Lytic Cycle Definition
The lytic cycle is named for the process of lysis, which occurs when a virus has infected a cell, replicated new virus particles, and bursts through the cell membrane. This releases the new virions, or virus complexes, so they can infect more cells.
As seen in the graphic above, the lytic cycle is often accompanied by the lysogenic cycle in many bacteria viruses, known as bacteriophages. After the virus injects its DNA or RNA into the host bacteria, the genetic material can enter either the lytic cycle or the lysogenic cycle.
In the lysogenic cycle, the bacteriophage DNA lies practically dormant. However, whenever the bacteria divides, the DNA of the virus is inadvertently copied. In this way, the virus can continue replicating within its host. As long as the bacteria are successful, the virus may remain dormant. At a certain point, conditions may change, and the virus will enter the lytic cycle.
In this cycle, the viral DNA or RNA is expressed by the host organism’s cellular mechanisms. In other words, the viral genes use the proteins within the cell to replicate themselves and produce viral proteins. These proteins and copies of the DNA will become new virions. The cell, helpless to its viral hijacker, simply waits until the pressure of these new virions is too high. Then, the cell membrane breaks. This lysis of the cell releases the virions created in the lytic cycle. Their final destination is a new cell, in which the lytic cycle can take place again. If conditions are favorable and the cell is dividing, the virus may stay in the lysogenic cycle for a time. Ultimately, to infect a greater number of cells, more virus genomes will enter the lytic cycle and produce thousands or millions of copies of themselves in a shorter amount of time.
Steps of the Lytic Cycle
Adsorption and Penetration
Adsorption is the process through which a bacteria gets its DNA or RNA into the host cell. This is labeled as 1 in the image above. The capsid, or protein coat around the viral genome, consists of very specific proteins. This sheild of proteins not only comes together to protect the viral genes, it serves as a sort of “key” to unlock a cell. The surface of the proteins are shaped to interact with proteins on the surface of the host cell.
When the “lock and key” align, the virion is bound to the cell membrane. When this happens, it also changes the shape of the capsid. This tears a hole or injects the viral DNA into the host cell. Here, it may travel into the nucleus or replicate in the cytoplasm. This depends on the virus itself, what type of genome it has, and the conditions of the cell.
During the lytic cycle, the replication of viral genes is carried out a number of times by a hijacked cellular system. Remember that the virus itself has imported few, if any, supporting proteins. Thus, the viral DNA must produce these in order to hijack the cell’s processes. The first proteins created are often created as the cell reads its own DNA and produces proteins. The viral genes simply sneak into the process. This creates what are called viral early proteins.
These early proteins have important functions (to the virus) of commandeering the cell’s machinery. They clear the cell’s normal metabolic agenda, and turn many of its activities toward the replication of viral genes and the production of viral proteins. The virus uses the raw products the cell has assembled (amino acids and nucleic acids) as building blocks for the parts it needs.
While this may seem like an overly complex process for such a small virus genome, consider first that there are really only a handful of proteins. Most viruses produce and code for only a handful of proteins. Unlike cells, a virus doesn’t need the complex proteins required to metabolize energy. As obligate parasites, a virus is dependent upon its host cell’s ability to provide raw materials. This makes it one of the most efficient forms of DNA replication that we know of.
Assembly and Release
As these parts are built, their natural evolutionary shapes help them come together in the proper way. Since most of the components are proteins, they have formed over evolutionary time to be able to come together with very little outside influence. The assembly of new virions is a hallmark of the lytic cycle. The other viral life cycle does not include producing and assembling new virions.
In this way, the lytic cycle resembles a small virus factory. All of the parts of the virus are produced independently, then assembled, and finally released into the environment. While the image above shows only 3 assembled virions at stage 6, in reality there would be millions. Compare the lytic cycle to the lysogenic cycle below it, in which an accurate 2 copies are shown after 1 bacterial division.
1. Which of the following represents the lytic cycle?
A. Viral DNA is replicated as the host cell divides.
B. The viral genome takes over the host cell, and creates a virus factory.
C. The viral genome is mostly dormant.
2. Which life cycle, the lytic cycle or the lysogenic cycle, produces the most virions?
A. It depends
B. The lytic cycle
C. The lysogenic cycle
3. Based on what you now know about the lytic cycle, why is it so hard to eradicate the common cold?
A. It shouldn’t be!
B. The virus changes too much.
C. In targeting the mechanisms viruses use, you target the mechanisms every cell uses.
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