Life Cycle and Replication of Viruses (A-level Biology)

Life Cycle and Replication of Viruses

Viral Life Cycle and Replication

Viruses do not undergo cell division. Instead, they infect a host cell, and use the host cell to replicate themselves and to produce new viral particles. Thus, the virus’s life cycle depends on its ability to successfully infect a host cell so that it may reproduce.

1. Virus binds to host cells. Virus binds to a host cell using its attachment proteins. Viruses can have different attachment proteins, which means that different viruses infect different types of cells. The HIV virus, for example, has attachment proteins which bind to receptors on the surface of human T-cells.
2. Injection of nucleic acids. When viruses infect infect a host cell, they inject their nucleic acids into the host cell. Remember that viruses can have either DNA based or RNA based genomes. Once the attachment proteins of a virus attach to a host cell, the virus latches on to the cell and injects the host cell with its nucleic acids (either DNA or RNA). This part of the viral life cycle is called the lysogenic cycle.
3. Injection of unique viral proteins. Certain viruses can also inject unique viral proteins into the host cell. These viral proteins help hijack the host cells machinery in order to replicate the viral genomes and to make new viral proteins. In this process, viruses can force the cell to give up using energy to make proteins and replicate DNA for the cell, and instead focus all its energy into replicating the viral genome and making viral proteins. Obviously, this is not good for the cells.
4. Release from host cell. Once the host cell has produced a sizeable number of viral particles, the viral particles will burst through the cell through a process called lytic release. Once they burst through the cell, the viruses will go on to infect other host cells.
5. The host cell dies. Meanwhile, the original host cell is now dead because of a gaping hole in its cell membrane. This part of the reproduction cycle is called the lytic cycle.

Latency

• Viruses can become latent during the lysogenic part of the viral reproduction cycle. A virus can become dormant (i.e. latent) within its host cell after the initial infection. This means that the virus has stopped reproducing viral particles.
• The virus can become reactivated. At some point in the future, the virus may become reactivated. This may be due to various factors e.g. stress. The virus will resume producing viral particles, and will continue onto the lytic cycle. This means that the host may become ill without having been reinfected by a new virus from the external environment.