DNA virus

DNA viruses comprise a diverse group of viruses whose genomes consist of deoxyribonucleic acid. They rely on DNA polymerase to replicate their genetic material and are found across a broad range of environments, particularly in marine ecosystems, where they infect organisms from prokaryotes to complex eukaryotes. DNA viruses exhibit considerable variation in genome structure, replication strategies, evolutionary history, and pathogenic potential.

Classification and Genome Types

DNA viruses are principally divided according to the number of DNA strands in their genome:

• Double-stranded DNA (dsDNA) viruses
• Single-stranded DNA (ssDNA) viruses

These two categories correspond to Groups I and II of the Baltimore classification, which groups viruses based on their method of mRNA synthesis. Although Baltimore classification is independent of evolutionary taxonomy, viruses within each group commonly share broad replication mechanisms.
Some DNA viruses are also placed in more complex taxonomic frameworks such as virus realms, which represent highly conserved molecular features rather than ancestral relationships.

Baltimore Group I: Double-Stranded DNA Viruses

Group I viruses possess a dsDNA genome and use host or viral RNA polymerase to transcribe mRNA. Transcription typically begins when a preinitiation complex assembles upstream of transcription start sites, facilitating polymerase recruitment and strand synthesis.
These viruses use several genome replication mechanisms:

• Bidirectional replication, featuring two replication forks moving in opposite directions from an origin
• Rolling circle replication, producing long linear concatemers from circular genomes
• Strand displacement replication, in which a new strand displaces the old one during synthesis
• Replicative transposition, involving copying of integrated viral DNA to new sites within the host genome

dsDNA viruses may replicate in either the nucleus, relying heavily on host enzymes, or the cytoplasm, where they possess their own transcription and replication machinery.
A major structural distinction divides dsDNA viruses into:

• Tailed bacteriophages of the realm Duplodnaviria, assigned to the order Caudovirales
• Nontailed dsDNA viruses of the realm Varidnaviria, with distinct capsid architectures

Prominent disease-causing dsDNA viruses include herpesviruses, papillomaviruses, and poxviruses.

Baltimore Group II: Single-Stranded DNA Viruses

Group II viruses have ssDNA genomes that must be converted to a dsDNA form by host DNA polymerase before transcription can occur. After conversion, mRNA synthesis proceeds similarly to dsDNA viruses.
Most ssDNA viruses possess circular genomes replicated via rolling circle replication (RCR). The process begins when a viral endonuclease cleaves the positive strand, enabling DNA polymerase to extend from the negative strand template. The displaced positive strand is cleaved again and circularised to form a new genome.
Some ssDNA viruses, notably parvoviruses, contain linear genomes and replicate through rolling hairpin replication (RHR). Their genomes contain terminal stem–loop structures that repeatedly unfold and refold, enabling synthesis to proceed back and forth along the DNA.
Nearly all ssDNA viruses have positive-sense genomes, but exceptions exist:

• Anelloviridae possess negative-sense circular genomes
• Parvoviruses package either strand depending on species
• Bidensoviruses package both strands

Major Realms in DNA Virus Taxonomy

The International Committee on Taxonomy of Viruses recognises three realms encompassing DNA viruses. Realms group viruses based on deeply conserved molecular traits rather than shared evolutionary ancestry.
DuplodnaviriaThis realm includes dsDNA viruses whose major capsid protein has the HK97 fold, a distinctive structural motif. Members include:

• Tailed bacteriophages (Caudovirales)
• Herpesviruses (Herpesvirales)

These viruses are ancient and may predate the last universal common ancestor of cellular life. Their evolutionary relationships remain incompletely resolved.
MonodnaviriaThis realm comprises ssDNA viruses encoding HUH endonucleases, enzymes that initiate rolling circle replication. The chief members are CRESS-DNA viruses, which have circular ssDNA genomes. Some linear ssDNA viruses and their dsDNA descendants are also included. Evidence suggests that Monodnaviria emerged multiple times from archaeal and bacterial plasmids.
VaridnaviriaVaridnaviria contains tailless dsDNA viruses with characteristic capsid protein structures distinct from those of Duplodnaviria. Members infect a wide range of hosts, including marine eukaryotes and unicellular organisms.

Ecological and Medical Importance

DNA viruses are globally distributed and are especially abundant in oceans, where they regulate microbial populations and influence nutrient cycles. Their interactions with both prokaryotic and eukaryotic hosts contribute significantly to ecological balance.
In humans and animals, DNA viruses are responsible for numerous diseases. Examples include:

• Herpes simplex virus, causing oral and genital herpes
• Varicella zoster virus, responsible for chickenpox and shingles
• Human papillomaviruses, implicated in cervical and other cancers
• Poxviruses, historically associated with smallpox and related infections

Replication Diversity and Viral Evolution

DNA viruses display a striking array of replication strategies, reflecting their diverse origins. While Monodnaviria appears to have arisen repeatedly from plasmid-like ancestors, the origins of Duplodnaviria and Varidnaviria remain less certain. Evolutionary pressures from host defences, ecological niches, and horizontal gene transfer have shaped the structure and behaviour of these viruses.