Nucleic Acid Biochemistry — MCQs
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Which of the following statements is true regarding the sigma factor?
C4, C5, and N7 in the purine ring are derived from which of the following?
What is the end product of purine metabolism in most mammals?
The gaps between Okazaki fragments on the lagging strand during DNA replication are rejoined and sealed by:
What are Okazaki fragments?
What is the first purine nucleotide synthesized in de novo purine biosynthesis?
In eukaryotic cells, where does the majority of functional RNA activity occur?
Which of the following is NOT a characteristic of the genetic code?
Which of the following molecular interactions are found in the structure of DNA?
Which of the following organs does not primarily utilize the salvage pathway of purine nucleotide synthesis?
Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs
Question 201: Which of the following statements is true regarding the sigma factor?
- A. It is a subunit of DNA polymerase.
- B. It is a subunit of RNA polymerase. (Correct Answer)
- C. It initiates DNA replication.
- D. It is a subunit of the 50s ribosome.
Explanation: ***It is a subunit of RNA polymerase.*** - The **sigma factor** is a crucial component of **bacterial RNA polymerase**, guiding it to specific promoter regions on the DNA. - It plays a vital role in **initiation of transcription** by recognizing and binding to the **-10 and -35 boxes** of the promoter. *It is a subunit of DNA polymerase.* - **DNA polymerase** is primarily involved in **DNA replication and repair**, not transcription. - Its subunits, such as the **beta clamp** or **alpha subunit**, are distinct from the sigma factor. *It initiates DNA replication.* - **DNA replication** is initiated by **DNA helicases** unwinding the double helix and **primase** synthesizing RNA primers. - The sigma factor's role is in **transcription**, the synthesis of RNA from a DNA template. *It is a subunit of the 50s ribosome.* - The **50S ribosomal subunit** is a component of the **ribosome**, responsible for **peptide bond formation** during translation. - Its subunits are ribosomal proteins and ribosomal RNA molecules, not the sigma factor.
Question 202: C4, C5, and N7 in the purine ring are derived from which of the following?
- A. CO₂
- B. Aspartate
- C. Glutamine
- D. Glycine (Correct Answer)
Explanation: ***Glycine*** - The entire **glycine molecule** contributes C4, C5, and N7 to the purine ring structure. - This amino acid provides a significant portion of the backbone to the imidazole ring within the purine. *Aspartate* - **Aspartate** contributes N1 to the purine ring. - It does not involve C4, C5, or N7, which are distinct atoms within the purine molecule. *CO₂* - **CO₂** contributes C6 to the purine ring through a carboxylation step. - It is not involved in providing the atoms at positions C4, C5, or N7. *Glutamine* - The nitrogen atoms N3 and N9 in the purine ring are derived from the **amide nitrogen of glutamine**. - Glutamine's contributions are different from the carbons and nitrogen provided by glycine.
Question 203: What is the end product of purine metabolism in most mammals?
- A. Glycogen
- B. Pyrimidine
- C. Histidine
- D. Allantoin (Correct Answer)
Explanation: ***Allantoin*** - **Allantoin** is the primary end product of **purine metabolism** in **most mammals** (except humans and higher primates), formed by the oxidation of uric acid by the enzyme **uricase**. - This conversion makes purine waste products more **water-soluble** and easier to excrete via the kidneys. - **Important clinical note:** Humans lack functional uricase, so **uric acid** is the end product in humans; this distinction is why hyperuricemia and gout occur in humans but not in most other mammals. *Glycogen* - **Glycogen** is a complex carbohydrate and serves as a primary **energy storage molecule** in animals, derived from glucose metabolism, not purine catabolism. - Its metabolism is regulated by hormones like **insulin** and **glucagon**, involved in maintaining blood glucose levels. *Pyrimidine* - **Pyrimidine** is a type of nitrogenous base, structurally distinct from purines, and is a component of DNA and RNA, not an end product of purine catabolism. - **Pyrimidine metabolism** involves the synthesis and breakdown of bases like cytosine, thymine, and uracil, which follows a separate biochemical pathway. *Histidine* - **Histidine** is an **essential amino acid**, a building block of proteins, and is involved in various metabolic processes, including histamine synthesis. - It plays no role as an end product of purine degradation; rather, its own metabolism leads to products like **urocanic acid**.
Question 204: The gaps between Okazaki fragments on the lagging strand during DNA replication are rejoined and sealed by:
- A. DNA Ligase (Correct Answer)
- B. DNA Helicase
- C. DNA Phosphorylase
- D. DNA Topoisomerase
Explanation: ***DNA Ligase*** - **DNA ligase** forms a **phosphodiester bond** between the **3'-OH group** of one Okazaki fragment and the **5'-phosphate group** of the adjacent fragment, effectively sealing the nicks. - After **DNA polymerase I** removes the **RNA primers** and fills in the gaps, DNA ligase completes the synthesis of the **lagging strand** during DNA replication. - This enzyme is essential for maintaining the **integrity of the DNA backbone**. *DNA Helicase* - **DNA helicase** functions to **unwind the DNA double helix**, separating the two strands to create a replication fork. - It does not participate in joining DNA fragments. *DNA Phosphorylase* - **DNA phosphorylase** is not a standard enzyme involved in the direct sealing of DNA fragments during replication. - This is not the enzyme responsible for ligating Okazaki fragments. *DNA Topoisomerase* - **DNA topoisomerase** relieves the **supercoiling tension** that builds up in the DNA double helix ahead of the replication fork due to unwinding. - It does not have a role in forming phosphodiester bonds between newly synthesized DNA fragments.
Question 205: What are Okazaki fragments?
- A. Long pieces of DNA on the lagging strand.
- B. Short pieces of DNA on the lagging strand. (Correct Answer)
- C. Short pieces of DNA on the leading strand.
- D. Long pieces of DNA on the leading strand.
Explanation: ***Short pieces of DNA on the lagging strand.*** - **Okazaki fragments** are the short, newly synthesized DNA fragments that are formed on the **lagging strand** during DNA replication. - The lagging strand is synthesized discontinuously because DNA polymerase can only add nucleotides in the **5' to 3' direction**, requiring it to move away from the replication fork as the DNA unwinds. *Long pieces of DNA on the lagging strand.* - The lagging strand is synthesized discontinuously in **short fragments**, not long continuous pieces. - The enzyme **DNA ligase** eventually joins these short fragments together to form a continuous strand. *Short pieces of DNA on the leading strand.* - The **leading strand** is synthesized continuously in one long stretch, moving towards the replication fork. - It does not require the synthesis of short fragments like the lagging strand. *Long pieces of DNA on the leading strand.* - While the leading strand is synthesized in a continuous, long piece, this statement does not accurately describe Okazaki fragments, which are specific to the lagging strand. - The leading strand's continuous synthesis is due to its **3' to 5' template orientation**, allowing DNA polymerase to proceed uninterrupted.
Question 206: 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.
Question 207: In eukaryotic cells, where does the majority of functional RNA activity occur?
- A. Nucleus
- B. Ribosome
- C. Cytoplasm (Correct Answer)
- D. None of the options
Explanation: ***Cytoplasm*** - The **cytoplasm** is the cellular compartment where the **majority of functional RNA activity** occurs, including **translation** (protein synthesis) involving mRNA, tRNA, and rRNA. - **Ribosomes** (the sites of translation) are located in the cytoplasm, either free-floating or bound to the endoplasmic reticulum. - Many types of **regulatory RNAs** such as microRNAs (miRNAs) and small interfering RNAs (siRNAs) exert their functions in the cytoplasm by targeting mRNAs for degradation or translational repression. - **mRNA degradation** and **RNA interference pathways** primarily operate in the cytoplasm. - The question asks for the broader location rather than the specific molecular machinery, making cytoplasm the most comprehensive answer. *Nucleus* - While RNA is **transcribed** from DNA and **processed** (capping, polyadenylation, splicing) in the nucleus, these are preparatory steps. - The nucleus is primarily the site of **RNA synthesis**, not where most RNA performs its functional roles. - Only a small fraction of functional RNA activity (like rRNA processing in the nucleolus) occurs here compared to the cytoplasm. *Ribosome* - While **ribosomes are the specific sites of translation** and are composed of rRNA and proteins, they represent molecular machinery rather than a cellular location. - Ribosomes themselves are located **within the cytoplasm**, making cytoplasm the more inclusive answer for where RNA activity occurs. - The question asks "where" in terms of cellular compartment, not which molecular complex. *None of the options* - This is incorrect as the cytoplasm is indeed the primary site where the majority of functional RNA activities occur in eukaryotic cells.
Question 208: Which of the following is NOT a characteristic of the genetic code?
- A. Overlapping (Correct Answer)
- B. Universal
- C. Degeneracy
- D. Nonambiguous
Explanation: ***Overlapping*** - The genetic code is generally **non-overlapping**, meaning each nucleotide is part of only one codon, and codons are read sequentially. - An overlapping code would mean that a single nucleotide could be part of multiple codons, which is not how protein synthesis typically occurs. *Nonambiguous* - This statement IS a characteristic; each codon specifies **only one amino acid**, meaning there is no ambiguity about which amino acid will be added. - While multiple codons can specify the same amino acid, a single codon never specifies more than one different amino acid. *Universal* - This statement IS a characteristic; the genetic code is largely **universal** across almost all organisms, from bacteria to humans. - The same codons typically specify the same amino acids in different species, which supports the idea of common ancestry. *Degeneracy* - This statement IS a characteristic; the genetic code is **degenerate**, meaning that most amino acids are specified by more than one codon. - This redundancy helps protect against the effects of single-nucleotide mutations.
Question 209: Which of the following molecular interactions are found in the structure of DNA?
- A. Hydrogen bond
- B. Glycosidic bond
- C. Covalent interactions
- D. All of the options (Correct Answer)
Explanation: ***All of the options*** - All three types of molecular interactions listed are present in DNA structure, making this the correct answer. - **Hydrogen bonds** hold together the two strands of the DNA double helix, forming between complementary base pairs (A-T with 2 hydrogen bonds, G-C with 3 hydrogen bonds). - **Glycosidic bonds** (N-glycosidic bonds) link the nitrogenous bases to the C1' carbon of the deoxyribose sugar in each nucleotide. - **Covalent interactions** (phosphodiester bonds) form the strong, stable sugar-phosphate backbone by linking the 3' hydroxyl group of one sugar to the 5' phosphate group of the next. *Hydrogen bond* - This is a **true statement** - hydrogen bonds are essential structural components of DNA. - However, this option alone is **incomplete** as DNA structure also contains glycosidic bonds and covalent phosphodiester bonds. - If only hydrogen bonds were present, there would be no nucleotides or backbone structure. *Glycosidic bond* - This is a **true statement** - glycosidic bonds are present in every nucleotide of DNA. - However, this option alone is **incomplete** as DNA also requires hydrogen bonds for base pairing and phosphodiester bonds for the backbone. - Without other bonds, individual nucleotides could not form a functional double helix. *Covalent interactions* - This is a **true statement** - covalent phosphodiester bonds form the DNA backbone within each strand. - However, this option alone is **incomplete** as it doesn't account for glycosidic bonds (nucleotide formation) or hydrogen bonds (strand pairing). - While the strongest bonds in DNA, they alone cannot create the complete double helix structure.
Question 210: Which of the following organs does not primarily utilize the salvage pathway of purine nucleotide synthesis?
- A. RBC
- B. Leukocytes
- C. Liver (Correct Answer)
- D. Brain
Explanation: ***Liver*** - The **liver** is capable of both *de novo* synthesis and the salvage pathway of purine nucleotides, but it primarily utilizes the **de novo pathway** due to its high metabolic capacity and central role in biosynthesis for the entire body. - While salvage pathways exist, the liver's robust *de novo* synthesis allows it to readily produce purines from simple precursors, making it less reliant on salvaging pre-formed bases. *Brain* - The **brain** relies heavily on the **salvage pathway** for purine nucleotide synthesis because it has a limited capacity for *de novo* purine synthesis. - This dependency makes the brain particularly vulnerable to deficiencies in salvage enzymes, such as in **Lesch-Nyhan syndrome** where HGPRT deficiency leads to severe neurological dysfunction. *RBC* - **Red blood cells (RBCs)** are anucleated and lack the machinery for *de novo* purine synthesis, making them entirely dependent on the **salvage pathway** to maintain their purine nucleotide pool. - They salvage pre-formed purine bases and nucleosides from the plasma to synthesize necessary adenine and guanine nucleotides. *Leukocytes* - **Leukocytes**, particularly lymphocytes, have a high turn-over rate and metabolic activity, and they primarily rely on the **salvage pathway** for purine nucleotide synthesis. - The **immune system's rapid proliferation** and response demand efficient nucleotide synthesis, and the salvage pathway offers a quick and energy-efficient way to achieve this.
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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