The analysis of the nitrogenous bases of the nucleic acid of a newly discovered virus showed 20% adenine, 40% guanine, 10% thymine, and 30% cytosine. The viral nucleic acid is most likely
double-stranded DNA.
double-stranded RNA.
single-stranded RNA.
single-stranded DNA.
The Correct Answer is C
A. double-stranded DNA: In double-stranded DNA, Chargaff’s rules apply: adenine pairs with thymine and guanine pairs with cytosine, so the percentages of A and T should be roughly equal, as should G and C. The viral nucleic acid shows 20% adenine and only 10% thymine, and 40% guanine with 30% cytosine, which violates these pairing rules, indicating it unlikely.
B. double-stranded RNA: Double-stranded RNA also follows base-pairing rules, with adenine pairing with uracil instead of thymine, and guanine pairing with cytosine. The unequal proportions of bases in this viral genome (e.g., A ≠ U, G ≠ C) suggest that it is not double-stranded RNA.
C. single-stranded RNA: Single-stranded RNA does not require complementary base pairing between strands, allowing unequal proportions of the four nucleotides. The observed percentages of adenine, guanine, cytosine, and thymine/uracil can vary widely, consistent with the data provided. This pattern supports the conclusion that the viral nucleic acid is single-stranded RNA.
D. single-stranded DNA: While single-stranded DNA does not require base pairing, thymine would still be present rather than uracil. However, the high guanine content (40%) and low thymine (10%) are unusual; still, the presence of thymine rather than uracil suggests that if it were RNA, thymine would be replaced with uracil, making single-stranded RNA more likely.
Nursing Test Bank
Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is A
Explanation
A. Pilus: During bacterial conjugation, a specialized structure called a sex pilus (or conjugation pilus) forms a direct connection between two bacterial cells. The donor cell transfers a copy of plasmid DNA or other genetic material through this pilus to the recipient cell, allowing for horizontal gene transfer and the spread of traits such as antibiotic resistance.
B. Capsule: The bacterial capsule is a polysaccharide layer that surrounds the cell wall and provides protection against phagocytosis and desiccation. It does not play a role in the transfer of DNA between bacterial cells, and therefore is not involved in conjugation.
C. Flagellum: The flagellum is a whip-like appendage that enables bacterial motility. While important for movement and chemotaxis, it is not used for DNA transfer during conjugation. DNA exchange occurs specifically through the pilus, not through flagella.
Correct Answer is D
Explanation
A. blocks penetration: Blocking penetration prevents viruses from entering host cells. Nucleotide analogs do not interfere with viral entry; instead, they act after the virus has already penetrated the host cell.
B. inhibits peptidoglycan cross-linking: Peptidoglycan is a component of bacterial cell walls, not viruses. Antibiotics like penicillin target this process, but nucleotide analogs target viral nucleic acid synthesis, not bacterial cell walls.
C. bonds to ergosterol in the cell membrane: Binding to ergosterol is the mechanism of action for antifungal drugs such as amphotericin B. Nucleotide analogs do not interact with membranes or sterols.
D. blocks DNA replication: Nucleotide analogs resemble natural nucleotides and are incorporated into viral DNA or RNA during replication. Once incorporated, they can terminate chain elongation or cause mutations, effectively inhibiting viral DNA replication and viral proliferation.
E. blocks maturation: Blocking maturation interferes with viral assembly or processing of viral proteins, a mechanism seen with protease inhibitors. Nucleotide analogs act earlier in the replication cycle by directly targeting nucleic acid synthesis rather than late-stage viral maturation.
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