What is a Virus?

WHAT IS A VIRUS?

Viruses are infectious agents that do not have the typical cellular structure of all other forms of life. By the end of the chapter, you will be able to decide for yourself if they represent life or not. They have been known about since about the 1900s, when the tobacco mosaic virus was discovered. These are the most numerous types of biological organisms on Earth and are found in every ecosystem.

Viruses can be independent but cannot replicate without infecting a cell of some type. Virus particles are referred to as virions. The three components of these particles are the following:

• Genetic material—these are long nucleic acids, which you’ve already discovered can be RNA or DNA, and can be double-stranded or single stranded.

These contain the particle’s genetic information. • Protein coat—this is referred to as the capsid and is what protects and surrounds the genetic material of the virus particle.

• Lipid envelope—not all viruses will have these but, if seen, it will surround the protein coat.

It is unclear how viruses came to be evolutionarily. They may be evolving structures that came from plasmids, which are stray pieces of DNA that travel from cell to cell. Some may have evolved from bacterial species. Interestingly, they may be involved in the evolution of other species by having their genes or the genes of one organism transferred to other organisms, causing a genetic change in the species receiving the nucleic acid through a viral infection. Viruses carry genetic material, evolve through natural selection, and reproduce, making them living in some scientists’ eyes, even though they are not cellular.