Higher order taxa
Viruses; Retro-transcribing viruses; Reoviridae
Alpharetrovirus, Betaretrovirus, Spumavirus (examples)
Description and Significance
The genome of retroviridae is dimeric, unsegmented and contains a single molecule of linear. The genome is -RT and a positive-sense, single-stranded RNA. Minor species of non-genomic nucleic acid are also found in virions. The encapsidated nucleic acid is mainly of genomic origin but virions may also contain nucleic acid of host origin, including host RNA and fragments of host DNA believed to be incidental inclusions. The complete genome of one monomer is 700-11000 nucleotides long. The 5'-end of the genome has a methylated nucleotide cap with a cap sequence type 1 m7G5ppp5'GmpNp. The 3'-terminus of each monomer has a poly (A) tract and the terminus has a tRNA-like structure. (source: ICTVdB Descriptions)
Virion Structure of a Retroviridae
The virions of a retroviridae consist of an encelope, a nucleocapsid and a nucleoid. The virus capsid is enveloped. The virions are spherical to pleomorphic and measure 80-100 nm in diameter. The surface projections are small or distinctive glycoprotein spikes that cover the surface evenly. The projections are densely dispersed and 8 nm long. The nucleoid is concentric or eccentric while the core is spherical. (source: ICTVdB Descriptions)
Reproduction Cycle of a Retroviridae in a Host Cell
The SU envelope glycoprotein binds to a specific receptor on the surface of the host targer cell to initiate the infection. The specificity of this interaction does much to determine the cell-tropism and pathogenesis of different retroviruses and even different isolates of the same virus. Murine reroviruses (MLVs) are sub-divided to three categories on the basis of receptor-determined host species specificity: ecotropic, xenotropic, and amphotropic. Ecotropic MLVs infect only mouse cells, xenotropic MLVs infect only non-mouse cells like rat and hamster, and amphotropic MLVs infect both mouse and non-mouse cells.
Interference between an exogenous virus and an endogenous virus of the same receptor specificity results in interference groups of viruses, as exemplified by ALVs. A number of retrovirus receptor molecules have been identified in recent years.