Key Concepts of Section 5.7

• Viruses are small parasites that can replicate only in host cells. Viral genomes may be either DNA (DNA viruses) or RNA (RNA viruses) and may be either single- or double-stranded.

• The capsid, which surrounds the viral genome, is composed of multiple copies of one or a small number of virus-encoded proteins. Some viruses also have an outer envelope, which is similar to the plasma membrane of the host cell but contains viral transmembrane proteins.

• Most animal and plant DNA viruses require host-cell nuclear enzymes to carry out transcription of the viral genome into mRNA and production of progeny genomes. In contrast, most RNA viruses encode enzymes that can transcribe the RNA genome into viral mRNA and produce new copies of the RNA genome.

• Host-cell ribosomes, tRNAs, and translation factors are used in the synthesis of all viral proteins in infected cells.

• Lytic viral infection entails adsorption, penetration, synthesis of viral proteins and progeny genomes (replication), assembly of progeny virions, and release of hundreds to thousands of virions, leading to death of the host cell (see Figure 5-45). Release of enveloped viruses occurs by budding through the host-cell plasma membrane (see Figure 5-46).

• Nonlytic infection occurs when the viral genome is integrated into the host-cell DNA and generally does not lead to cell death.

• Retroviruses are enveloped animal viruses containing a single-stranded RNA genome. After a host cell is penetrated, reverse transcriptase, a viral enzyme carried in the virion, converts the viral RNA genome into double-stranded DNA, which is integrated into chromosomal DNA by an integrase enzyme that enters the cell inside the virion (see Figure 5-48).

• Unlike infection by other retroviruses, HIV-1 infection eventually kills host cells, causing the defects in the immune response characteristic of AIDS.

• Tumor viruses, which contain oncogenes, may have an RNA genome (e.g., human T-cell lymphotrophic virus) or a DNA genome (e.g., human papillomaviruses). Integration of the genomes of these viruses into a host-cell chromosome can cause transformation of the host cell into a tumor cell.