Nucleic Acid Biochemistry Practice Questions

Q: Which one of the following bonds links two nucleotides in a nucleic acid?

3'-5' phosphodiester bond. ### Explanation **1. Why the Correct Answer is Right:** In nucleic acids (DNA and RNA), nucleotides are linked together to form a linear polymer through **3'-5' phosphodiester bonds**. This bond forms when the phosphate group attached to the **5' carbon** of one pentose sugar (ribose or deoxyribose) reacts with the hydroxyl (-OH) group on the **3' carbon** of the adjacent nucleotide's sugar. This linkage creates the "sugar-phosphate backbone," providing the molecule with a distinct polarity (a 5' end and a 3' end). This directionality is crucial for processes like DNA replication and transcription, which always occur in a 5' to 3' direction. **2. Why the Other Options are Incorrect:** * **A & C (3'-3' and 5'-5' bonds):** These bonds do not occur in the standard backbone of DNA or RNA. Such linkages would disrupt the linear polarity required for genetic coding and enzyme recognition. (Note: A 5'-5' triphosphate bridge is found uniquely in the **mRNA 5' cap**). * **D (5'-5' phosphotriester bond):** A phosphotriester involves three ester bonds to a single phosphate. While these occur in some synthetic oligonucleotides or chemical intermediates, they are not the natural linkage in biological nucleic acids. **3. High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** DNA polymerase adds new nucleotides only to the **3' -OH group**; hence, DNA synthesis is always **5' → 3'**. * **Cleavage:** **Phosphodiesterases** (like snake venom or restriction enzymes) are enzymes that hydrolyze these 3'-5' bonds. * **Charge:** The phosphate group in the phosphodiester bond is acidic, giving DNA/RNA an overall **negative charge**, which is why they migrate toward the anode (+) during electrophoresis. * **Stability:** RNA is more susceptible to alkaline hydrolysis than DNA because of the additional 2' -OH group on the ribose sugar, which can attack the 3'-5' phosphodiester bond.

Q: Which method is used to monitor DNA denaturation in vitro?

UV absorption. **Explanation:** **1. Why UV Absorption is Correct:** DNA denaturation (the separation of double-stranded DNA into single strands) is monitored using the **Hyperchromic Effect**. Nitrogenous bases in DNA strongly absorb UV light at a wavelength of **260 nm**. In a double helix, these bases are "stacked" and shielded, which limits their light absorption. When DNA denatures (due to heat or alkali), the stacking interactions are disrupted, exposing the bases. This results in a significant increase (approx. 30-40%) in UV absorbance. The temperature at which 50% of the DNA is denatured is known as the **Melting Temperature (Tm)**. **2. Why Other Options are Incorrect:** * **A. Centrifugation:** This technique (e.g., Ultracentrifugation) separates molecules based on density, size, or shape (Svedberg units). While it can separate DNA from other organelles, it is not a real-time monitor for the denaturation process. * **C. Chromatography:** This is a broad term for separating mixtures (e.g., ion-exchange or size-exclusion). While it can purify DNA, it does not measure the transition from double-stranded to single-stranded states. * **D. HPLC (High-Performance Liquid Chromatography):** Used for high-resolution separation and quantification of nucleotides or drugs, but it is not the standard kinetic tool for monitoring DNA melting curves. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tm and GC Content:** The Melting Temperature (Tm) is directly proportional to the **G-C content** because G-C pairs have three hydrogen bonds, while A-T pairs have only two. * **Hyperchromicity:** An increase in absorbance upon denaturation. * **Hypochromicity:** A decrease in absorbance (occurs during DNA renaturation/annealing). * **Purity Ratio:** A pure DNA sample has an **A260/A280 ratio of ~1.8**. A lower ratio suggests protein contamination.

Q: Enzymatic activity of RNA is involved in all of the following EXCEPT?

RNA-Aptamer binding to cognate small molecules. ### Explanation The question tests the distinction between **Ribozymes** (catalytic RNA) and **RNA Aptamers** (binding RNA). **1. Why Option A is the Correct Answer:** An **RNA Aptamer** is a short, single-stranded nucleic acid molecule that folds into a specific 3D structure to bind to a target molecule (like a protein or small metabolite) with high affinity and specificity. This is a **binding/recognition** function, similar to an antibody-antigen interaction, and does **not** involve the chemical transformation of a substrate. Therefore, it lacks enzymatic (catalytic) activity. **2. Analysis of Incorrect Options (Catalytic Functions):** * **B. Cleavage of nucleic acids:** Ribozymes like **RNase P** (involved in tRNA processing) and **Hammerhead ribozymes** catalyze the phosphodiester bond cleavage of RNA. * **C. Peptide bond formation:** The **23S rRNA** (in prokaryotes) or **28S rRNA** (in eukaryotes) of the large ribosomal subunit acts as a **Peptidyl transferase**. It catalyzes the formation of peptide bonds during translation. This is a classic example of a ribozyme. * **D. RNA splicing:** **Small nuclear RNAs (snRNAs)** within the spliceosome catalyze the transesterification reactions required to remove introns and join exons. **3. High-Yield Facts for NEET-PG:** * **Ribozyme Definition:** RNA molecules that accelerate chemical reactions without being consumed. * **Thomas Cech & Sidney Altman:** Awarded the Nobel Prize for discovering the catalytic properties of RNA. * **Clinical Pearl:** **Macugen (Pegaptanib)** is an FDA-approved **RNA Aptamer** used in the treatment of Neovascular (Wet) Age-related Macular Degeneration (AMD) by binding to VEGF. * **Riboswitch:** A regulatory segment of an mRNA molecule that binds a small molecule (aptamer domain), resulting in a change in production of the proteins encoded by the mRNA.

Q: PRPP participates in purine biosynthesis in which step?

Serves as a scaffold for assembly of the purine ring. **Explanation:** In **Purine biosynthesis**, the assembly of the heterocyclic ring occurs **directly onto** a pre-existing ribose-5-phosphate molecule. This is a fundamental distinction from pyrimidine synthesis. **Why Option A is correct:** The process begins with **PRPP (5-Phosphoribosyl-1-pyrophosphate)**. In the first committed step, the enzyme *PRPP glutamyl amidotransferase* replaces the pyrophosphate group of PRPP with an amino group from glutamine. This ribose-5-phosphate unit then acts as the **scaffold (foundation)** upon which atoms from various donors (glycine, aspartate, glutamine, folate, and $CO_2$) are added sequentially to build the Inosine Monophosphate (IMP) ring. **Why other options are incorrect:** * **Option B:** This describes **Pyrimidine biosynthesis**. In pyrimidines, the base (Orotic acid) is synthesized first as a free ring and is only *subsequently* attached to PRPP. * **Option C & D:** These are incorrect because the biochemical sequence is specific to the "scaffold" method for purines. **High-Yield Clinical Pearls for NEET-PG:** * **Rate-limiting enzyme:** PRPP glutamyl amidotransferase (inhibited by AMP, GMP, and IMP via feedback inhibition). * **PRPP Synthetase:** The enzyme that forms PRPP from Ribose-5-P. Overactivity of this enzyme leads to **Hyperuricemia and Gout** due to overproduction of purines. * **Von Gierke’s Disease:** Increased G-6-P shunts into the HMP pathway, increasing Ribose-5-P and PRPP levels, which explains the associated hyperuricemia. * **Lesch-Nyhan Syndrome:** Failure of the salvage pathway (HGPRT deficiency) leads to an accumulation of PRPP, which further stimulates *de novo* purine synthesis.

Q: A dideoxynucleotide does not contain which of the following?

2' and 3' OH groups. **Explanation:** The core concept here is the structure of nucleotides used in DNA synthesis versus DNA sequencing. A standard **Deoxynucleotide (dNTP)** lacks an -OH group at the 2' position but possesses a hydroxyl (-OH) group at the 3' position, which is essential for forming phosphodiester bonds. **1. Why Option A is Correct:** A **Dideoxynucleotide (ddNTP)** is a synthetic nucleotide that lacks hydroxyl groups at **both the 2' and 3' positions** (hence "di-deoxy"). In DNA synthesis, the 3' -OH group acts as a nucleophile to attack the incoming nucleotide. Without the 3' -OH group, no further nucleotides can be added to the chain. This leads to **obligatory chain termination**, a principle utilized in Sanger sequencing. **2. Analysis of Incorrect Options:** * **Options B, C, and D:** These are incorrect because all nucleotides (including ddNTPs) must retain their **5' group** (usually attached to phosphates) to initiate a bond with the previous nucleotide, and the **4' position** in the pentose sugar ring does not typically carry a free hydroxyl group in this context. The specific modification that defines a "dideoxy" nucleotide is strictly at the 2' and 3' carbons. **3. NEET-PG High-Yield Pearls:** * **Sanger Sequencing:** Also known as the "Dideoxy chain termination method." It relies on ddNTPs to stop synthesis at specific bases. * **Mechanism of Action:** Drugs like **Zidovudine (AZT)** and **Didanosine (ddI)** used in HIV treatment are nucleoside reverse transcriptase inhibitors (NRTIs). They function similarly to ddNTPs by lacking a 3' -OH group, thereby terminating the viral DNA chain. * **Structure:** Remember: Ribose (RNA) has 2' and 3' -OH; Deoxyribose (DNA) has only 3' -OH; Dideoxyribose (Sequencing/Drugs) has neither.

Q: Which type of RNA has the highest percentage of modified bases?

tRNA. **Explanation:** **Why tRNA is the correct answer:** Transfer RNA (tRNA) contains the highest percentage of modified bases, accounting for approximately **10–15%** of its total nucleotides. These modifications occur post-transcriptionally and are essential for the structural stability, L-shaped folding, and accurate codon-anticodon recognition. Common modified bases include **Pseudouridine (ψ)**, **Dihydrouridine (D)**, **Inosine (I)**, and **Ribothymidine (T)**. These modifications are concentrated in specific regions like the D-loop and TψC loop, which are critical for the tRNA's interaction with the ribosome and aminoacyl-tRNA synthetases. **Analysis of Incorrect Options:** * **mRNA (Messenger RNA):** Contains the least amount of modified bases. While it undergoes 5' capping (7-methylguanosine) and 3' polyadenylation, the internal sequence remains largely unmodified to ensure accurate translation of the genetic code. * **rRNA (Ribosomal RNA):** While rRNA does undergo some modifications (like ribose methylation and pseudouridylation), they are significantly less frequent than those found in tRNA. rRNA is the most **abundant** RNA in the cell (80%), but not the most modified. * **snRNA (Small Nuclear RNA):** Involved in splicing (spliceosomes), snRNAs contain some modifications to assist in RNA-protein interactions, but the density does not match that of tRNA. **High-Yield Clinical Pearls for NEET-PG:** * **Abundance Rule:** **r**RNA is the most **r**ampant (80%), **t**RNA is the **t**iniest/smallest, and **m**RNA is the **m**essy/most heterogeneous in size. * **Pseudouridine:** Known as the "fifth nucleotide," it is a hallmark of tRNA and is excreted in urine; elevated levels can be a biomarker for high cell turnover (e.g., leukemia). * **Wobble Hypothesis:** Inosine (a modified base) at the 5' end of the tRNA anticodon allows for non-traditional base pairing, enabling one tRNA to recognize multiple codons.

Q: Which statement is not true regarding tRNA?

The D arm is involved in ribosomal attachment.. **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 1

Which one of the following bonds links two nucleotides in a nucleic acid?

Accepted Answer

3'-5' phosphodiester bond

Answer

3'-3' phosphodiester bond

Answer

5'-5' phosphodiester bond

Answer

5'-5' phosphotriester bond

Question 2

Which method is used to monitor DNA denaturation in vitro?

Accepted Answer

UV absorption

Answer

Centrifugation

Answer

Chromatography

Answer

HPLC

Question 3

Enzymatic activity of RNA is involved in all of the following EXCEPT?

Accepted Answer

RNA-Aptamer binding to cognate small molecules

Answer

Cleavage of nucleic acids

Answer

Peptide bond formation

Answer

RNA splicing

Question 4

A ten-year-old child presents with aggressive behavior, poor concentration, joint pain, reduced urinary output, and a history of self-mutilative behavior, specifically mutilating his fingers. Which of the following enzymes is likely to be deficient in this child?

Accepted Answer

Hypoxanthine-guanine phosphoribosyltransferase (HGPrtase)

Answer

Adenosine deaminase

Answer

Alkaline phosphatase (APase)

Answer

Acid maltase

Question 5

Azaserine inhibits which of the following enzymes?

Accepted Answer

Formyl glycinamide ribonucleotide amidotransferase

Answer

Glycinamide ribonucleotide synthetase

Answer

Glycinamide ribonucleotide transformylase

Answer

Inosine monophosphate synthase

Question 6

PRPP participates in purine biosynthesis in which step?

Accepted Answer

Serves as a scaffold for assembly of the purine ring

Answer

Post assembly of the purine ring

Answer

Both

Answer

None

Question 7

In a DNA molecule, one strand contains 20 thymine (T), 25 cytosine (C), 30 guanine (G), and 22 adenine (A) residues. How many of each base pair are found in the complete double-stranded molecule?

Accepted Answer

T=42, C=55, G=55, A=42

Answer

T=44, C=60, G=50, A=40

Answer

T=22, C=30, G=25, A=20

Answer

T=40, C=50, G=60, A=44

Question 8

A dideoxynucleotide does not contain which of the following?

Accepted Answer

2' and 3' OH groups

Answer

3' and 4' OH groups

Answer

4' and 5' OH groups

Answer

2' and 5' OH groups

Question 9

Which type of RNA has the highest percentage of modified bases?

Accepted Answer

tRNA

Answer

mRNA

Answer

rRNA

Answer

snRNA

Question 10

Which statement is not true regarding tRNA?

Accepted Answer

The D arm is involved in ribosomal attachment.

Answer

The TpsiC arm is involved in ribosomal attachment.

Answer

A CCA trinucleotide sequence is attached to the acceptor arm.

Answer

The first nucleotide of the anticodon of tRNA is not specific.

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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