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Gene products and regulatory signals required for

 replication of the genome

 efficient expression of the genome

 assembly and packaging of the genome

 regulation and timing of the reproduction cycle

 modulation of host defenses

 spread to other cells and hosts

Information not contained in viral genomes:

 genes encoding a complete protein synthesis machine (e.g., no ribosomal RNA and no ribosomal or translation proteins)

 genes encoding proteins of membrane biosynthesis

 telomeres (to maintain genomes) or centromeres (to ensure segregation of genomes)

 this list becomes shorter with each new edition of this textbook!

Figure 3.1 The Baltimore classification. All viruses must produce mRNA that can be translated by cellular ribosomes. This classification system traces the pathways from viral genomes to mRNA for the seven classes of viral genomes.

The seven strategies for expression and replication of viral genomes are illustrated in Fig. 3.2 to 3.8. In some cases, genomes can enter the replication cycle directly, but in others, genomes must first be repaired, and viral gene products that participate in the replication cycle must first be synthesized. Examples of specific viruses in each class are provided.