ABSTRACT
Viruses are the last and simplest form of life whose genomes we will investigate. In fact, viruses are so simple in biological terms that we have to ask ourselves if they can really be thought of as living organisms. Doubts arise partly because viruses are constructed along lines different from all other forms of life—viruses are not cells—and partly because of the nature of the viral life cycle. Viruses are obligate parasites of the most extreme kind: they reproduce only within a host cell, and in order to replicate and express their genomes, they must subvert at least part of the host’s genetic machinery to their own ends. Some viruses possess genes coding for their own DNA polymerase and RNA polymerase enzymes, but many depend on the host enzymes for genome replication and transcription. All viruses make use of the host’s ribosomes and translation apparatus for synthesis of the polypeptides that make up the protein coats of their progeny. This means that viral genes must be matched to the host genetic system. Viruses are therefore quite specific for particular organisms, and individual types cannot infect a broad spectrum of species.
