Cold and Flu
Viruses (biological)
Cell Biology (cytology)

What are the steps of the lytic cycle?


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The lytic cycle is one of the two cycles of viral reproduction, the other being the lysogenic cycle. These cycles should not, however, be seen as separate, but rather as somewhat interchangeable. The lytic cycle is typically considered the main method of viral replication, and it results in the immediate destruction of the infected cell.

A generalized scheme is presented here. There are variations in the process depending on the type of virus (its genome) and the type of host (bacteria or plant cells or animal cells).

The six steps of the lytic cycle operative in viral pathogenesis can be summarized as follows:

  1. The virus finds a host cell/ Contact (or initial infection):

    Viruses require a host to replicate. To infect a cell, the virion links on to a specific region (like a receptor or a glycoprotein) on the surface of the host cell. Viruses do so by either attaching to a receptor on the cell's surface or by simple mechanical force.

  2. The virus enters the cell/ Injection (or in some cases the virus' genes are injected into the cell while the virion remains outside the cell): Once a virus attaches, it enters the cell through the plasma membrane and (if present) the cell wall. The virus then releases its genetic material (either single- or double-stranded DNA or RNA) into the cell. In doing this, the cell is infected and can also be targeted by the immune system.

  3. The virus takes over the host cell/Integration: The viral genetic material integrates with the host genetic material and uses it to express viral genes instead of the usual cell function. In other words, the virus hijacks the gene expression machinery of the host. Technically, the virus' nucleic acid uses the host cell's machinery to make large amounts of viral components. In the case of DNA viruses, the DNA transcribes itself into messenger RNA (mRNA) molecules that are then used to direct the cell's ribosomes. One of the first polypeptides to be translated is one that destroys the host cell's DNA. In retroviruses (which inject an RNA strand), a unique enzyme called reverse transcriptase transcribes the viral RNA into DNA, which is then transcribed again into mRNA.

  4. Biosynthesis/ Viral gene expression: Viral genes are expressed and parts (building blocks) of new virions are formed. After many copies of viral components are made, they are ready to be assembled into complete virus particles.

  5. The genes from the virus turn the cell into a virus factory/ Packaging and maturation: Copies of the viral genetic material are packaged into the newly formed virions and the parts are assembled to form many new complete virus particles.

  6. The new viruses break out and find a new host to repeat the process/

    Lysis and infection: Once new complete virions are fully formed, the production of an enzyme that breaks down the cell wall and allows fluid to enter is begun. The cell eventually becomes filled with typically 100-200 virions and liquid. It then bursts open, which is called host cell lysis. This is how the lytic cycle got its name. Once the host cell is lysed, a huge number of new viruses are released into the inter cellular spaces of the host. The new viruses are then free to attach to and infect other cells in the same host, or to shed from the first host and infect others. This process repeats cell by cell and host by host.

Note about a "Lytic" cycle without lysis: Some viruses escape the host cell without bursting the cell membrane. Instead, they bud off from it by taking a portion of the membrane with them to package the new virion. Eventually the host cell's membrane can be totally used up in the budding process, so it is ultimately destroyed by this other mechanism, just as host cells that are destroyed by lysis. Hepatitis C viruses presumably use this method.