Overview

Viruses are acellular, intracellular parasites whose reproduction depends in part on the host's genetic machinery. Viral infection may kill a cell; at the very least viruses are a burden to their host. All organisms are susceptible to viral infection, and by conservative estimate there are 108 different kinds of virus. The ubiquity, variety, and destructiveness of viruses have motivated study of their reproduction. In addition, viruses are extremely useful per se in the study of genetics (replication, mutation, recombination, and gene expression), as tools in molecular biology, medicine, and agriculture, and as “windows” into the biology of the host organisms.

This chapter summarizes viral life cycles. A virus's host and genetic material strongly influence its mode of reproduction; accordingly, the chapter is organized both by host and by genetic material – RNA or DNA, single-or double-stranded.

A Little Terminology

The (+) strand of an RNA virus genome can be translated into proteins, and the (-) strand is complementary to the (+) strand; essentially, the (+) strand is mRNA. The (+) strand of a DNA virus corresponds to the mRNA sequence, while the (-) strand is the template for RNA synthesis. The chromosome, which may be circular or linear, may contain a small amount of protein.

Bacteriophages, the Viruses of Bacteria

Bacteriophages (also simply phages) are viruses that infect bacteria. Phages consist of a chromosome contained within a protein capsid, ranging in length between 10 and 300 nm.