adminA very simple microorganism that infects cells and may cause disease. Viruses are not considered living organisms because they do not contain the essential cellular components that are required for life. Instead, they use the host cell’s machinery to produce copies of themselves.
A virus consists of nucleic acid (either DNA or RNA) inside a protein coat, called the capsid, and, in some cases, a lipid envelope. The capsid is designed to protect the genetic material from physical, chemical or enzymatic damage. Depending on the type of virus, the capsid can take on a variety of shapes. Virions can range from short, highly rigid rods to long, flexible filaments. Typically, the outer surface of the capsid has binding sites for a specific receptor molecule found on the cellular membrane. Once a virus binds to a host cell, it releases its DNA or RNA into the cell. The nucleic acid then directs the cellular protein-synthesis machinery to make more viruses, or to pass the virus on to new cells.
Viruses reproduce very rapidly, with thousands of viral particles (also known as virion) produced per minute in cell culture or blood. This feature distinguishes them from bacteria, fungi and protozoa, which also replicate very quickly. Like other living organisms, a virus must be injected into a host cell to replicate and cannot survive outside the body. During this replication process, a single-strand of nucleic acid (like a typo in a document) is occasionally inserted into the new copy, resulting in a defective virus. This defect is usually not harmful, but it can lead to serious diseases such as cancer or hemophilia.
