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A DNA virus is a virus that has DNA as its genetic material and replicates using a DNA-dependent DNA polymerase. The nucleic acid is usually double-stranded DNA (dsDNA) but may also be single-stranded DNA (ssDNA). DNA viruses belong to either ''Group I'' or ''Group II'' of the Baltimore classification system for viruses. Single-stranded DNA is usually expanded to double-stranded in infected cells. Although ''Group VII'' viruses such as hepatitis B contain a DNA genome, they are not considered DNA viruses according to the Baltimore classification, but rather reverse transcribing viruses because they replicate through an RNA intermediate. Notable diseases like smallpox, herpes, and chickenpox are caused by such DNA viruses. ==Group I: dsDNA viruses== Genome organization within this group varies considerably. Some have circular genomes (''Baculoviridae'', ''Papovaviridae'' and ''Polydnaviridae'') while others have linear genomes (''Adenoviridae'', ''Herpesviridae'' and some phages). Some families have circularly permuted linear genomes (phage T4 and some ''Iridoviridae''). Others have linear genomes with covalently closed ends (''Poxviridae'' and ''Phycodnaviridae''). A virus infecting archaea was first described in 1974. Several others have been described since: most have head-tail morphologies and linear double-stranded DNA genomes. Other morphologies have also been described: spindle shaped, rod shaped, filamentous, icosahedral and spherical. Additional morphological types may exist. Orders within this group are defined on the basis of morphology rather than DNA sequence similarity. It is thought that morphology is more conserved in this group than sequence similarity or gene order which is extremely variable. Three orders and 31 families are currently recognised. A fourth order – Megavirales – for the nucleocytoplasmic large DNA viruses has been proposed.〔 Four genera are recognised that have not yet been assigned a family. The species ''Sulfolobus turreted icosahedral virus'' is so unlike any previously described virus that it will almost certainly be placed in a new family on the next revision of viral families. Fifteen families are enveloped. These include all three families in the order ''Herpesvirales'' and the following families: ''Ascoviridae'', ''Ampullaviridae'', ''Asfarviridae'', ''Baculoviridae'', ''Fuselloviridae'', ''Globuloviridae'', ''Guttaviridae'', ''Hytrosaviridae'', ''Iridoviridae'', ''Lipothrixviridae'', ''Nimaviridae'' and ''Poxviridae''. Bacteriophages (viruses infecting bacteria) belonging to the families ''Tectiviridae'' and ''Corticoviridae'' have a lipid bilayer membrane inside the icosahedral protein capsid and the membrane surrounds the genome. The crenarchaeal virus ''Sulfolobus turreted icosahedral virus'' has a similar structure. The genomes in this group vary considerably from ~10 kilobases to over 2.5 megabases in length. The largest bacteriophage known is Klebsiella Phage vB_KleM-RaK2 which has a genome of 346 kilobases. A recently proposed clade is the Megavirales which includes the nucleocytoplasmic large DNA viruses. This proposal has yet to be ratified by the ICTV. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「DNA virus」の詳細全文を読む スポンサード リンク
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