Overview:
Viruses are very basic organisms that are on the border between living and nonliving things. When they are dormant, they are similar to crystals. They are only able to reproduce within a living host cell. Although they contain protein and nucleic acid, they do not grow or respond to stimuli.
What Do Viruses Contain?
Some viruses are shaped like a tetrahedron, while others are rod-shaped. Viruses that invade bacteria combine both shapes, with a hard tetrahedral head portion and a hollow tail portion. No matter what their shape, viruses contain either DNA or RNA surrounded by a protective coating of protein and enzymes. They do not contain both, or other structures common to living cells. Viruses draw energy from host cells to reproduce, so they do not need the other structures. The smallest forms, called viroids, contain a short strand of RNA without a protective coating. They reproduce within the nuclei of host cells, and interfere with their normal function.
How Do They Reproduce?
Each virus is shaped in such a way that it can invade a specific type of host cell. Many viruses can be reproduced within a single cell, and most host cells are destroyed in the process. Some viruses go dormant within the cell, to activate later when the host cell itself begins to divide. That way, even more copies of the virus can be made, long after the original cell was invaded by the virus. The nucleic acid reproduces along with the DNA and RNA of the host cells.
How Do They Cause Disease?
Even when viruses appear dormant, they can cause disease by invading host cells and disrupting their function. For example, the herpes virus causes sores that disappear during the dormant part of their cycle but reappear during the active phase. In addition, they can be passed on to other types of organisms. One of the earliest viruses to be discovered, the tobacco mosaic virus, passed on an ability to infect other plants even when the synthesized virus resembled dead crystals.
What Is Immunity?
The body of a plant or animal has natural defenses against being invaded by viruses. For example, some white blood cells attack and destroy viruses themselves, while antibodies are produced as a specific defense against them. The reason vaccines are effective against many viruses is that they stimulate the body to produce more antibodies as a result of the wakened or killed pathogens they contain. Interferon is a protein that interferes with replication of viruses by producing enzymes that attack them.
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