The orthoretroviral replication cycle. Infection is initiated when the mature, infectious virion binds to a receptor on the surface of the new host cell. The Env protein of the virus induces fusion between the viral membrane and the cell membrane (Step 1). Within the cytoplasm, the mature core dissociates (Step 2) and the dimeric viral RNA (shown in orange) is copied (Step 3) into double-stranded DNA (shown in green). The DNA copy enters the nucleus (probably when the nuclear membrane breaks down during mitosis) and is inserted into the chromosomal DNA of the cell (Step 4). The DNA is transcribed and the RNA product is exported from the nucleus (Step 5); within the cytoplasm, some molecules will be translated into viral proteins, and others are destined for encapsidation into progeny virus particles. The viral components assemble into budding virions (Step 6), which are released from the cell as immature particles (Step 7). Finally, PR cleaves the viral proteins, converting immature particles into mature, infectious virions (Step 8). It is possible that DNA synthesis actually occurs within the mature core rather than after dissociation of the core as shown here.
