Retroviridae is a taxonomic family of enveloped viruses whose replication involves reverse transcription and integration of the resulting DNA into host-cell DNA. The integrated form is a provirus, from which host machinery can make viral RNA. Thus the replication cycle links RNA and DNA forms rather than remaining entirely in one nucleic-acid form.[ref]
Retroviridae report by the International Committee on Taxonomy of Viruses. Summary and Table 1, replication
A virion contains a viral envelope, an internal core, genomic nucleic acid and replication enzymes. Orthoretrovirinae package a dimer of single-stranded RNA. In Spumaretrovirinae, reverse transcription can already be partly completed in the particle, so a family-wide description should not require every virion to contain only RNA.[ref]
Retroviridae report by the International Committee on Taxonomy of Viruses. Table 1, virion and genome
HIV-1, in the genus Lentivirus, is one member of this broader family. The family name therefore supplies a classification shared by multiple genera, rather than another name for HIV-1 alone.
The RNA-to-DNA-to-RNA cycle also clarifies measurement. A test detecting a proviral DNA sequence addresses a different molecular form from a test detecting packaged RNA. Demonstrating one intermediate can support the proposed replication pathway, but its abundance alone does not count every completed infectious particle. The assay must identify which stage and material it measures.
Spumaretrovirinae is a subfamily of Retroviridae that includes the foamy viruses. The name refers to a characteristic foamy appearance that many isolates produce in cell culture. That visible effect is one observation within a classification also supported by molecular features and phylogenetic analysis.[ref]
Retroviridae report by the International Committee on Taxonomy of Viruses. Derivation of names, Spumaretrovirinae; Relationships within the family
A notable feature concerns the timing of reverse transcription. Released virions can already contain substantial reverse-transcribed DNA. In Orthoretrovirinae, the usual packaged genome is instead two RNA copies and reverse transcription follows entry into a new host cell. Both groups use a DNA stage in their replication cycles.[ref]
Spumaretrovirinae report by the International Committee on Taxonomy of Viruses. Distinguishing features, reverse-transcribed DNA in virions
[ref]Orthoretrovirinae report by the International Committee on Taxonomy of Viruses. Distinguishing features, RNA packaging and post-entry reverse transcription
The ICTV account also describes cytoplasmic capsid assembly and a genome of roughly 11.6 kilobases for a typical spumavirus. Its viral enzyme precursor is expressed from a spliced transcript, rather than the same fused precursor arrangement emphasized for Orthoretrovirinae.[ref]
Spumaretrovirinae report by the International Committee on Taxonomy of Viruses. Distinguishing features, assembly, typical genome and Pro/Pol expression
The timing distinction gives a useful experimental question: at which point in the cycle is a particular nucleic-acid form present? Detecting DNA in released particles and detecting newly made DNA after entry describe different stages. A single positive measurement, without identifying the sampled stage and material, cannot reconstruct when conversion occurred. Molecular form, location and sequence in time are complementary evidence.
Orthoretrovirinae is the subfamily of Retroviridae containing Lentivirus and several other retroviral genera. It includes human immunodeficiency virus and feline immunodeficiency virus. The family placement describes a replication strategy; it does not imply one host range or one disease outcome for all members.[ref]
Orthoretrovirinae report by the International Committee on Taxonomy of Viruses. Subfamily classification and Distinguishing features
A typical virion packages two copies of its RNA genome. After entry and release of the viral core into a host cell, reverse transcription produces DNA that can become an integrated provirus. Proteolytic processing of a viral polyprotein also releases functional components during maturation.[ref]
Orthoretrovirinae report by the International Committee on Taxonomy of Viruses. Distinguishing features, RNA copies, reverse-transcription timing and protease processing
[ref]Retroviridae report by the International Committee on Taxonomy of Viruses. Table 1, replication and integration
An invented counting example clarifies the two-copy description. If each packaged RNA molecule has 10,000 nucleotides, the pair contains 20,000 RNA nucleotides in total. The length of one genome copy is still 10,000 nucleotides. A corresponding 10,000-base-pair double-stranded DNA molecule contains 20,000 DNA nucleotides, but its length is conventionally reported in base pairs. Copy count, nucleotide count and base-pair length answer different questions.
The subfamily's genera are distinguished in part by phylogenetic analysis of conserved reverse-transcriptase regions. A shared enzyme mechanism can coexist with differences in accessory proteins, host distribution and cellular targets. Observing the shared mechanism therefore provides a classification clue while leaving those other biological properties to be determined.[ref]
Orthoretrovirinae report by the International Committee on Taxonomy of Viruses. Genus demarcation criteria and differences among members
Lentivirus is a genus within Retroviridae that includes HIV-1 and feline immunodeficiency virus. Members infect several mammalian host groups. Their virions characteristically have an internal core that appears bar-like or cone-shaped in structural observations.[ref]
Lentivirus report by the International Committee on Taxonomy of Viruses. Distinguishing features, Virion and Biology
The name covers a shared evolutionary grouping rather than one human disease. Primate lentiviruses use CD4 together with a chemokine receptor for recognition and entry; that statement is narrower than claiming that every member of the genus uses the same host receptor.[ref]
Lentivirus report by the International Committee on Taxonomy of Viruses. Distinguishing features
The genus illustrates complementary kinds of evidence. Electron microscopy reveals particle morphology, while genome comparisons examine sequence divergence and distinctive genetic features. The ICTV account describes species demarcation increasingly in terms of genome evidence, alongside historical criteria such as host range and antigenic properties.[ref]
Lentivirus report by the International Committee on Taxonomy of Viruses. Species demarcation criteria
A cone-shaped core can consequently be an informative structural observation without uniquely identifying a viral species. Conversely, sequence-based placement does not by itself measure the entry properties of a particle in a particular cell. Classification and functional characterization answer connected but different scientific questions.
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