DNA viruses have only deoxyribonucleic acid (DNA) molecules as their nucleic acid. Viruses unlike other microorganisms only have one type of nucleic acid (DNA or RNA) molecule in their genome or genetic make. And on this basis, viruses can be classified as either DNA viruses or RNA viruses depending on the type of nucleic acid molecule that makes up its genome. Depending on the type of DNA virus, the DNA can either be double-stranded (ds) or single-stranded (ss). 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.
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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 ssDNA viruses) cause a variety of infections and diseases in man and other vertebrates.
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, Dependoparvovirus (formerly, Dependovirus), Erythrovirus, Iteravirus, and Contravirus||Nucleus||ssDNA (no envelope)|
|Hepadnaviridae||Hepatitis B virus (HBV)||Nucleus||ssDNA (no envelope)|
The replication of the DNA viruses is usually not as complex as is the case for RNA viruses. And this is usually because DNA viruses unlike RNA viruses require no need for the reverse-transcription of their genome before the replication of their genetic information can take place. DNA viruses already contain DNA as their genetic material or genome. 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 – which is mainly geared towards the production of viral particles or components such as viral proteins and viral genome required for the coupling and release of new virions from the infected host cells.
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 with the exception of Parvoviridae and Hepadnaviridae whose genome or DNA is single-stranded (ss) in nature. This implies that, both Parvoviridae and Hepadnaviridae have ssDNA genome.
Those viral families with dsDNA genome that parasitize non-vertebrate hosts such as bacteria, Archaea, algae and mycoplasmas include: Myoviridae (infects bacteria), Siphoviridae (infects bacteria), Podoviridae (infects bacteria), Tectiviridae (infects bacteria), Corticoviridae (infects bacteria), Plasmaviridae (infects mycoplasma), Lipothrixviridae (infects Archaea), Rudiviridae (infects Archaea), Fuselloviridae (infects Archaea), Guttaviridae (infects Archaea), and Phycodnaviridae (infects algae). The ssDNA viruses (with their host in parenthesis) include: Microviridae (infects bacteria), Inoviridae (infects Mycoplasma and bacteria), Circoviridae (infects vertebrates), and Densovirinae (infects invertebrates, mosquitoes and silkworm).
The characteristics of some DNA-containing viruses especially those that are of clinical or medical importance to humans are quite unique and different from some RNA viruses (whose genome is mainly made up of RNA as their own nucleic acid molecule). Finally, DNA viruses have DNA as the major component of their genome, and they are of medical importance to humans and animals i.e., DNA viruses also cause infections and diseases in both human and animal population. However, some of the DNA viruses (e.g., adenoviruses) are apathogenic and are used for a wide variety of industrial applications. The adenoviruses could serve as vectors for vaccination purposes and they have also been employed in gene therapy techniques because of their seeming apathogenic nature.
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