“A virus is a piece of bad news wrapped in protein.” This quote from immunologist Sir Peter Medawar is certainly true for the cells that viruses infect. As we described in Chapter 13, a bacterial virus (bacteriophage) injects its genetic material into a host bacterium and turns that cell into a virus factory (see Figure 13.3). Other viruses enter cells intact and then shed their coats and take over the cell’s replication machinery. Viral life cycles can be very efficient. An example is the poliovirus: a single poliovirus infecting a mammalian cell can produce more than 100,000 new virus particles!
focus your learning
A virus life cycle may be lytic or lysogenic.
Understanding the infection cycles of a virus makes it possible to design therapeutic agents to fight infections.
Viruses are small infectious agents that infect cellular organisms and that cannot reproduce outside their host cells. Most virus particles, called virions, consist of only two or three components: the genetic material made up of DNA or RNA, a protein coat that protects the genetic material, and in some cases, an envelope of lipids that surrounds the protein coat. As we will see in this section, viral genomes include sequences that encode regulatory proteins. These proteins “hijack” the host cells’ transcriptional machinery, allowing the viruses to complete their reproductive cycles.
346