Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 191: To which site of the ribosome does aminoacyl tRNA attach during protein synthesis?

A. mRNA binding site
B. P site of the ribosome
C. A site of the ribosome (Correct Answer)
D. E site of the ribosome

Explanation: **A site of the ribosome** - The **A (aminoacyl) site** is where the **incoming aminoacyl-tRNA** first binds to the ribosome during protein synthesis, carrying the next amino acid to be added to the polypeptide chain. - This binding is guided by the **codon-anticodon interaction** between the mRNA at the A site and the anticodon on the tRNA. *P site of the ribosome* - The **P (peptidyl) site** is where the **tRNA carrying the growing polypeptide chain** resides. - After the new amino acid is added from the A site, the tRNA from the A site translocates to the P site. *E site of the ribosome* - The **E (exit) site** is where the **deacylated tRNA** (which has released its amino acid) is released from the ribosome. - It is the final stop for tRNA before it leaves the ribosome to be recharged with another amino acid. *mRNA binding site* - The **mRNA binding site** refers to the general region on the ribosome where the messenger RNA molecule associates. - While the mRNA provides the codons for protein synthesis, it is not the specific site where the **aminoacyl-tRNA** attaches.

Question 192: Which of the following statements about tRNA is correct?

A. 80% of total RNA
B. Contains 50-60 nucleotides
C. Longest RNA
D. The CCA sequence is added post-transcriptionally. (Correct Answer)

Explanation: ***The CCA sequence is added post-transcriptionally.*** - The **CCA motif** at the 3' end of tRNA is crucial for amino acid attachment and is added by the enzyme **tRNA nucleotidyltransferase** after transcription. - This **post-transcriptional modification** ensures that the tRNA is fully functional for protein synthesis. *80% of total RNA* - **Ribosomal RNA (rRNA)**, not tRNA, constitutes the majority (approximately 80%) of cellular RNA. - tRNA typically makes up about **15%** of total cellular RNA. *Contains 50-60 nucleotides* - Transfer RNA molecules are relatively small, typically containing between **70 to 90 nucleotides**, not 50-60. - This specific length is important for their characteristic **cloverleaf secondary structure** and L-shaped tertiary structure. *Longest RNA* - **Messenger RNA (mRNA)** molecules are generally the longest type of RNA, varying greatly in length depending on the protein they encode. - tRNA molecules are among the **shortest** RNA molecules.

Question 193: All of the following are true about Nucleic Acid Sequence Based Amplification except which of the following?

A. It is not a replacement for reverse transcriptase PCR.
B. It requires three enzymes: reverse transcriptase, RNase H, and T7 RNA polymerase.
C. It is a specific amplification of RNA
D. Denaturation is carried out at 94°C (Correct Answer)

Explanation: ***Denaturation is carried out at 94°C*** - This statement is **incorrect** because **Nucleic Acid Sequence Based Amplification (NASBA)** is an **isothermal** amplification method, meaning it does not require high-temperature denaturation steps. - NASBA operates at a constant temperature (typically around **41°C**), making it distinct from PCR which involves thermal cycling with high-temperature denaturation at 94°C. - This is the **EXCEPT answer** - the only false statement among the options. *It is a specific amplification of RNA* - NASBA is indeed designed for the **specific amplification of RNA targets**, making it particularly useful for detecting RNA viruses or mRNA transcripts. - It utilizes **T7 RNA polymerase** to produce multiple copies of RNA from an RNA template. *It is not a replacement for reverse transcriptase PCR.* - While both detect RNA, NASBA is an **alternative to**, not a replacement for, **reverse transcriptase PCR (RT-PCR)**. - NASBA has advantages in certain settings, such as **isothermal operation** and continuous RNA amplification, but RT-PCR remains the gold standard for many applications. *It requires three enzymes: reverse transcriptase, RNase H, and T7 RNA polymerase.* - This statement is **correct**. NASBA employs **three key enzymes** in its amplification process: - **Reverse transcriptase (AMV-RT)**: Synthesizes cDNA from RNA template - **RNase H**: Degrades the RNA strand in RNA-DNA hybrids - **T7 RNA polymerase**: Generates multiple RNA copies from the DNA template

Question 194: Which of the following is a nucleoside?

A. Adenine
B. Uridine (Correct Answer)
C. Thymine
D. Guanine

Explanation: ***Uridine*** - Uridine is a **nucleoside** composed of the nitrogenous base **uracil** covalently attached to the sugar **ribose** via a β-N1-glycosidic bond. - Nucleosides consist of a **nitrogenous base** (purine or pyrimidine) linked to a **pentose sugar** (ribose or deoxyribose). *Adenine* - Adenine is a **purine nitrogenous base**, not a nucleoside. - It is a component of both DNA and RNA, and when combined with ribose, it forms the nucleoside **adenosine**. *Thymine* - Thymine is a **pyrimidine nitrogenous base**, not a nucleoside. - When combined with deoxyribose, it forms the deoxynucleoside **thymidine**, which is found in DNA. *Guanine* - Guanine is a **purine nitrogenous base**, not a nucleoside. - When combined with ribose, it forms the nucleoside **guanosine**.

Question 195: Which enzyme is responsible for synthesizing RNA during transcription?

A. RNA polymerase (Correct Answer)
B. Primase
C. Ligase
D. Topoisomerase

Explanation: ***RNA polymerase (Correct Answer)*** - **RNA polymerase** is the central enzyme responsible for synthesizing an **RNA strand** from a DNA template during transcription. - It unwinds the DNA helix, reads the nucleotide sequence, and adds complementary RNA nucleotides to form a new RNA molecule. - This is a fundamental process in **gene expression**, occurring in the nucleus of eukaryotic cells. *Primase (Incorrect)* - **Primase** is an enzyme involved in **DNA replication**, not transcription. - Its function is to synthesize short **RNA primers** (8-12 nucleotides) that provide a starting point for DNA polymerase. *Ligase (Incorrect)* - **Ligase** is an enzyme that joins DNA fragments together by forming **phosphodiester bonds**. - It is primarily involved in **DNA replication** and repair processes, connecting Okazaki fragments on the lagging strand or repairing nicks in DNA strands. *Topoisomerase (Incorrect)* - **Topoisomerases** are enzymes that regulate the **supercoiling** of DNA. - They relieve **torsional stress** that builds up during DNA replication and transcription by cutting and rejoining DNA strands, preventing tangling.

Question 196: For which tissue/organ is the salvage pathway of purine biosynthesis particularly important?

A. Liver
B. RBCs (Correct Answer)
C. Kidney
D. Lung

Explanation: ***RBCs*** - **Red blood cells (RBCs)** lack a nucleus and the machinery for *de novo* purine synthesis, making them entirely dependent on the **salvage pathway** to acquire purines. - The **salvage pathway** reuses pre-existing purine bases and nucleosides to synthesize new purine nucleotides via enzymes like **HGPRT** (hypoxanthine-guanine phosphoribosyltransferase), which is crucial for RBC function. - **Brain tissue** is another organ critically dependent on salvage pathways, but among the given options, RBCs represent the classic example of absolute salvage pathway dependence. *Liver* - The liver is a major site of **_de novo_ purine synthesis** and is not primarily dependent on the salvage pathway for its purine requirements. - While the liver does utilize the salvage pathway, it also has robust **_de novo_ synthesis** capabilities, making it less critical than for RBCs. *Kidney* - The kidney performs both **_de novo_ purine synthesis** and utilizes the salvage pathway, similar to most other metabolically active tissues. - It is not uniquely or predominantly reliant on the salvage pathway for its purine needs compared to _de novo_ synthesis. *Lung* - The lung tissue, like most tissues with active metabolism and cell division, has the capacity for both **_de novo_ purine synthesis** and the salvage pathway. - It does not have a specific or heightened dependence on the salvage pathway that would make it particularly important compared to other tissues.

Question 197: If a sample of DNA has adenine at 28%, what will be the amount of Cytosine present?

A. 20%
B. 24%
C. 22% (Correct Answer)
D. 26%

Explanation: ***22%*** - According to **Chargaff's rules**, in a double-stranded DNA molecule, the amount of **adenine (A)** is approximately equal to the amount of **thymine (T)**, and the amount of **guanine (G)** is approximately equal to the amount of **cytosine (C)**. - If adenine (A) is 28%, then thymine (T) is also 28%. The total percentage of A and T is 28% + 28% = 56%. The remaining percentage for G and C is 100% - 56% = 44%. Since G = C, cytosine (C) will be 44% / 2 = 22%. *20%* - This value is not consistent with the given **adenine percentage** when applying **Chargaff's rules** for DNA base pairing. - If cytosine were 20%, then guanine would also be 20%, making the total G+C content 40%. This would leave 60% for A+T, meaning A would be 30%, not 28%. *24%* - This percentage does not align with the fundamental **base-pairing rules** of DNA. - If cytosine were 24%, then guanine would also be 24%, totaling 48% for G+C. This would imply 52% for A+T, meaning adenine would be 26%, which contradicts the given 28%. *26%* - This would only be correct if the **adenine percentage** was lower, as it suggests a different **G+C content**. - If cytosine were 26%, then guanine would also be 26%, making the total G+C content 52%. This would imply 48% for A+T, meaning adenine would be 24%, not 28%.

Question 198: What is the mechanism responsible for the intestine-specific expression of apoprotein B-48?

A. DNA rearrangement
B. RNA alternative splicing
C. RNA editing (Correct Answer)
D. Protein synthesis

Explanation: ***RNA editing*** - In the intestine, a **cytidine deaminase enzyme (APOBEC-1)** deaminates a specific **cytidine to uridine** at position 6666 in the apoB mRNA. - This C-to-U change creates a **premature stop codon (UAA)**, resulting in the truncated **apoB-48 protein** (48% of the full-length apoB-100). *DNA rearrangement* - This mechanism involves permanent changes in the **genomic DNA sequence**, often seen in immune gene diversification (e.g., V(D)J recombination). - It would lead to a different gene product at the DNA level, which is not how apoB-48 is generated. *RNA alternative splicing* - This process involves the selective inclusion or exclusion of **exons** during mRNA processing, leading to different protein isoforms from a single gene. - While it generates multiple protein products, it does not involve a nucleotide change within the mRNA sequence to create a new stop codon. *Protein synthesis* - This is the process of translating mRNA into protein, directed by the codons in the mRNA sequence. - While apoB-48 is a product of protein synthesis, the mechanism for its *intestine-specific expression* lies in the modification of the mRNA *before* translation, not in the synthesis process itself.

Question 199: Which of the following statements about DNA polymerase I is correct?

A. Participates in the synthesis of Okazaki fragments.
B. Is the primary enzyme for DNA replication in bacteria
C. Involved in DNA repair processes. (Correct Answer)
D. Not essential for DNA replication in bacteria.

Explanation: ***Involved in DNA repair processes.*** - **DNA polymerase I** possesses **5' to 3' exonuclease activity**, which is crucial for removing **RNA primers** and damaged DNA segments during DNA repair. - Its **DNA repair function** is essential for maintaining genome integrity by excising incorrect nucleotides and filling the gaps. - DNA pol I plays a key role in **nick translation** and **gap filling** after primer removal during DNA replication. *Participates in the synthesis of Okazaki fragments.* - **DNA polymerase III** is the primary enzyme responsible for synthesizing **Okazaki fragments** on the lagging strand during bacterial DNA replication. - While DNA polymerase I does **process** Okazaki fragments by removing RNA primers and filling gaps, it does not *synthesize* them. *Is the primary enzyme for DNA replication in bacteria* - **DNA polymerase III** is the main enzyme responsible for the bulk of DNA synthesis during replication in **bacteria**. - DNA polymerase I plays a more specialized role in **primer removal** and **gap filling** rather than primary elongation. *Not essential for DNA replication in bacteria.* - **DNA polymerase I** is **essential** for bacterial viability despite not being the primary replicative polymerase. - Its crucial role in **primer removal** and **gap filling** after primer excision is indispensable for completing DNA replication and repair processes.

Question 200: What is the first purine nucleotide synthesized in de novo purine biosynthesis?

A. AMP
B. GMP
C. IMP (Correct Answer)
D. UMP

Explanation: ***IMP (Inosine Monophosphate)*** - **IMP** is the first complete purine nucleotide synthesized during the **de novo purine biosynthesis pathway**. - It serves as a branch point, from which **AMP** and **GMP** are subsequently synthesized through separate pathways. *AMP (Adenosine Monophosphate)* - **AMP** is a derivative of **IMP**, synthesized by the addition of an amino group from **aspartate** to IMP. - This step occurs after the formation of the complete purine ring structure in IMP. *GMP (Guanosine Monophosphate)* - **GMP** is also derived from **IMP**, through a pathway involving the oxidation of IMP to **XMP** (xanthosine monophosphate) and subsequent amination. - Its synthesis occurs downstream from IMP. *UMP (Uridine Monophosphate)* - **UMP** is a **pyrimidine nucleotide**, not a purine, and is synthesized via a completely different de novo pathway. - Pyrimidine biosynthesis involves forming the ring structure first, then attaching it to ribose-phosphate, unlike purine synthesis which builds the ring on a pre-existing ribose-phosphate.

Practice by Chapter

Nucleotide Structure and Function

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DNA Structure and Replication

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RNA Structure and Types

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Transcription: RNA Synthesis

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Post-Transcriptional Modifications

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Translation: Protein Synthesis

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Genetic Code and Codon Usage

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Regulation of Gene Expression

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Mutations and DNA Repair

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Purine Metabolism and Disorders

Practice Questions

Pyrimidine Metabolism and Disorders

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Nucleotide Degradation and Salvage Pathways

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