A virus is an infective agent that typically consists of a nucleic acid molecule in a protein coat and is able to multiply only within the living cells of a host.
The genetic material is held in the capsid or head of the virus. The head is made up of proteins arranged in an icosahedron pattern. Under the head is the sheath. The sheath is a long chain of proteins arranged into a tube like structure that leads from the capsid to the base plate of the virus. The pins around the base plate are long coils of proteins that are wide at one end and become slimmer forming a “spike.” The tail fibers are also long protein chains that help attach to the host cell. Once the tail fibers have attached to the membrane of the host cell, the spikes begin to drill an opening. The genetic material moves down the sheath tube into the host cell. The genetic material is integrated into the host DNA’s and later the cell will build the viruses babies for it.
Once in the cell, a protein called retroviral integrase protein cuts and pastes the viral DNA into sequence with the host DNA. The infected host can go into two cycles: the lytic cycle and the lysogenic cycle.
The lytic cycle is the virus’ life cycle. A virus finds a host cell, injects its material and the cell creates several new viruses. Once the new phages have been made, they exit the cell and leave behind large holes that kill the cell. The other cycle is the lysogenic cycle where the viral DNA stay transcribed in the host’s DNA and the host goes through meiosis, spreading the viral DNA into future generations. Occasionally, some of these new infected cells will undergo the lytic cycle. Because it is the cell doing all the work for the virus, the cell’s mistakes do effect the offspring. The reason viruses have been evolving over time is due the the fact that cells occasionally make mistakes during replication and if one of those mistakes happens to be on the viral DNA it will be present in all new viruses produced.
Our bodies can defend themselves from viruses however its no easy task. When you are infected with the flu, you could be sick in bed for 3-5 days. During this time the viruses are reeking havoc on your cells killing many in the process. Our bodies can defend because our specialized white blood cells produce these little protein sticks called antibodies which basically work as a tire clamp that grabs onto the virus and stops it in its tracks. Although this can stop many of the viruses we encounter it can take time, or not be able to stop them at all. A well known viral disease that our bodies cannot fight is the AIDS virus. This virus only attacks specific cells, white blood cells. For most patients, its not the AIDS virus that kills them, but instead other common, usually harmless viruses such as the common cold.The AIDS virus kills a large amount of the bodies white blood cells effectively destroying the body’s defense system. Without white blood cells the body cannot produce antibodies to fight off a disease.