Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 231: What is the function of primase?

A. Joining DNA fragments
B. Synthesizing small RNA fragments during translation
C. Unwinding of DNA
D. Synthesizing small RNA fragments during DNA synthesis (Correct Answer)

Explanation: ***Synthesizing small RNA fragments during DNA synthesis*** - **Primase** is an enzyme that synthesises short **RNA primers** which are crucial for initiating DNA replication. - These **RNA primers** provide a free 3'-hydroxyl group, which **DNA polymerase** requires to start adding deoxyribonucleotides. *Joining DNA fragments* - This function is primarily carried out by **DNA ligase**, which forms phosphodiester bonds between adjacent nucleotides to join DNA fragments. - **DNA ligase** is essential for repairing DNA breaks and joining **Okazaki fragments** on the lagging strand during replication. *Synthesising small RNA fragments during translation* - Small RNA fragments are generally involved in **gene regulation** (e.g., microRNAs) or structural components of ribosomes (e.g., ribosomal RNA) during translation, but primase is not involved in their synthesis for this purpose. - The synthesis of **mRNA**, **tRNA**, and **rRNA** during translation is carried out by **RNA polymerases**, not **primase**. *Unwinding of DNA* - The **unwinding of the DNA double helix** is primarily performed by an enzyme called **DNA helicase**. - **DNA helicase** breaks the hydrogen bonds between complementary base pairs, separating the two strands to allow replication or transcription to proceed.

Question 232: Uric acid is formed by?

A. Catabolism of proteins
B. Catabolism of ketones
C. Catabolism of purines (Correct Answer)
D. Catabolism of pyrimidines

Explanation: ***Catabolism of purines*** - **Uric acid** is the final breakdown product of **purine metabolism** in humans. - **Purines** (adenine and guanine) are components of nucleic acids (DNA and RNA) and are broken down through a series of enzymatic steps, ultimately forming uric acid. *Catabolism of proteins* - The catabolism of proteins leads to the formation of **amino acids**, which are then further broken down to produce **urea** (via the urea cycle) as the main nitrogenous waste product, not uric acid. - Protein breakdown primarily provides energy or building blocks for new proteins. *Catabolism of ketones* - The catabolism of ketones occurs when the body uses **fatty acids** for energy, often during fasting or uncontrolled diabetes. - Key products are **acetyl-CoA** and energy, not uric acid. *Catabolism of pyrimidines* - The catabolism of pyrimidines (cytosine, thymine, and uracil) produces compounds like **beta-alanine** and **ammonia**. - Unlike purines, pyrimidine breakdown does not result in uric acid formation.

Question 233: Which RNA is used in RNA splicing?

A. mRNA
B. tRNA
C. rRNA
D. Small nuclear RNA (snRNA) (Correct Answer)

Explanation: ***Small nuclear RNA (snRNA)*** - **snRNAs** are key components of **spliceosomes**, the molecular machines that catalyze the removal of introns from pre-mRNA. - They bind to specific sequences within the pre-mRNA and facilitate the splicing reactions. *mRNA* - **mRNA (messenger RNA)** carries the genetic code from DNA to the ribosomes for **protein synthesis**. - While it is the molecule that gets spliced, it does not directly participate in the splicing machinery itself. *rRNA* - **rRNA (ribosomal RNA)** is a structural and catalytic component of **ribosomes**, where protein synthesis occurs. - It plays no direct role in the process of RNA splicing. *tRNA* - **tRNA (transfer RNA)** molecules are responsible for carrying specific **amino acids** to the ribosome during protein synthesis. - They are involved in translation, not in the processing of RNA by splicing.

Question 234: What is the function of DNA ligase?

A. Unwinding (denaturation) of dsDNA to provide an ssDNA template
B. Initiation of DNA synthesis and elongation
C. Initiates synthesis of RNA primers
D. Joins the Okazaki fragments on the lagging strand by sealing the nicks between them. (Correct Answer)

Explanation: ***Joins the Okazaki fragments on the lagging strand by sealing the nicks between them.*** - **DNA ligase** catalyzes the formation of a **phosphodiester bond** between adjacent nucleotides, specifically joining the 3'-hydroxyl of one fragment to the 5'-phosphate of another. - This enzyme is crucial for completing **DNA replication** on the lagging strand by connecting the discontinuous **Okazaki fragments**. *Unwinding (denaturation) of dsDNA to provide an ssDNA template* - This function is primarily carried out by **DNA helicase**, which unwinds the double helix, separating the two strands. - DNA ligase plays no direct role in the initial unwinding of the DNA molecule. *Initiation of DNA synthesis and elongation* - The initiation of DNA synthesis is performed by **DNA primase** (laying down RNA primers) and then extended by **DNA polymerase**. - DNA ligase's role is to seal gaps, not to initiate or elongate new DNA strands. *Initiates synthesis of RNA primers* - The synthesis of **RNA primers** is the specific function of **DNA primase**. - These primers provide a free 3'-hydroxyl group for DNA polymerase to begin synthesizing new DNA.

Question 235: Which enzyme is required for cutting the DNA strand during synthesis?

A. DNA polymerase
B. DNA ligase
C. Topoisomerase (Correct Answer)
D. Helicase

Explanation: ***Topoisomerase*** - **Topoisomerases** are enzymes essential for DNA replication; they induce temporary **single- or double-strand breaks** in DNA to relieve **supercoiling** ahead of the replication fork. - This cutting and rejoining activity prevents the DNA from becoming excessively tangled and facilitates the unwinding process required for synthesis. *DNA polymerase* - **DNA polymerase** is responsible for **synthesizing new DNA strands** by adding nucleotides, not for cutting the DNA backbone. - It works by moving along the template strand, reading the bases, and then adding complementary nucleotides to the growing DNA strand. *DNA ligase* - **DNA ligase** functions to **join DNA fragments** together by forming phosphodiester bonds, especially in sealing Okazaki fragments during lagging strand synthesis. - Its role is to ligate (join) cut strands, not to initiate cuts in the DNA. *Helicase* - **Helicase** unwinds the DNA double helix into single strands using ATP hydrolysis; it **separates the two strands** but does not cut the phosphodiester backbone. - This enzyme creates the replication fork by disrupting hydrogen bonds between base pairs, making the DNA accessible for replication machinery.

Question 236: The anticodon region is an important part of which type of RNA?

A. r-RNA
B. m-RNA
C. t-RNA (Correct Answer)
D. hn-RNA

Explanation: **t-RNA** - The **anticodon region** is a critical component of **transfer RNA (tRNA)**, responsible for recognizing and binding to the complementary codon on mRNA during protein synthesis. - This interaction ensures that the correct **amino acid** is delivered to the growing polypeptide chain according to the genetic code. *r-RNA* - **Ribosomal RNA (rRNA)** is a structural and enzymatic component of **ribosomes**, which are the cellular machinery for protein synthesis. - While rRNA plays a crucial role in forming **peptide bonds** and facilitating translation, it does not possess an anticodon region. *m-RNA* - **Messenger RNA (mRNA)** carries the **genetic code** from DNA to the ribosomes in the form of codons, which specify the sequence of amino acids for protein synthesis. - mRNA molecules have codons, but they do not have an **anticodon region**; instead, they are read by the anticodons of tRNA. *hn-RNA* - **Heterogeneous nuclear RNA (hnRNA)** is a precursor to mRNA in eukaryotic cells, containing both exons and introns. - It undergoes extensive processing, including **splicing**, to become mature mRNA, but it does not have an **anticodon region**.

Question 237: Which of the following is a termination codon?

A. AUG
B. UAA (Correct Answer)
C. AUA
D. AGG

Explanation: ***UAA*** - **UAA** is one of the three **stop codons** (UAA, UAG, UGA) that signals the termination of protein synthesis during translation. - When the ribosome encounters a UAA codon, no corresponding tRNA with an anticodon binds, and release factors bind instead, leading to the dissociation of the polypeptide chain. *AUG* - **AUG** is the universal **start codon** in most organisms, encoding for methionine in eukaryotes and N-formylmethionine in prokaryotes. - Its presence signals the initiation of protein synthesis, not its termination. *AUA* - **AUA** is a codon that codes for the amino acid **Isoleucine**. - It is a **sense codon** and does not act as a signal for termination. *AGG* - **AGG** is a codon that codes for the amino acid **Arginine**. - Similar to AUA, it is a **sense codon** and participates in elongating the polypeptide chain, rather than terminating it.

Question 238: Which of the following usually require a RNA intermediate for cloning/replication?

A. Cosmids
B. Retroviruses (Correct Answer)
C. Plasmids
D. Transposons

Explanation: ***Retroviruses*** - **All retroviruses require an RNA intermediate** for their replication cycle, making this the correct answer. - Retroviruses have an **RNA genome** that must be **reverse transcribed into DNA** by reverse transcriptase enzyme before integration into the host genome. - The integrated DNA (provirus) is then transcribed back to RNA, which serves both as mRNA for viral proteins and as genomic RNA for new virions. - Examples include **HIV, HTLV**, and other retroviruses that definitively use this RNA → DNA → RNA replication strategy. *Transposons* - This option is **too broad** to be correct. Only **retrotransposons** (Class I transposons) use RNA intermediates via a "copy-and-paste" mechanism involving reverse transcription. - However, **DNA transposons** (Class II) move by a "cut-and-paste" DNA mechanism **without any RNA intermediate**. - Since the question asks what "usually requires" RNA intermediate, and many common transposons (like bacterial Tn5, Tn10) are DNA transposons, this answer is imprecise. *Cosmids* - Cosmids are **hybrid cloning vectors** containing cos sites from bacteriophage lambda combined with plasmid sequences. - They replicate as **DNA plasmids** in bacteria using DNA-dependent DNA polymerase. - No RNA intermediate is involved in their replication mechanism. *Plasmids* - Plasmids are **extrachromosomal circular DNA molecules** that replicate independently within bacterial or yeast cells. - Replication occurs via **DNA-to-DNA synthesis** using DNA polymerase. - No RNA intermediate is required for plasmid propagation.

Question 239: What is the role of adenine phosphoribosyl transferase (APRT) in purine metabolism?

A. Breakdown of purines
B. Salvage pathway of purine nucleotide synthesis (Correct Answer)
C. Not involved in purine metabolism
D. De novo synthesis of purines

Explanation: ***Salvage pathway of purine nucleotide synthesis*** - **Adenine phosphoribosyl transferase (APRT)** catalyzes the reaction of **adenine** with **5-phosphoribosyl-1-pyrophosphate (PRPP)** to form **adenosine monophosphate (AMP)**. - This reaction is a crucial step in the **purine salvage pathway**, which reclaims pre-formed purine bases and converts them back into nucleotides, conserving energy. *Breakdown of purines* - The breakdown of purines (catabolism) typically involves enzymes like **adenosine deaminase** and **xanthine oxidase**, leading to the formation of **uric acid**. - APRT is involved in synthesizing nucleotides, not their degradation. *Not involved in purine metabolism* - APRT is an enzyme specifically involved in the **anabolic processes** of purine metabolism, as it contributes to the formation of purine nucleotides. - Its role is well-established within the **salvage pathway**. *De novo synthesis of purines* - The **de novo synthesis pathway** builds purine nucleotides from simpler precursors like **amino acids**, **CO2**, and **THF derivatives**. - While both pathways produce purine nucleotides, APRT is exclusively part of the **salvage pathway**, which recycles existing purine bases.

Question 240: What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?

A. 3' TAC 5' (Correct Answer)
B. 3' TAG 5'
C. 3' TAA 5'
D. 3' ATG 5'

Explanation: ***3' TAC 5'*** - The **start codon** for protein synthesis on **mRNA** is **5'-AUG-3'**, which codes for **methionine** (or N-formylmethionine in prokaryotes) and signals the initiation of translation. - To produce an mRNA codon of **5'-AUG-3'**, the complementary sequence on the **template DNA strand** must be **3'-TAC-5'** (adenine pairs with uracil/thymine, guanine pairs with cytosine, and the strands are antiparallel). - During transcription, RNA polymerase reads the template strand in the 3' to 5' direction and synthesizes mRNA in the 5' to 3' direction. *3' TAG 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUC-3'**, which codes for **isoleucine**, not methionine. - Therefore, this sequence does not correspond to the first amino acid inserted into a protein. *3' TAA 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUU-3'**, which also codes for **isoleucine**, not methionine. - This is not the initiation codon sequence. *3' ATG 5'* - While **ATG** appears in this sequence, when presented as the **template strand** in the 3' to 5' orientation, it would be transcribed to produce mRNA **5'-UAC-3'**, which codes for **tyrosine**, not methionine. - The sequence **ATG** on the **coding strand** (non-template strand) corresponds to the start codon, but this option incorrectly presents it as the template strand sequence.

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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