Describe the structural components common to all viruses.
Viruses lack all features of cellular organisms except for their possession of nucleic acid. A virus's nucleic acid may be either DNA or RNA but not both. Viral DNA and RNA may be linear or circular, single- or double-stranded. Further, a virus usually contains only a single molecule of nucleic acid that has anywhere from thousands to hundreds of thousands of nucleotides, or from five to several hundred genes. The nucleic acids are contained by a protein coat, or capsid, which can range in complexity from simple polyhedrons or rod shapes to more intricate arrangements consisting of a head, tail, and leg-like fibers. Surrounding their capsid, some viruses have an envelope, derived from a host's cell membrane or synthesized by the host's translational machinery based on viral genomic information. In addition to the proteins found in their capsid and envelope, some viruses contain genes for enzymes, but no virus has been found that possesses multi-enzyme systems.
