In regard to viruses with large genomes, which is correct?

Question

BlackTom AI · Accepted Answer

No virus codes for every enzyme needed to replicate, so they all must replicate inside cells.

Relevant knowledge points:
- Viruses are obligate intracellular parasites: they require a living host cell to carry out key steps of their life cycle.
- Viral replication steps include genome replication, transcription, translation, assembly and release.
- Cellular machinery required for protein synthesis (ribosomes, most tRNAs, many translation factors) is not fully encoded by any virus, even giant ones (e.g., Mimivirus, pandoravirus).
- Some large viruses encode many replication and transcription enzymes (viral polymerases, helicases, some tRNAs, aminoacyl-tRNA synthetases), but they never encode a complete translation system.

Step-by-step reasoning:
1) Identify required functions for producing progeny virions: (a) replicate nucleic acid, (b) transcribe mRNA, (c) translate proteins, (d) assemble virions. Steps (b) and (c) require RNA polymerases and ribosomes/translation factors.
2) Check viral genomes: large DNA viruses (poxviruses, mimiviruses) often carry their own DNA-dependent RNA polymerases and many replication enzymes, enabling cytoplasmic transcription or DNA replication. That satisfies some functions.
3) Crucially, no known virus encodes functional ribosomes or the full complement of translation machinery. Translation therefore must use host ribosomes and cytoplasmic factors.
4) Because translation (and other host-dependent processes such as energy metabolism, membrane synthesis, chaperones) cannot proceed without living host cell systems, viral replication must occur inside cells.

Why the correct answer is correct:
- It directly follows from the absence of a complete, virus-encoded translation apparatus. Even the largest viral genomes fall short of encoding ribosomal RNAs/proteins and all necessary translation cofactors, so intracellular replication is required.

Why other possible choices would be incorrect:
- Any choice claiming some viruses can fully replicate independently outside cells or that any virus encodes a complete protein synthesis system is incorrect, because no virus encodes ribosomes or all translation factors.
- Claims that large viruses are essentially free-living cells are false for the same reason.

Conclusion: The statement "No virus codes for every enzyme needed to replicate, so they all must replicate inside cells" is correct and matches the provided answer.

In regard to viruses with large genomes, which is correct?单项选择题

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