The virus genome covered by the capsid penetrates the host cell. Once inside, the virus is uncoated as the envelope and capsid are removed. Free of its covering, the viral genome (DNA or RNA) proceeds with biosynthesis. Newly assembled viral particles are released by budding. Components of viral envelopes (i.e., lipids, proteins, and carbohydrates) are obtained from the plasma or nuclear membrane as the viruses leave the cell. Budding does not necessarily kill the host cell. A retrovirus is a virus which has a genome consisting of two plus sense RNA molecules, which may or may not be identical. It relies on reverse transcriptase to perform the reverse transcription of its genome from RNA into DNA, which can then be integrated into the host's genome with an integrase. The virus itself is a storage form for its nucleic acid genome as well as a means of delivery of its genome into targeted cells, which constitute the infection. Once in the host's cell, the RNA strands undergo reverse transcription in the cytosol. Once integrated into the host's genome, the retroviral DNA, is then referred to as a provirus.

While transcription was classically thought to only occur from DNA to RNA, reverse transcriptase transcribes RNA into DNA. The term "retro" in retrovirus refers to this reversal of the central dogma of molecular biology. Reverse transcriptase activity outside of retroviruses has been found in almost all eukaryotes, enabling the generation and insertion of new copies of retrotransposons into the host genome.

Because reverse transcription is missing the usual "proofreading" of DNA transcription, this kind of virus mutates very often. This enables the virus to quickly grow resistant to antiviral pharmaceuticals, and is one of...

Certain groups speculate that the processes followed by retroviruses (that is, RNA>DNA>RNA>Protein) may be the key to the evolution of DNA; thus, that in the "primordial soup," retroviruses evolved to create DNA from the RNA templates, and it was subsequently adopted by cellular organisms due to the increased chemical stability of DNA.
The number of proteins required to form a spherical virus capsid is denoted by the "T-number" whereby 60t proteins are necessary. In the case of the hepatitis B virus, the T-number is 4, therefore 240 proteins assemble to form the capsid. As in the helical viruses, the spherical virus capsid may be enclosed in a lipid envelope, although frequently spherical viruses are not enveloped, and the capsid proteins themselves are directly involved in attachment and entry into the host cell. The complete virus particle is referred to as a virion. A virion is little more than a gene transporter, and components of the envelope and capsid provide the mechanism for injecting the viral genome into a host cell.

A virus makes use of existing enzymes and other molecules of a host cell to create more virus particles. Viruses are neither unicellular nor multicellular organisms; they are somewhere between being living and non-living. "Viruses can be considered to be on the threshold between living and non-living. In a sense, they can cause themselves to be reproduced, so they could be considered as being 'alive'. But they can only do this by