DNA VIRUSES

DNA viruses have only the deoxyribonucleic acid (DNA) molecules as their nucleic acid; and the DNA can either be double-stranded or single-stranded as the case may be. The replication site of all DNA viruses is the nucleus of their host cell. Nevertheless, Poxviruses (which are also DNA viruses) replicate outside the nucleus of their host cell. Poxviruses replicate in the cytoplasm of their host cell.

All DNA viruses are double-stranded with the exception of Parvoviruses that are single-stranded. Double stranded DNA (dsDNA) viruses infect man, animals, mycoplasmas, algae, fungi and protozoa. However, viruses in this category rarely infect plants. Some DNA viruses have double stranded DNA genome while others have single stranded DNA (ssDNA) genome; and both category of DNA viruses cause infections in humans (Table 1).

A handful of viruses in this category (inclusive of dsDNA and ssRNA viruses) cause a variety of infections and diseases in man and other vertebrates. The replication of the DNA viruses is usually not as complex as is the case for RNA viruses. During the replication process of most DNA-containing viruses, the genome of the host cell (i.e. the DNA) is stimulated by the viral genome which overpowers it and causes it to drive their own DNA replication.

Some of the viral families that make up the DNA containing viruses and that parasitize humans and other vertebrates include: Parvoviridae, Adenoviridae, Herpesviridae, Poxviridae, Hepadnaviridae, Polyomaviridae, Papillomaviridae, Iridoviridae, and Asfarviridae. These viral families have viruses that contain dsDNA genome.

Those viral families with dsDNA genome that parasitize non-vertebrate hosts such as bacteria, Archaea, algae and mycoplasmas include: Myoviridae (bacteria), Siphoviridae (bacteria), Podoviridae (bacteria), Tectioviridae (bacteria), Corticoviridae (bacteria), Plasmaviridae (mycoplasma), Lipothrixviridae (Archaea), Rudiviridae (Archaea), Fuselloviridae (Archaea), Guttaviridae (Archaea), and Phycodnaviridae (algae). The ssDNA viruses (with their host in parenthesis) include: Microviridae (bacteria), Inoviridae (Mycoplasma and bacteria), Circoviridae (vertebrates), and Densovirinae (invertebrates, mosquitoes and silkworm).

The characteristics of some DNA-containing viruses especially those that are of clinical or medical importance to man is shown in Table 1.

Table 1. DNA viruses that parasitize humans

Viral family	Representative viruses	Replication site	Genome
Poxviridae	Cowpox, smallpox, monkey pox viruses and other poxviruses	Outside the nucleus	dsDNA (plus envelope)
Herpesviridae	Herpes simplex viruses (HSV) types 1 and 2, Epstein-Barr virus (EBV), cytomegalovirus (CMV), Varicella-zoster virus (VZV), Kaposi’s sarcoma-associated herpes virus (KSHV) and human herpes viruses 6 and 7	Nucleus	dsDNA (plus envelope)
Adenoviridae	Adenoviruses	Nucleus	dsDNA (no envelope)
Papillomaviridae	Papilloma viruses	Nucleus	dsDNA (no envelope)
Polyomaviridae	Polyomaviruses	Nucleus	dsDNA (no envelope)
Parvoviridae	Human parvovirus B19, Dependovirus, Erythrovirus, Iteravirus, and contravirus	Nucleus	ssDNA (no envelope)
Hepadnaviridae	Hepatitis B virus (HBV)	Nucleus	ssDNA (no envelope)
dsDNA=double stranded DNA; ssDNA=single stranded DNA

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