Virus replication — Which event is essential for a virus to replicate inside a host cell?

Introduction:
All viruses are obligate intracellular parasites. Despite diverse entry pathways, a universal requirement is delivery of the viral genome to a permissive intracellular site where replication and expression can proceed.

Given Data / Assumptions:

• Different viruses enter cells by endocytosis, membrane fusion, or direct injection.
• Some uncoat at the plasma membrane, others in endosomes, and some at nuclear pores.
• Mitotic state of the host is not universally required.

Concept / Approach:
Replication cannot begin until the viral nucleic acid is uncoated and becomes accessible to host or viral enzymes. Whether the capsid itself enters fully is not a constant; bacteriophages inject nucleic acid, leaving capsids outside. Therefore, the reliable, generalizable requirement is genome release inside the cell.

Step-by-Step Solution:

Verification / Alternative check:
Uncoating is a named step in all standard viral life cycle diagrams, preceding replication, transcription, and translation.

Why Other Options Are Wrong:

• Capsid must enter intact: not required for many viruses such as tailed phages.
• Host mitosis required: many viruses replicate in nondividing cells.
• Host must lack membrane: biologically impossible for animal cells; membranes mediate entry.

Common Pitfalls:
Equating endocytosis of a capsid with a universal rule; mechanisms vary widely across virus families.

Final Answer:
The viral genome must be released inside the cell to access replication machinery