Answer:
Step-by-step explanation:
a. Attachment. The first step in the cycle involves the virus actually coming into contact with the correct type of host cell and attaching itself to the cell wall or cell membrane of that host cell. This is accomplished by the matching up of specialized structures on both the virion and the host cell. If the receptor site on the host cell does not match perfectly with the viral attachment protein of the virion, the virion will not attach to the cell and infection will not take place. The receptor site may be any number of structures on the outside of a potential host cell, which structure is very specific to that type of a cell. The viral attachment protein is part of the protein capsid of a virion. This mechanism is what keeps the common cold virus, which will only infect the cells of the respiratory tract, from infecting our blood cells, our bone cells, or the cells of our nervous system.
b. Penetration. Once a virion has attached itself to the outside of a cell, the next step in the cycle is getting inside the host cell. In order for the virion to take over the host cell’s “machinery,” it is necessary to get the genetic material of the virion inside the host cell. This may happen a number of different ways. For example, the T4 bacteriophage injects only the DNA genetic material into the E. coli cell, while the capsid remains outside the host cell. In other cases, the entire virion, including the capsid, enters the cell in a manner similar to phagocytosis in amoeba proteus.
c. Uncoating. Once inside the host cell, the viral genetic material must get into place within the host cell where it will be used to dictate the “manufacturing and assembly” functions of the host cell “machinery.” In some cases, the capsid is removed completely and the genetic material either remains in the host cell cytoplasm or the genetic material makes its way into the host cell nucleus where it may join the host cell genetic material. In other cases, the capsid is only partially removed and the new “command center” is set up in the region of the host cell where the viral genetic material and modified capsid remain.
d. Replication. This next step in the viral replication cycle is when the “manufacturing and assembly” actually take place. During the replication step in a lytic cycle, the “hi-jacked” host cell structures create new copies of the viral genetic material and new proteins which will become the viral capsid. The viral parts are then put together within the cell to form new virions which will be released during the release step in the viral replication cycle.
At this point in the replication cycle, there are some viruses that may simply cause their genetic material to join the genetic material of the host cell, but not immediately begin the manufacture of new material for new virions. These viruses enter what is called a lysogenic cycle. During this cycle, a viral infection is often referred to as a latent infection. No cells are being destroyed; thus the typical symptoms of infection are lacking in the host organism. However, because the viral genetic material has been incorporated into the host cell genetic material, as the host cell grows and reproduces, the new daughter cells also contain the viral genetic material and viral replication continues.
e. Release. If a virus has caused a latent infection, then this final step in a viral replication cycle is postponed indefinitely until some outside stimulant causes the virus to enter a lytic cycle. In the case of a viral infection already in a lytic cycle, this step typically ends with the death of the host cell and the release of the replicated virions.
The release of the newly assembled virions may take place through a number of different processes. Two of the most basic and common processes are cell lysis and budding. During cell lysis, the host cell simply breaks apart, releasing all cell contents, including the new virions. This common mechanism of releasing these new virions is not used when the virion includes an envelope. Instead, enveloped virions are typically released through viral budding. During budding in the viral replication cycle, an envelope is formed around the viral capsid using lipids from the host cell membrane. Repeating this process over and over often leaves the host cell damaged enough that in time it will rupture or die.