Question at position 5 RNA viruses require their own supply of certain enzymes because ________.these enzymes translate viral mRNA into proteinshost cells do not have enzymes that can make copies of the viral genomethese enzymes cannot be made in host cellsthese enzymes penetrate host-cell membraneshost cells rapidly destroy the virusesClear my selection

Question

Question at position 5 RNA viruses require their own supply of certain enzymes because ________.these enzymes translate viral mRNA into proteinshost cells do not have enzymes that can make copies of the viral genomethese enzymes cannot be made in host cellsthese enzymes penetrate host-cell membraneshost cells rapidly destroy the virusesClear my selection

BlackTom AI · Accepted Answer

RNA viruses often carry an RNA-dependent RNA polymerase (RdRp) because host cells lack enzymes capable of copying RNA genomes.

1. Relevant knowledge points and concepts
- Cellular organisms use DNA-dependent DNA polymerases and DNA-dependent RNA polymerases (to transcribe DNA into RNA). They do not possess RdRp activity for copying RNA genomes. 
- Viral replication requires synthesis of new genome copies and, for negative-sense RNA viruses, synthesis of mRNA from the viral RNA. RNA viruses therefore need RdRp or reverse transcriptase (for retroviruses) to produce templates for translation and replication. Some viruses package these enzymes in the virion; others encode them and synthesize them after infection.

2. Step-by-step reasoning
- The viral genome must be replicated to produce progeny viruses. 
- If the genome is RNA, replication requires an enzyme that uses RNA as a template (RdRp). 
- Host cells do not have RdRp activity, so the virus must provide or synthesize an RdRp. 
- Negative-sense RNA viruses cannot use their genome directly as mRNA, so they must supply a polymerase to make positive-sense mRNA immediately upon infection. Positive-sense RNA viruses can be translated directly, but replication of their RNA still needs RdRp.

3. Calculations
- None required.

4. Why the correct answer is correct
- The statement "host cells do not have enzymes that can make copies of the viral genome" accurately identifies the fundamental reason RNA viruses require their own polymerases: hosts lack RNA-dependent RNA polymerases necessary for copying RNA genomes.

5. Why the other options are incorrect
- "These enzymes translate viral mRNA into proteins": Translation is performed by host ribosomes and translation factors, not viral enzymes. 
- "These enzymes cannot be made in host cells": Host translational machinery can produce viral proteins (including polymerases) after infection; the issue is absence of the enzyme class before/for immediate replication, not absolute inability to produce them. 
- "These enzymes penetrate host-cell membranes": Irrelevant and incorrect description. 
- "Host cells rapidly destroy the viruses": While host defenses exist, this is not the reason RNA viruses need their own replication enzymes.

6. Conclusion
The correct answer is: host cells do not have enzymes that can make copies of the viral genome, which matches the provided answer.

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