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The typical retrovirus genome consists of a single-stranded RNA of about 8500 nucleotides. The enzyme reverse transcriptase is a multifunctional enzyme that first makes a DNA copy of the viral RNA molecule. It then acts as a nuclease to remove the RNA, and then makes a second DNA strand, generating a double-stranded DNA copy of the RNA genome. The integration of this DNA into the host chromosome, catalyzed by a viral protein called integrase, is required for the synthesis of new viral RNA molecules by the host cell RNA polymerase. Retroviruses are examples of enveloped viruses, in which the protein shell is further enclosed by an outer lipid bilayer membrane. The envelope contains proteins that enable the virus to bind to cells, and that aid its entry into a cell. As indicated, the lipid membrane is acquired when the virus is released from the cell by a process of budding from the plasma membrane, taking some of the plasma membrane with it. The budding process is reversed when the virus reinfects a cell.