FIGURE 5-48 Retroviral life cycle. Retroviruses have a genome of two identical copies of single-stranded RNA and an outer envelope. Step 1: After viral glycoproteins in the retroviral envelope interact with a specific host-cell membrane protein, the envelope fuses directly with the plasma membrane, allowing entry of the nucleocapsid into the cytoplasm of the cell. Step 2: Viral reverse transcriptase and other proteins copy the viral ssRNA genome into a double-stranded DNA. Step 3: The viral dsDNA is transported into the nucleus and integrated into one of many possible sites in the host-cell chromosomal DNA. For simplicity, only one host-cell chromosome is depicted. Step 4: The integrated viral DNA (provirus) is transcribed by the host-cell RNA polymerase, generating viral mRNAs (dark red) and viral genomic RNA molecules (bright red). The host-cell machinery translates the viral mRNAs into glycoproteins and nucleocapsid proteins. Step 5: Progeny virions then assemble and are released by budding, as illustrated in Figure 5-46.