The virus that replicates its genome by directing a cellular DNA-directed DNA polymerase to make RNA from an RNA template is

Question

BlackTom AI · Accepted Answer

DNA-dependent DNA polymerases copy DNA into DNA; they cannot synthesize RNA from an RNA template.

1) Relevant knowledge points and concepts
- Polymerase specificity: enzymes are named for template and product. DNA-dependent DNA polymerase (DDDP) uses a DNA template and adds deoxyribonucleotides (dNTPs) to synthesize DNA. DNA-dependent RNA polymerase (DDRP) uses DNA template to synthesize RNA (mRNA) using NTPs. RNA-dependent RNA polymerase (RdRp) uses an RNA template to synthesize RNA. Reverse transcriptase (RNA-dependent DNA polymerase) uses RNA template to make DNA.  
- Viral replication strategies: RNA viruses encode RdRp to copy RNA. Retroviruses use reverse transcriptase to make DNA from RNA, then host machinery transcribes that DNA. No known virus co-opts a cellular DDDP to produce RNA from RNA.

2) Step-by-step reasoning
- The proposed mechanism requires a cellular DNA-directed DNA polymerase to (a) accept an RNA template and (b) polymerize ribonucleotides (NTPs) to make RNA.  
- By definition, DDDP recognizes DNA templates and incorporates deoxyribonucleotides; its active site geometry and substrate-binding pockets exclude ribonucleotides and do not accommodate an RNA template reliably.  
- Therefore a DDDP cannot both read RNA and synthesize RNA.

3) Intermediate "steps" (molecular reasoning)
- Template recognition: DDDP requires deoxyribose backbone geometry; RNA has 2'‑OH altering conformation.  
- Substrate specificity: DDDP uses dNTPs; RNA synthesis requires NTPs.  
- Result: biochemical incompatibility prevents the described reaction.

4) Why the correct answer is correct
- Because cellular DNA-directed DNA polymerases are biochemically incapable of making RNA from an RNA template, no virus uses this mechanism. Known viral mechanisms use RdRp, reverse transcriptase, or host DDRP (DNA→RNA), not DDDP acting on RNA.

5) Why other plausible options are incorrect (brief)
- RNA viruses: use viral RdRp (not cellular DDDP).  
- Retroviruses: use reverse transcriptase to make DNA from RNA (not DDDP making RNA from RNA).  
- DNA viruses: may use host DDRP to transcribe DNA→RNA (template is DNA, not RNA).

6) Conclusion
- The mechanism described is impossible; the correct answer is: "this would be impossible, no virus does this," which matches the provided answer.

The virus that replicates its genome by directing a cellular DNA-directed DNA polymerase to make RNA from an RNA template is单项选择题

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