2 What Is a Virus? Abstract Viruses are built from short sequences of nucleic acid, either DNA or RNA wrapped in a protein shell. Until the invention of the electron microscope, it was impossible to visualize a virus. The first viruses to be visualized were bacteriophage, which appeared to have a head and tail-like structure. Only the nucleic acid entered the bacterial cell through the tail. Animal viruses were described as spherical or rod-shaped; they were bound to receptors and were taken up by the cell. After the crystallization of the tobacco mosaic virus, there was much discussion as to whether viruses were ‘‘living’’ organisms; the controversy continues to this day. Although viruses were defined in part on the basis of size and filterability, viruses much larger than the traditional viruses have recently been isolated. Studies of viral replication indicate that most viruses self-assemble as a result of interactions between the viral proteins to form a viral capsid that interacts with the nucleic acid to form the whole. The viral replication cycle and synthesis is presented in this chapter. Viral classification into a Linnaean scheme has been proposed, but newer methods using nucleic acid homologies are changing classification. Viruses are spread in the human population by various means, including airborne particles, fecal-oral contact, clothing, insects, and contact with other animals (zoonosis). 2.1 Definition of a Virus Although viruses tend to be diverse in terms of the diseases they cause and the organs they attack, all viruses have a unity of structure and consist of proteins and nucleic acid. Some viruses are also encased in a lipid membranous envelope (Fig. 2.1 ). Their mode of replication is not binary (one divides into two, two divide into four, etc.), as in most other organisms, but occurs as a burst of thousands of virus particles from a single virus over a short time. The number of viruses M. W. Taylor, Viruses and Man: A History of Interactions , DOI: 10.1007/978-3-319-07758-1_2, ȑ Springer International Publishing Switzerland 2014 23
produced in cell culture or the blood is in the tens of millions per milliliter of media or blood. This mode of replication alone makes viruses unique. All other life forms contain DNA as genetic material and RNA as a message or intermediate for the formation of proteins or other structures. Viruses are also unique in that they contain either RNA or DNA as the genetic material. To date, no virus has been discovered that contains both types of nucleic acid as genetic material, although both types are used during virus replication in the cell. Viruses do not contain ribosomes, mitochondria or other cell-like organelles, and are thus com- pletely parasitic. Since they cannot replicate without the metabolic processes of the host cell, they are genetic parasites. This differentiates viruses from bacteria or other single-cell microorganisms (e.g., protozoa), most of which can replicate in culture on their own, although they can also be parasitic. One other feature dis-
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