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Viral replication cycles

Viral replication can be divided into two categories based on the mechanism by which the viral genome is spread:

  • The lytic cycle destroys the host cell. This is the main form of viral replication. The cell lyses (breaks apart) when the virus is released.

    In the lytic cycle, the viral genome is spread through the production of many viral particles within the host.

  • The lysogenic cycle preserves the host cell. Viral DNA is inserted into the host genome, where it remains and replicates along with the host.

    The viral genome is spread through the reproduction of the host cell. This happens most often with bacteriophages, but some eukaryotic viruses, such as HIV can replicate through the lysogenic cycle.

The lytic cycle is the most common type of viral replication. It results in the lysis (splitting apart) of the host cell.

The T4 phage, which infects E. coli, is able to replicate via the lytic cycle but not through the lysogenic cycle.

The lifecycle occurs in the following steps:

  1. Entry: The bacteriophage attaches to the outside of the bacterial cell and injects its genetic material. Only the bacteriophage's genetic material enters the cell.
  2. Replication: Once inside the cell, the viral DNA replicates and the proteins that make up the capsid are synthesised. New bacteriophages assemble inside the cell.
  3. Exit: The new bacteriophages break out of the cell, destroying it in the process.

Many viruses only replicate through the lytic cycle. Other viruses can switch from the lytic cycle to the lysogenic cycle.

HIV can remain dormant in the lysogenic cycle for years.

Under certain conditions, it is able to switch to the lytic cycle and disperse; at this point, the person infected usually develops acquired immunodeficiency syndrome (AIDS).

The lysogenic cycle is the viral replication cycle that poreserves the host cell. Unlike the lytic cycle, not all viruses undergo the lysogenic cycle.

The lambda ($$\lambda $$) phage, which infects E. coli, can replicate via the lysogenic cycle. It has a DNA genome.

The $$\lambda $$ phage lysogenic cycle occurs in the following steps:

  1. Entry: The lambda phage injects its genetic material into the cell. The capsid remains on the outside of the host cell.
  2. Replication: The genome is inserted into the bacterial chromosome, forming part of the host genome.

Every time the host replicates and divides, the phage is also replicated and its genome is present in the daughter cells.