Hepadnaviridae

The family Hepadnaviridae comprises enveloped viruses with a circular, partially double-stranded DNA as genome. The name of the family is composed of the two terms "hepar" (Latin for liver) and "DNA" (for genome). This indicates that the preferred site for replication of the virus is the liver cell. The virus species have so far only been found in mammals and birds. The family Hepadnaviridae is divided into two genera:

• Orthohepadnavirus ("true" hepadnaviruses, viruses pathogenic to mammals).

and

• Avihepadnavirus (viruses pathogenic to birds).

Numerous other virus species in primates and various birds have not yet been definitively assigned to a genus or defined as a separate species. TheOrthohepadnaviruses with the human pathogenic hepatitis Bvirus belong to the most important genus in this family.

The family Hepadnaviridae comprises enveloped viruses with a circular, only partially double-stranded DNA as genome associated with a protein P that has domains for polymerase, reverse transcriptase and RNAse. Genome and P protein are packaged in a capsid formed from the HBc protein. The envelope contains 3 variants of the HBs protein: LHBs, MHBs and SHBs. They serve to adsorb the virus on its host cell and subsequently to penetrate into the cell interior.

The virions of the Hepadnaviridae are between 42 and 50 nm in diameter and have an icosahedral capsid. As the viruses use genomic RNA during replication of the viral genome, which they transcribe into DNA using a viral reverse transcriptase, they are closely related to retroviruses. Very characteristic of hepadnaviruses is the binding of a short piece of RNA, the original primer piece of the reverse transcription, to the DNA genome. This makes them the only viruses that contain both viral DNA and viral RNA in the virion.