The
Baltimore classification system is used to group viruses together based on their manner of
messenger RNA (mRNA) synthesis and is often used alongside standard virus taxonomy, which is based on evolutionary history. DNA viruses constitute two Baltimore groups: Group I: double-stranded DNA viruses, and Group II: single-stranded DNA viruses. While Baltimore classification is chiefly based on
transcription of mRNA, viruses in each Baltimore group also typically share their manner of replication. Viruses in a Baltimore group do not necessarily share genetic relation or morphology.
Double-stranded DNA viruses The first Baltimore group of DNA viruses are those that have a double-stranded DNA genome. All dsDNA viruses have their mRNA synthesized in a three-step process. First, a
transcription preinitiation complex binds to the DNA upstream of the site where transcription begins, allowing for the recruitment of a host
RNA polymerase. Second, once the RNA polymerase is recruited, it uses the negative strand as a template for synthesizing mRNA strands. Third, the RNA polymerase terminates transcription upon reaching a specific signal, such as a
polyadenylation site. dsDNA viruses make use of several mechanisms to replicate their genome. Bidirectional replication, in which two replication forks are established at a replication origin site and move in opposite directions of each other, is widely used. A rolling circle mechanism that produces linear strands while progressing in a loop around the circular genome is also common. Some dsDNA viruses use a strand displacement method whereby one strand is synthesized from a template strand, and a complementary strand is then synthesized from the prior synthesized strand, forming a dsDNA genome. Lastly, some dsDNA viruses are replicated as part of a process called
replicative transposition whereby a viral genome in a host cell's DNA is replicated to another part of a host genome. dsDNA viruses can be subdivided between those that replicate in the
cell nucleus, and as such are relatively dependent on host cell machinery for transcription and replication, and those that replicate in the
cytoplasm, in which case they have evolved or acquired their own means of executing transcription and replication. dsDNA viruses are also commonly divided between tailed dsDNA viruses, referring to members of the realm
Duplodnaviria, usually the tailed bacteriophages of the order
Caudovirales, and tailless or non-tailed dsDNA viruses of the realm
Varidnaviria.
Single-stranded DNA viruses is an ssDNA virus. The second Baltimore group of DNA viruses are those that have a single-stranded DNA genome. ssDNA viruses have the same manner of transcription as dsDNA viruses. However, because the genome is single-stranded, it is first made into a double-stranded form by a
DNA polymerase upon entering a host cell. mRNA is then synthesized from the double-stranded form. The double-stranded form of ssDNA viruses may be produced either directly after entry into a cell or as a consequence of replication of the viral genome. Eukaryotic ssDNA viruses are replicated in the nucleus. Most ssDNA viruses contain circular genomes that are replicated via
rolling circle replication (RCR). ssDNA RCR is initiated by an
endonuclease that bonds to and cleaves the positive strand, allowing a DNA polymerase to use the negative strand as a template for replication. Replication progresses in a loop around the genome by means of extending the 3'-end of the positive strand, displacing the prior positive strand, and the endonuclease cleaves the positive strand again to create a standalone genome that is
ligated into a circular loop. The new ssDNA may be packaged into virions or replicated by a DNA polymerase to form a double-stranded form for transcription or continuation of the replication cycle.
Parvoviruses contain linear ssDNA genomes that are replicated via
rolling hairpin replication (RHR), which is similar to RCR. Parvovirus genomes have
hairpin loops at each end of the genome that repeatedly unfold and refold during replication to change the direction of DNA synthesis to move back and forth along the genome, producing numerous copies of the genome in a continuous process. Individual genomes are then excised from this molecule by the viral endonuclease. For parvoviruses, either the
positive or negative sense strand may be packaged into capsids, varying from virus to virus. Nearly all ssDNA viruses have positive sense genomes, but a few exceptions and peculiarities exist. The family
Anelloviridae is the only ssDNA family whose members have negative sense genomes, which are circular. ==ICTV classification==