a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) Endonucleolytic cleavage to 5'-phosphomonoester. The RT polymerase active site binds 2 magnesium ions. Intracisternal A particles (IAPs) are defective retroviral elements. Due to extensive mutations in the envelope coding sequence, IAPs can only form defective viral particles confined to the intracisternae of the Golgi. IAPs are an important class of transposable elements that induce genomic mutations and cell transformation by disrupting gene expression. Reverse transcriptase/ribonuclease HThe reverse transcriptase is an error-prone enzyme that lacks a proof-reading function. High mutations rate is a direct consequence of this characteristic. RT also displays frequent template swiching leading to high recombination rate. Recombination mostly occurs between homologous regions of the two copackaged RNA genomes. If these two RNA molecules derive from different viral strains, reverse transcription will give rise to highly recombinated proviral DNAs.