How Do Viruses Replicate?

Viruses replicate by entering a living host cell and using its machinery to make new virus particles. They cannot replicate on their own: a virus has genetic material and a protein coat, but it is not a cell and has no ribosomes to make proteins. The protein coat is called a capsid. It protects the viral genetic material, which may be DNA or RNA. Attachment proteins recognise particular host-cell surface molecules; this is why a virus does not infect every type of cell.

What happens inside the host cell?

1. Attachment: Proteins on the virus bind to specific molecules on the surface of a host cell. This helps determine which cells the virus can infect.
2. Entry: The viral genetic material enters the cell. How this happens depends on the virus; in some cases, the whole virus enters first.
3. Replication and protein synthesis: The host cell is used to copy the viral genetic material and make viral proteins. Its ribosomes make the proteins needed for new virus particles.
4. Assembly and release: Copies of the genetic material and viral proteins are assembled into new virus particles. These leave the cell, sometimes by causing it to burst and sometimes by budding from its membrane.

For example, HIV infects helper T cells. Its genetic material is RNA, which is converted into DNA by the viral enzyme reverse transcriptase. That DNA can become part of the host cell’s DNA, allowing the cell to produce components of new HIV particles. This is an example of one virus’s replication process, not a step shared by all viruses. As HIV damages helper T cells, the immune response becomes less effective; severe immune-system damage can lead to AIDS.

Exam mistake to avoid: Do not say that viruses reproduce by cell division. Unlike bacteria, viruses do not divide by binary fission. The host cell makes the components, which are then assembled into new virus particles.