Retroviral replication cycle.

Infection begins when the virus envelope glycoprotein recognizes specific receptors located on the cell surface, followed by fusion of the viral envelope and cell membrane and release of the virus core into the cytoplasm. Viral RNA is uncoated, copied into a double-stranded DNA molecule, and transported to the nucleus. Although they are shown in the diagram as occurring in a stepwise manner, the exact timing and mechanism of these events are unclear. Integration into chromosomal DNA enables the viral genome (now called the provirus) to be stably maintained, replicated during DNA synthesis, and passed to progeny cells. Viral RNA is produced and translated to produce viral proteins and enzymes. Viral RNA also associates with viral proteins to form new core particles. Mature progeny virions capable of infecting new cells are formed when the cores obtain their envelopes, consisting of cellular membrane and envelope glycoprotein, when they are released from the cell. The above describes portions of the retroviral replication cycle relevant for the discussion of retroviral vectors. Not included is discussion of the lentiviral accessory genes and how they might affect timing and other aspects of viral replication.