Characteristics

The classification of retroviruses is based on comparisons of the size of the genome and morphologic characteristics (see Table 1). The genomic RNA

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Genus

Distinguishing feature

Example

Host

Diseases/pathologies

Alpha-retrovirus

genome <8kb; assembly at cell membrane

avian leukosis virus

birds

malignancies

Beta-retrovirus

intracytoplasmic assembly (B- or D-type)

mouse mammary tumor virus

mice

mammary and ovarian carcinoma; lymphomas

Gamma-retrovirus

genome >8kb; assembly at cell membrane

murine leukemia virus

mice

malignancies

Delta-retrovirus

genomes <9kb; C-type

bovine leukemia virus

cows

malignancies

Epsilon-retrovirus

assembly at cell membrane; hosts: fish

walleye dermal sarcoma virus

fish

solid tumors

Lentivirus

genome >9kb; bar-shaped concentric core

human immunodeficiency virus

humans

immunodeficiency and neurologic disease

Spumavirus

assembly as intracytoplasmic particles

chimpanzee foamy spumavirus

simians

none apparent

Retrovirus

öc of retroviruses is single-stranded and possesses "positive" polarity similar to Table 1 Viruses of the that found in messenger RNA (mRNA). Virions (virus particles) contain two family Retroviridae. 5' ("five prime"), end-linked, identical copies of the genome RNA, and are therefore said to be diploid.

Three genes are universally present in the genomes of retroviruses that are capable of replication, such as murine (mouse) leukemia virus. The gag (group antigen) gene encodes proteins that make up the nucleocapsid of the virus as well as a matrix layer, the two of which surround the RNA. The pol gene (a type of polymerase) encodes reverse transcriptase, which copies the RNA into DNA, and integrase, which integrates the DNA into the host chromosome. Depending on the species, pol can also encode protease, a protein that cleaves the initial multiprotein products of retrovirus translation to make functional proteins. Some retroviruses have incorporated viral oncogene sequences. An example of this is reticuloendotheliosis virus strain T. The genome of complex retroviruses, such as HTLV, can contain several other genes that regulate genome expression or replication and are not present in simple retroviruses.

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