Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 171: In DNA replication, which enzyme fills gaps left by RNA primer removal?

A. Primase
B. DNA polymerase I (Correct Answer)
C. DNA polymerase III
D. Helicase

Explanation: ***DNA polymerase I*** - This enzyme possesses **5' to 3' exonuclease activity** which is used to remove the RNA primers - It also has **5' to 3' polymerase activity** to synthesize DNA and fill the gaps left by primer removal - This dual function makes it uniquely suited for **"nick translation"** - the process of removing primers and filling gaps in Okazaki fragments *Primase* - **Synthesizes short RNA primers** (8-12 nucleotides) that provide a free 3'-OH group for DNA polymerase to initiate DNA synthesis - It does not fill gaps or remove primers during replication - Functions only at the initiation phase of DNA synthesis *DNA polymerase III* - The **primary replicative enzyme** responsible for synthesizing the bulk of new DNA in both the leading and lagging strands - It lacks the 5' to 3' exonuclease activity needed for primer removal and gap filling - Extends RNA primers but cannot remove them *Helicase* - Responsible for **unwinding the DNA double helix** at the replication fork by breaking hydrogen bonds between complementary base pairs - Creates the replication bubble but does not participate in primer processing or gap filling - Works ahead of the DNA polymerases

Question 172: Which enzyme unwinds DNA during replication?

A. Helicase (Correct Answer)
B. Ligase
C. Topoisomerase
D. Polymerase

Explanation: ***Helicase*** - **Helicase** is the enzyme responsible for **unwinding the double helix** of DNA by breaking the hydrogen bonds between complementary base pairs. - This action creates the **replication fork**, allowing other enzymes to access the single-stranded DNA templates. *Ligase* - **Ligase** is involved in joining DNA fragments, particularly **Okazaki fragments** on the lagging strand, by forming phosphodiester bonds. - It does not participate in the initial unwinding of the DNA helix. *Topoisomerase* - **Topoisomerase** enzymes relieve the **supercoiling** tension that builds up ahead of the replication fork as DNA unwinds. - While it's crucial for replication, it doesn't directly unwind the double helix itself; rather, it handles the topological stress. *Polymerase* - **DNA Polymerase** is responsible for synthesizing new DNA strands by adding nucleotides complementary to the template strand. - It plays a role in extending the DNA chain, not in the initial unwinding process.

Question 173: Which RNA modification at the 3' end stabilizes mRNA by preventing exonuclease degradation?

A. Polyadenylation (Correct Answer)
B. Capping
C. Splicing
D. Methylation

Explanation: ***Polyadenylation*** - The addition of a **poly-A tail** (a long chain of adenine nucleotides) to the **3' end** of mRNA is the primary modification that protects it from degradation by **3' to 5' exonucleases**. - This tail also plays crucial roles in mRNA export from the nucleus, translation initiation, and determining mRNA half-life. - The poly-A tail is progressively shortened by deadenylases, and once critically shortened, the mRNA becomes susceptible to degradation. *Capping* - **5' capping** involves the addition of a **7-methylguanosine cap** to the **5' end** of mRNA. - While this protects mRNA from **5' to 3' exonuclease** degradation, the question specifically asks about the **3' end** modification, which is polyadenylation. - The cap structure is also essential for ribosome binding and translation initiation. *Splicing* - **Splicing** is the process of removing **introns** (non-coding regions) and joining **exons** (coding regions) in pre-mRNA. - Its main function is to produce a mature mRNA sequence that can be translated into a functional protein, not to directly prevent exonuclease degradation. *Methylation* - **Methylation** can occur on various nucleotides within RNA molecules (e.g., N6-methyladenosine or m6A). - While methylation can influence mRNA stability, translation efficiency, and splicing, it is primarily a regulatory modification rather than a direct structural protection against exonuclease degradation like polyadenylation.

Question 174: DNA Methylation is not related to?

A. DNA Replication
B. Gene silencing
C. Capping (Correct Answer)
D. Mismatch repair

Explanation: ***Capping*** - **Capping** is a modification of messenger RNA (mRNA) that occurs during **mRNA processing** in eukaryotes, involving the addition of a 7-methylguanosine cap to the 5' end of the mRNA molecule. - This process is crucial for mRNA stability, translation initiation, and nuclear export, and is entirely **independent of DNA modifications** like DNA methylation. *DNA Replication* - DNA methylation plays a role in **DNA replication** to distinguish newly synthesized strands from parental strands during **DNA repair**. - In bacteria, methylation at specific sites (**Dam methylase**) helps in **mismatch repair** by identifying the parental strand. *Gene silencing* - **DNA methylation** of CpG islands in promoter regions is a well-established mechanism for **gene silencing** by altering chromatin structure and preventing transcription factor binding. - This epigenetic modification leads to stable transcriptional repression and is critical for processes like X-chromosome inactivation and genomic imprinting. *Mismatch repair* - In prokaryotes, **DNA methylation** marks the parental strand, which is used by the **mismatch repair system** to correct errors on the newly synthesized, unmethylated strand. - In eukaryotes, while not directly marking strands, DNA methylation can influence the efficiency of mismatch repair pathways by altering chromatin accessibility.

Question 175: Which of the following is not associated with post-transcription modification?

A. 5' capping
B. Glycosylation (Correct Answer)
C. Methylation
D. Endonuclease cleavage

Explanation: ***Glycosylation*** - **Glycosylation** is a type of post-translational modification, which involves the enzymatic addition of carbohydrates to proteins or lipids, not RNA. - This process is crucial for protein folding, stability, and function in the cell, occurring after translation has been completed. *5' capping* - **5' capping** is a crucial post-transcriptional modification of eukaryotic pre-mRNA, involving the addition of a 7-methylguanosine cap to the 5' end. - This cap protects the mRNA from degradation, facilitates nuclear export, and is essential for translation initiation. *Methylation* - **Methylation** can occur as a post-transcriptional modification, affecting various RNA types including tRNA, rRNA, and mRNA. - For mRNA, internal methylation, particularly of adenosine residues (m6A), plays a role in mRNA stability, splicing, and translation regulation. *Endonuclease cleavage* - **Endonuclease cleavage** is a significant post-transcriptional modification, particularly in the maturation of rRNA and tRNA, where larger precursor molecules are cut into functional smaller units. - In mRNA processing, endonuclease cleavage is involved in the formation of the 3' end, signaling for the addition of the poly-A tail.

Question 176: How does the diphtheria toxin inhibit protein synthesis?

A. Inhibits RNA polymerase
B. Interferes with tRNA
C. Inactivates EF-2 (Correct Answer)
D. Blocks ribosome binding

Explanation: ***Inactivates EF-2*** - The **diphtheria toxin** is an A-B toxin, where the A subunit acts as an enzyme. - The A subunit **ADP-ribosylates** and thereby inactivates **elongation factor-2 (EF-2)**, which is essential for eukaryotic protein synthesis. *Inhibits RNA polymerase* - **RNA polymerase inhibition** would prevent **transcription**, the synthesis of RNA from a DNA template. - The diphtheria toxin specifically targets **translation (protein synthesis)**, not transcription. *Interferes with tRNA* - **tRNA (transfer RNA)** is critical for carrying specific amino acids to the ribosome during protein synthesis. - While tRNA is involved in protein synthesis, the diphtheria toxin's primary mechanism is not direct interference with tRNA itself, but rather with an elongation factor. *Blocks ribosome binding* - Blocking **ribosome binding** would prevent the initiation of protein synthesis. - The diphtheria toxin allows for initial ribosome binding but then halts the **elongation phase** of protein synthesis by targeting EF-2.

Question 177: A patient presents with hyperuricemia and gout. Which enzyme's overactivity is most likely associated?

A. HGPRT
B. Xanthine oxidase
C. Adenosine deaminase
D. PRPP synthetase (Correct Answer)

Explanation: ***PRPP synthetase*** - **Overactivity** of **PRPP synthetase** leads to increased production of **5-phosphoribosyl-1-pyrophosphate (PRPP)**, a key substrate for *de novo* purine synthesis. - This increased purine synthesis results in an **overproduction of uric acid**, causing **hyperuricemia** and **gout**. *HGPRT* - **Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)** deficiency, not overactivity, is associated with hyperuricemia and gout, as seen in **Lesch-Nyhan syndrome**. - Its normal function is in the **salvage pathway**, recycling purine bases; deficiency leads to increased *de novo* purine synthesis. *Xanthine oxidase* - **Xanthine oxidase** is involved in the catabolism of purines, converting **hypoxanthine to xanthine** and then **xanthine to uric acid**. - While inhibition of this enzyme (e.g., by allopurinol) is a treatment for gout, its **overactivity alone is not typically the primary cause** of hereditary hyperuricemia; rather, altered purine metabolism leading to excess substrates for xanthine oxidase is the issue. *Adenosine deaminase* - Deficiency of **adenosine deaminase (ADA)** is primarily associated with **severe combined immunodeficiency (SCID)**, due to the accumulation of toxic metabolites that impair lymphocyte development. - It is not directly linked to the pathogenesis of **hyperuricemia** or **gout**.

Question 178: Which of the following amino acids directly contributes atoms to the purine ring structure?

A. Glycine (Correct Answer)
B. Arginine
C. Tyrosine
D. Cysteine

Explanation: ***Glycine*** - **Glycine** is a direct precursor for the purine ring structure, providing carbons at positions **C4 and C5**, and the nitrogen at position **N7**. - Its carbon and nitrogen atoms are incorporated directly into the **inosine monophosphate (IMP)** backbone during de novo purine synthesis. - Glycine is the only amino acid that contributes its **entire structure** to the purine ring. *Arginine* - **Arginine** does not directly contribute atoms to the purine ring structure. - While arginine is involved in the urea cycle, **aspartate** (which can be formed from arginine metabolism) does contribute N1 to the purine ring. - However, arginine itself is not a direct precursor. *Tyrosine* - **Tyrosine** is a precursor for **catecholamines** (dopamine, norepinephrine, epinephrine) and **thyroid hormones**. - It is not involved in purine synthesis and does not contribute to the purine ring structure. *Cysteine* - **Cysteine** is important for protein structure (disulfide bonds) and is a precursor for **glutathione** synthesis. - It does not contribute carbon or nitrogen atoms to the purine ring structure.

Question 179: Which of the following is NOT a characteristic of the DNA replication process in eukaryotes?

A. Involves the formation of Okazaki fragments on the lagging strand
B. Continuous synthesis on the leading strand
C. Requires primase to synthesize RNA primers
D. Single origin of replication (Correct Answer)

Explanation: ***Single origin of replication*** - Eukaryotic DNA replication begins at **multiple origins of replication** along each chromosome to efficiently replicate their larger genomes. - A single origin of replication is characteristic of **prokaryotic DNA replication**, which have smaller, circular chromosomes. *Continuous synthesis on the leading strand* - This is a **characteristic feature** of DNA replication in both prokaryotes and eukaryotes. - The **leading strand** is synthesized continuously in the 5' to 3' direction, following the replication fork. *Requires primase to synthesize RNA primers* - **Primase** is an essential enzyme in eukaryotic DNA replication that synthesizes **short RNA primers**. - These primers provide a free 3'-OH group for **DNA polymerase** to begin synthesizing new DNA strands. *Involves the formation of Okazaki fragments on the lagging strand* - This is a definite characteristic of eukaryotic DNA replication. - The **lagging strand** is synthesized discontinuously in short segments called **Okazaki fragments**, due to its antiparallel orientation relative to the leading strand.

Question 180: In the context of DNA replication, which of the following is NOT a function of the enzyme DNA polymerase?

A. Elongating DNA by adding nucleotides
B. Proofreading newly synthesized DNA
C. Repairing DNA damage during replication
D. Initiating synthesis of new DNA strands (Correct Answer)

Explanation: ***Initiating synthesis of new DNA strands*** - DNA polymerase **cannot initiate** the synthesis of a DNA strand from scratch; it requires a pre-existing **3'-hydroxyl group**, which is provided by an **RNA primer** synthesized by **primase**. - Its role is to **elongate** a pre-existing polynucleotide chain, not to start a new one. *Proofreading newly synthesized DNA* - DNA polymerase possesses **3' to 5' exonuclease activity**, allowing it to remove incorrectly paired nucleotides during replication. - This **proofreading function** significantly reduces the error rate during DNA synthesis. *Elongating DNA by adding nucleotides* - The primary function of DNA polymerase is to add **deoxyribonucleotides** to the 3' end of a growing DNA strand, synthesizing a new strand complementary to the template. - This process is essential for the accurate **duplication** of genetic material. *Repairing DNA damage during replication* - Beyond basic replication, DNA polymerase enzymes (e.g., **DNA Pol I** in prokaryotes, various polymerases in eukaryotes) are involved in **DNA repair mechanisms**, such as filling gaps created by excision repair. - They can replace damaged or incorrect nucleotides and synthesize the correct sequence during replication and repair processes.

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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