Unlike human cells and bacteria, viruses do not contain the components required to carry out the chemical reactions necessary to replicate and reproduce. A virus must therefore “invade” a host cell (bacteria, plant or animal) and appropriate its bio-synthetic machinery to make more viruses.
How does this happen? Viruses lay dormant in the environment all of the time. They can gain access to a host through the nose, mouth or breaks in the skin and then find host cells to infect. For example, cold and flu viruses commonly attack cells that line the respiratory or digestive tracts while HIV assaults the immune system.
How do Viruses Replicate?
The Lytic Cycle
All viruses, regardless of what type of host cell they have infected, follow the same sequence of steps from infection to replication:
1. The virus attaches to the host cell.
2. The virus injects its DNA (or RNA) into the host cell.
3. Once inside the host cell, the viral DNA (or RNA) commandeers the biosynthetic machinery of the host cell to replicate its viral parts and assemble them into new viral particles. In this way, about 200 new viruses are formed every 25 min after infection!
4. As the host cell fills with new virus particles, they are eventually released in one of three ways:
◎Lysis: they break open the host cell thereby destroying it.
◎Exocytosis: they exit through the host cell’s own export pathways.
◎Budding: they pinch out through the cell membrane, thereby preserving the host cell.