Now we come to viral genomes—but not because they are particularly complex. Viral genomes are actually rather small and compact with little or no repetitive DNA, and their sequences are relatively easy to assemble, as we have seen in Fig. 13.7 for HIV. What sets viral genomes apart from those of cellular organisms are the types of nucleic acid that make up their genomes and the manner in which these genomes are replicated in the infected cells. In their nucleic-acid composition and manner of replication, viral genomes are far more diverse than the genomes of cellular organisms: All cellular organisms have genomes of double-stranded DNA that replicate by means of the processes described in Chapter 12. Not so with viruses. Furthermore, whereas biologists classify cellular organisms according to their degree of evolutionary relatedness, they classify viruses based on their type of genome and mode of replication.

We mostly think of viruses as causing disease, and indeed they do. Familiar examples include influenza (“flu”), polio, and HIV. In some cases, they cause cancer (Case 2: Cancer). In fact, the term “virus” comes from the Latin for “poison.” But viruses play other roles as well. Some viruses transfer genetic material from one cell to another. This process is called horizontal gene transfer to distinguish it from parent-to-offspring (vertical) gene transfer. Horizontal gene transfer has played a major role in the evolution of bacterial and archaeal genomes, as well as in the origin and spread of antibiotic-resistance genes (Chapter 26). Molecular biologists have learned to make use of this ability of viruses to deliver genes into cells.

In this section, we focus on viral diversity with an emphasis on viral genomes. Thousands of viruses have been described in detail, and probably millions more have yet to be discovered. It has been estimated that life on Earth is host to 10³¹ virus particles—ten hundred thousand million more virus particles than grains of sand! Most of these viruses infect bacteria, archaeons, and unicellular eukaryotes, and they are they are especially abundant in the ocean (10¹¹ viruses per liter). Viruses are small, consisting of little more than a genome in a package, and they can reproduce only by hijacking host-cell functions, but as a group they have had amazing evolutionary success.

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