The Life Cycle as a Sequence of Problems | AI Learning Guide

Read the list as a chain of problems rather than a set of labels. Attachment solves the problem of a particle that cannot enter on its own, and it is also where tropism is decided, because only cells displaying the right receptor can be bound. Entry solves the membrane barrier. Uncoating solves the problem that the genome is still locked inside a capsid. Replication and gene expression solve the need for more genomes and more proteins. Assembly solves the problem of scattered parts. Release solves the problem of progeny trapped in a cell. Now look at the note. It matters that receptor binding is necessary but not sufficient. A cell can have the receptor and still fail to support the virus if it lacks a protease or the right compartment. That is why tropism is a property of the whole sequence, not just the first step.

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The ordered stages and the problem each solves

Attachment — binding a specific receptor; this is where tropism is set.
Entry — crossing the membrane by fusion or penetration.
Uncoating — releasing the genome from the capsid.
Replication and gene expression — copying the genome and making viral proteins.
Assembly — packaging new genomes into new capsids.
Release — exiting by budding or by lysis.

Tropism is not a single gate

Receptor binding is necessary but not sufficient. A cell can display the right receptor and still fail to support replication if it lacks a required protease, a suitable nucleotide pool, or the right intracellular compartment. Tropism is the product of the whole sequence, not just the first step.