Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 201: If the content of adenine (A) is 15%, what is the percentage of guanine (G) in the DNA?

A. 15%
B. 85%
C. 70%
D. 35% (Correct Answer)

Explanation: ***35%*** - According to **Chargaff's rules**, in a DNA molecule, the amount of **adenine (A) is equal to the amount of thymine (T)**, and the amount of **guanine (G) is equal to the amount of cytosine (C)**. - If A = 15%, then T must also be 15%. This means A + T = 30%. Since the total percentage of all bases is 100%, G + C must be 100% - 30% = 70%. As G = C, then G = 70% / 2 = 35%. *15%* - This would only be correct if guanine paired with adenine, which it does not; guanine pairs with **cytosine**. - This answer incorrectly assumes that all four bases are present in equal proportions, or that G equals A, which violates **Chargaff's rules**. *85%* - This percentage would imply an incorrect base pairing or an imbalanced ratio of purines and pyrimidines, violating the fundamental structure of DNA. - An 85% guanine content would mean that G + C far exceeds 100% or that T is extremely low, which is biologically impossible. *70%* - This represents the combined percentage of **guanine and cytosine**, not guanine alone. - While it correctly acknowledges the remaining proportion of bases, it fails to divide this sum between the two equal components, **G and C**.

Question 202: 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 203: 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 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: 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 206: 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 207: What does salvage purine synthesis refer to?

A. Synthesis of purine nucleotides from purine bases (Correct Answer)
B. Synthesis of purine nucleotides from ribose-5-phosphate
C. Synthesis of purine nucleotides from simple precursors (de novo synthesis)
D. Synthesis of purine nucleotides from degraded RNA

Explanation: ***Synthesis of purine nucleotides from purine bases*** - **Salvage pathways** recycle pre-existing purine or pyrimidine bases (from nucleic acid degradation) by re-attaching them to a **ribose phosphate** to form a new nucleotide. - This process is energy-efficient as it bypasses several steps of the de novo synthesis pathway, utilizing enzymes like **adenine phosphoribosyltransferase (APRT)** and **hypoxanthine-guanine phosphoribosyltransferase (HGPRT)**. *Synthesis of purine nucleotides from ribose-5-phosphate.* - While **ribose-5-phosphate** is a precursor, the complete synthesis from this molecule is part of the **de novo pathway**, which starts with PRPP (phosphoribosyl pyrophosphate) formation from ribose-5-phosphate. - This option does not specify the direct reuse of a pre-formed purine base, which is the hallmark of salvage. *Synthesis of purine nucleotides from simple precursors (de novo synthesis).* - **De novo synthesis** is the creation of nucleotides from scratch using simple metabolic precursors like amino acids (glycine, aspartate, glutamine), CO2, and THF derivatives. - This contrasts with salvage pathways, which recycle existing bases. *Synthesis of purine nucleotides from degraded RNA.* - Degraded RNA breaks down into **nucleotides**, which can then be further broken down into **purine bases** and ribose phosphates. - The direct synthesis of purine nucleotides from *degraded RNA* involves recovering the individual bases or nucleosides, then converting them to nucleotides via salvage, not directly using the entire degraded RNA.

Question 208: Which type of RNA is most commonly associated with pseudouridine?

A. messenger RNA (mRNA)
B. ribosomal RNA (rRNA)
C. transfer RNA (tRNA) (Correct Answer)
D. DNA

Explanation: ***Transfer RNA (tRNA)*** - **Pseudouridine (ψ)** is one of the most abundant modified nucleosides in RNA, and **tRNA contains the highest proportion** of pseudouridine modifications among all RNA types. - **tRNA molecules can contain up to 10-15% modified bases**, with pseudouridine being particularly abundant in the **TψC arm** (thymine-pseudouridine-cytosine loop). - These modifications are critical for **tRNA stability, proper folding, and accurate codon-anticodon recognition** during translation. - Pseudouridine enhances base stacking and stabilizes RNA structure through additional hydrogen bonding capability. *Ribosomal RNA (rRNA)* - While rRNA does contain pseudouridine modifications, they are present in **lower proportions compared to tRNA**. - rRNA pseudouridine modifications do play important roles in **ribosomal assembly and function**, but tRNA remains the RNA type most commonly associated with this modification. *Messenger RNA (mRNA)* - **mRNA is generally much less modified** than tRNA or rRNA. - Pseudouridine modifications in mRNA are relatively rare in prokaryotes and were only recently discovered to be more common in eukaryotic mRNA. - When present, they may affect **mRNA stability and translation efficiency**. *DNA* - **DNA does not contain pseudouridine** as this is an RNA-specific modification. - Pseudouridine is formed by **post-transcriptional isomerization** of uridine residues in RNA.

Question 209: Which type of DNA polymerase is responsible for the replication of mitochondrial DNA?

A. DNA polymerase delta
B. DNA polymerase alpha
C. DNA polymerase gamma (Correct Answer)
D. DNA polymerase beta

Explanation: ***DNA polymerase gamma*** - **DNA polymerase gamma** is the sole DNA polymerase responsible for replicating and repairing the mitochondrial DNA in eukaryotic cells. - It consists of a large catalytic subunit and two smaller accessory subunits that provide **proofreading** and processivity functions. *DNA polymerase alpha* - **DNA polymerase alpha** is primarily involved in initiating DNA replication on both the leading and lagging strands of nuclear DNA. - It forms a complex with **primase** to synthesize short RNA primers followed by a short stretch of DNA. *DNA polymerase delta* - **DNA polymerase delta** is a key enzyme in nuclear DNA replication, primarily responsible for the **elongation of the lagging strand**. - It also plays a significant role in various DNA repair pathways, including **nucleotide excision repair**. *DNA polymerase beta* - **DNA polymerase beta** is mainly involved in **DNA repair processes**, specifically **base excision repair (BER)** in the nucleus. - It has a low processivity and lacks **proofreading activity**, making it unsuitable for bulk DNA replication.

Question 210: 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.

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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