According to this hypothesis, viruses originated through a progressive process. Mobile genetic elements, pieces of genetic material capable of moving within a genome, gained the ability to exit one cell and enter another. To conceptualize this transformation, let's examine the replication of retroviruses, the family of viruses to which HIV belongs.
Retroviruses have a single-stranded RNA genome. When the virus enters a host cell, a viral enzyme, reverse transcriptase, converts that single-stranded RNA into double-stranded DNA. This viral DNA then migrates to the nucleus of the host cell. Another viral enzyme, integrase, inserts the newly formed viral DNA into the host cell's genome. Viral genes can then be transcribed and translated. The host cell's RNA polymerase can produce new copies of the virus's single-stranded RNA genome. Progeny viruses assemble and exit the cell to begin the process again (Figure 2).
This process very closely mirrors the movement of an important, though somewhat unusual, component of most eukaryotic genomes: retrotransposons. These mobile genetic elements make up an astonishing 42% of the human genome (Lander et al. 2001) and can move within the genome via an RNA intermediate. Like retroviruses, certain classes of retrotransposons, the viral-like retrotransposons, encode a reverse transcriptase and, often, an integrase. With these enzymes, these elements can be transcribed into RNA, reverse-transcribed into DNA, and then integrated into a new location within the genome (Figure 3). We can speculate that the acquisition of a few structural proteins could allow the element to exit a cell and enter a new cell, thereby becoming an infectious agent. Indeed, the genetic structures of retroviruses and viral-like retrotransposons show remarkable similarities.
