Nucleic Acid Biochemistry — MCQs
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The term "modified base" refers to:
In E. coli, Arthur Kornberg found which enzyme?
Which of the following correctly describes the subunits of eukaryotic ribosomes?
Which of the following pairs are purines that utilize the salvage pathway?
Which discovery in gene regulation was awarded the Nobel Prize in Physiology or Medicine in 2006?
Which type of cellular component is most susceptible to damage from radiation?
Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs
Question 271: The term "modified base" refers to:
- A. A purine or pyrimidine that has been altered (Correct Answer)
- B. A purine or pyrimidine attached to a sugar by O-glycoside linkage
- C. A standard nucleotide consisting of a purine or pyrimidine attached to deoxyribose
- D. A nitrogen-containing base that is not a standard purine or pyrimidine
Explanation: ***A purine or pyrimidine that has been altered*** - A **modified base** refers to a purine (adenine, guanine) or pyrimidine (cytosine, thymine, uracil) that has undergone **post-transcriptional or post-replicative chemical alteration**. - These modifications can include **methylation**, deamination, or other structural changes, leading to changes in their properties and functions within nucleic acids. *A standard nucleotide consisting of a purine or pyrimidine attached to deoxyribose* - This describes a **standard deoxyribonucleotide** (the building block of DNA), which is composed of a nitrogenous base (purine or pyrimidine), a deoxyribose sugar, and one to three phosphate groups. - A **modified base** is distinct from a standard nucleotide because it involves an *alteration* to the base itself, not just its assembly into a nucleotide. *A purine or pyrimidine attached to a sugar by O-glycoside linkage* - In nucleic acids, **purine and pyrimidine bases** are attached to the C1' carbon of the sugar (deoxyribose or ribose) via an **N-glycosidic bond**, not an O-glycosidic linkage. - An **O-glycosidic linkage** is typically found in carbohydrates, connecting sugars to other molecules, but is not characteristic of the bond between a base and a sugar in nucleotides. *A nitrogen-containing base that is not a standard purine or pyrimidine* - While modified bases are indeed **nitrogen-containing bases** that often are not the *standard* A, T, C, G, or U, this definition is incomplete. - The key aspect of a **modified base** is that it *originated* from a standard purine or pyrimidine and was subsequently *altered*, differentiating it from completely novel nitrogenous bases.
Question 272: In E. coli, Arthur Kornberg found which enzyme?
- A. DNA polymerase (Correct Answer)
- B. Glucose 6 phosphate dehydrogenase
- C. Fatty acid synthase
- D. Topoisomerase
Explanation: ***DNA polymerase*** - Arthur Kornberg was awarded the Nobel Prize in Physiology or Medicine in 1959 for his discovery of **DNA polymerase I** in *Escherichia coli*. - This enzyme is crucial for **DNA replication and repair** in bacteria, catalyzing the synthesis of new DNA strands. *Fatty acid synthase* - This enzyme complex is responsible for the **biosynthesis of fatty acids** in living organisms. - While essential for *E. coli*, its discovery is not attributed to Arthur Kornberg. *Glucose 6 phosphate dehydrogenase* - This enzyme is key in the **pentose phosphate pathway**, producing NADPH and ribose-5-phosphate. - It is critical for cellular metabolism but was not the enzyme discovered by Kornberg. *Topoisomerase* - Topoisomerases are enzymes that regulate the **supercoiling of DNA** by transiently breaking and rejoining DNA strands. - Their discovery postdates Kornberg's work on DNA polymerase.
Question 273: Which of the following correctly describes the subunits of eukaryotic ribosomes?
- A. 80S & 30S
- B. 50S & 30S
- C. 60S & 40S (Correct Answer)
- D. 50S & 40S
Explanation: ***60S & 40S*** - Eukaryotic ribosomes are 80S, which are composed of two subunits: a **large 60S subunit** and a **small 40S subunit**. - The "S" refers to **Svedberg units**, which are a measure of sedimentation rate and are not directly additive. *50S & 30S* - These subunits (50S and 30S) combine to form a **70S ribosome**, which is characteristic of **prokaryotic cells** (e.g., bacteria). - This combination is not found in eukaryotic ribosomes. *80S & 30S* - This combination is incorrect; while 80S is the size of a **complete eukaryotic ribosome**, the small subunit is **40S**, not 30S. - 30S is the small subunit of **prokaryotic ribosomes**. *50S & 40S* - This combination is incorrect; the large subunit of a eukaryotic ribosome is **60S**, not 50S. - 50S is the large subunit of **prokaryotic ribosomes**.
Question 274: Which of the following pairs are purines that utilize the salvage pathway?
- A. Hypoxanthine & Thymine
- B. Adenine & Guanine (Correct Answer)
- C. Xanthine & Guanine
- D. Hypoxanthine & Xanthine
Explanation: ***Correct: Adenine & Guanine*** - **Adenine** and **guanine** are the two major **purine bases** that are actively salvaged through dedicated salvage pathways to form their respective nucleotides - **Adenine** is salvaged by **APRT (adenine phosphoribosyltransferase)** to form AMP - **Guanine** is salvaged by **HGPRT (hypoxanthine-guanine phosphoribosyltransferase)** to form GMP - The salvage pathway conserves energy by recycling free bases rather than synthesizing nucleotides de novo - This is the **most complete and accurate pair** for the question *Incorrect: Hypoxanthine & Thymine* - **Thymine** is a **pyrimidine**, not a purine, making this pair incorrect - While hypoxanthine is a salvageable purine (via HGPRT to form IMP), the pair fails the "both purines" requirement *Incorrect: Xanthine & Guanine* - While **guanine** is correctly salvaged by HGPRT, **xanthine** is primarily a **degradation product** rather than a salvage substrate - Xanthine is an intermediate in purine catabolism that is oxidized to **uric acid** by xanthine oxidase - Xanthine is **not a significant substrate for salvage pathways** to form functional nucleotides *Incorrect: Hypoxanthine & Xanthine* - **Hypoxanthine** is indeed salvaged by HGPRT to form IMP and is an important salvage substrate - However, **xanthine** is primarily a **catabolic intermediate** converted to uric acid, not a major salvage pathway substrate - While both are purines, xanthine does not significantly participate in salvage to form nucleotides, making this pair incomplete for the question's requirement
Question 275: Which discovery in gene regulation was awarded the Nobel Prize in Physiology or Medicine in 2006?
- A. Discovery of RNA interference (RNAi) (Correct Answer)
- B. Role of mitochondrial DNA in cellular functions
- C. Involvement of lipoxins in inflammation
- D. Function of transcription factors in gene regulation
Explanation: ***Discovery of RNA interference (RNAi)*** - The 2006 Nobel Prize in Physiology or Medicine was awarded to **Andrew Z. Fire** and **Craig C. Mello** for their discovery of **RNA interference (RNAi)** in the nematode *C. elegans*. - This seminal work revealed a fundamental mechanism for **controlling gene flow** in cells, involving double-stranded RNA molecules that silence gene expression. *Role of mitochondrial DNA in cellular functions* - While the role of **mitochondrial DNA** is crucial for cellular energy production and has been extensively studied, it was not the subject of the 2006 Nobel Prize in Physiology or Medicine. - The discovery and understanding of mitochondrial DNA's function emerged over several decades, with significant contributions from various researchers, predating 2006 for its fundamental elucidation. *Involvement of lipoxins in inflammation* - **Lipoxins** are important lipid mediators involved in the resolution of **inflammation**, and their discovery and characterization are significant. - However, the Nobel Prize in 2006 specifically recognized a discovery in **gene regulation**, not lipid mediators of inflammation. *Function of transcription factors in gene regulation* - **Transcription factors** play a critical role in regulating gene expression by binding to specific DNA sequences. - While vital to understanding gene regulation, the discovery and characterization of transcription factors was not the particular focus of the 2006 Nobel Prize, which centered on RNAi.
Question 276: Which type of cellular component is most susceptible to damage from radiation?
- A. Nucleic acids (Correct Answer)
- B. Carbohydrates
- C. Lipids
- D. Proteins
Explanation: ***Nucleic acids*** - **DNA** is the most critical target for radiation damage due to its central role in cell function, repair, and replication. Mutations or breaks in DNA can lead to cell death or uncontrolled growth. - Ionizing radiation can cause **single-strand and double-strand breaks** in DNA, leading to chromosomal aberrations and ultimately affecting cell viability and division. *Proteins* - While radiation can cause damage to proteins, such as **denaturation** or alteration of their structure, the cell has numerous repair mechanisms and redundant proteins, making this damage less lethal compared to DNA damage. - Protein damage is often a secondary effect of radiation, resulting from free radicals generated by water radiolysis, rather than a primary direct hit. *Lipids* - **Lipids**, particularly those in cell membranes, can undergo **lipid peroxidation** when exposed to radiation, affecting membrane integrity and function. - However, cells have antioxidant defense systems and membrane repair mechanisms that can mitigate lipid damage, making it less immediately critical for cell survival than DNA damage. *Carbohydrates* - Carbohydrates, such as **glycoproteins** and **glycolipids**, can be damaged by radiation, affecting cell surface recognition and signaling. - This damage is usually less significant in terms of immediate cellular lethality compared to DNA damage, as carbohydrate structures can often be repaired or replaced.
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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