Viruses: Key Ideas Why is a virus not considered a living organism? What two structures are characteristic of viruses? What are two ways that a virus can reproduce?

Is a Virus Alive? Viruses are not considered living because they are missing key characteristics of living organisms. Viruses do have genetic material, but they cannot reproduce on their own. Viruses reproduce by infecting cells. Viruses use the cell’s ribosomes, ATP, enzymes, and other molecules to make more viruses.

Is a Virus Alive?, continued Viruses do not grow. Instead, they are assembled into their full size within a cell. Viruses do not carry out any metabolic activities, do not have any cytoplasm or organelles, and do not maintain homeostasis.

Viral Structure The structure of a virus is relatively simple with two structures that are characteristic of all viruses. All viruses have nucleic acid and a capsid. A capsid is a protein covering that surrounds the nucleic acid core in a virus. In addition to a capsid, viruses may have an envelope. An envelope is a membranelike layer that covers the capsid of some viruses. Viruses that infect bacteria also have tail fibers.

Types of Viruses

Plant Viruses They are similar to animal viruses in most basic characteristics but they can also be markedly different. Most plant viruses have RNA as the genetic material. Tobacco Mosaic Virus

Enteroviruses cause many common illnesses such as polio, meningitis, encephalitis, common cold, and tonsillitis,

Rotavirus Viral gastroenteritis is the second most common clinical disease in developed countries

Bacteriophages viruses that attack bacteria

Virus Reproduction A viral infection begins when the genetic material of a virus enters a host cell. Once inside the cell, a virus can reproduce by two different processes. Viruses can reproduce by a lytic life cycle and a lysogenic life cycle.

The Lytic and Lysogenic Life Cycle