ABSTRACT
Viruses are submicroscopic infectious particles that are incapable of growing or reproducing outside host cells. Each viral particle (or virion) is composed of a genetic core in the form of DNA or RNA and a protective protein coat called capsid, which often measures between 10 and 300 nm in diameter. The viral capsid may be of simple helical and icosahedral (polyhedral or near-spherical) forms, or of more complex structures with tails (or envelope). The viral envelope often protects a virion from enzymes and certain chemicals, and also functions as receptor molecules to allow recognition and uptake by host cells. This puts enveloped virus in an advantaged position over other capsid-only virions. Being noncellular organisms, viruses possess genes that enable them to reproduce in host cells by creating multiple copies of themselves through self-assembly and to evolve in infected cells by natural selection. While many more viruses contain RNA and belong to RNA virus category (see Chapter 3), some viruses possess a double-stranded DNA (dsDNA) or singlestranded DNA (ssDNA), and are thus called DNA viruses. DNA viruses often have larger genomes due to the high fi delity of their replication enzymes-DNA polymerases.
