Figure 6-22

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   Retroviral life cycle

Retroviruses have two identical copies of a plus single-stranded RNA genome and an outer envelope containing protruding viral glycoproteins. After envelope glycoproteins on a virion interact with a specific host-cell membrane protein or group of proteins, the retroviral envelope fuses directly with the plasma membrane without first undergoing endocytosis (step 1). Following fusion, the nucleocapsid enters the cytoplasm of the cell; then deoxynucleoside triphosphates from the cytosol enter the nucleocapsid, where viral reverse transcriptase and other proteins copy the ssRNA genome of the virus into a dsDNA copy (step 2). The viral DNA copy is transported into the nucleus (only one host-cell chromosome is depicted) and integrated into one of many possible sites in the host-cell chromosomal DNA (step 3). The integrated viral DNA, referred to as a provirus, is transcribed by the host-cell RNA polymerase, generating mRNAs (light red) and genomic RNA molecules (dark red). The host-cell machinery translates the viral mRNAs into glycoproteins and nucleocapsid proteins (step 4). The latter assemble with genomic RNA to form progeny nucleocapsids, which interact with the membrane-bound viral glycoproteins, as illustrated in Figure 6-17. Eventually the host-cell membrane buds out and progeny virions are pinched off (step 5). See Figures 9-20 and 9-21 for details of the reverse transcription process and the transcription and processing of viral RNA.