, with a distance of 0.34 nm between successive base pairs.

Question 59: All of the following are true about Lesch-Nyhan syndrome, EXCEPT:

A. Affects almost solely males
B. Excessive synthesis of purines is a feature
C. Characterized by gouty arthritis
D. Febuxostat is routinely used for treatment (Correct Answer)

Explanation: **Explanation:** Lesch-Nyhan Syndrome (LNS) is an X-linked recessive disorder caused by a complete deficiency of the enzyme **Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT)**. This enzyme is crucial for the purine salvage pathway. **Why Option D is the Correct Answer (The Exception):** While hyperuricemia is a hallmark of LNS, **Allopurinol** is the standard pharmacological treatment used to reduce uric acid levels. **Febuxostat** is a potent xanthine oxidase inhibitor used in adult refractory gout, but it is **not routinely used** or considered the first-line standard of care in pediatric LNS patients due to limited clinical data and the established efficacy of Allopurinol. Furthermore, treating the hyperuricemia does not resolve the devastating neurological or behavioral symptoms of the syndrome. **Analysis of Incorrect Options:** * **Option A:** As an **X-linked recessive** disorder, it affects males almost exclusively; females are typically asymptomatic carriers. * **Option B:** HGPRT deficiency leads to an accumulation of PRPP (Phosphoribosyl pyrophosphate). PRPP is a potent activator of *glutamine-PRPP amidotransferase*, the rate-limiting enzyme of **de novo purine synthesis**, leading to massive overproduction of purines. * **Option C:** The end product of excessive purine degradation is uric acid. Severe hyperuricemia leads to the deposition of sodium urate crystals in joints, causing **gouty arthritis** and nephrolithiasis. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for HGPRT deficiency:** **H**yperuricemia, **G**out, **P**issed off (Self-mutilation/Aggression), **R**etardation (Intellectual disability), **T**one (Dystonia/Choreoathetosis). * **Self-mutilation** (biting of lips and fingers) is the pathognomonic behavioral feature. * **Orange sand-like crystals** (sodium urate) are often found in the diapers of affected infants. * **Enzyme Diagnosis:** Assaying HGPRT activity in erythrocytes or fibroblasts.

Question 60: Pseudouridine is a modified nucleoside found in which type of RNA molecule?

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

Explanation: **Explanation:** **Correct Answer: D. tRNA** Pseudouridine ($\psi$) is the most abundant post-transcriptionally modified nucleoside found in RNA. It is a structural isomer of uridine where the ribose sugar is attached to the **C5 position** of the uracil ring via a **C-C bond**, rather than the standard N1-C1 glycosidic bond. It is a hallmark feature of **tRNA**, specifically located in the **T$\psi$C arm** (the "T-loop"), which is responsible for binding the tRNA to the ribosomal surface during protein synthesis. **Analysis of Incorrect Options:** * **A. DNA:** DNA contains deoxyribonucleose sugars and standard bases (A, T, G, C). While it undergoes methylation, pseudouridine is not a constituent of DNA. * **B. mRNA:** Eukaryotic mRNA undergoes modifications like 5' capping and 3' polyadenylation. While trace amounts of pseudouridine have been discovered via high-throughput sequencing, it is not a defining structural component like it is for tRNA. * **C. rRNA:** Although rRNA does contain some pseudouridine residues that assist in ribosome folding, the question asks for the molecule where it is a classic, diagnostic feature. In the context of NEET-PG, pseudouridine is most strongly associated with the "T$\psi$C arm" of tRNA. **High-Yield NEET-PG Pearls:** * **Unique Bond:** Pseudouridine is the only nucleoside with a **C-C glycosidic bond**. * **tRNA Structure:** tRNA contains other unusual bases like **Dihydrouridine (D-arm)** and **Inosine (Wobble position)**. * **Clinical Significance:** Urinary excretion of pseudouridine is a marker of **high RNA turnover**, often seen in malignancies (e.g., leukemia) because pseudouridine cannot be recycled by salvage pathways and is excreted unchanged.

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

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