Introduction. Retroviruses are single-stranded RNA viruses of eukaryotes. Different subfamilies have been described. Avian or murine oncoviruses induce neoplasms, whereas lentiviruses, typified by human immunodeficiency virus and spumaviruses, produce persistent infections. Lentiviral infections may cause chronic disease whereas spumaviruses are apparently non-pathogenic. The retroviral life cycle is characterized by reverse transcription of their single-stranded plus (i.e. coding) RNA genome into a double-stranded DNA intermediate that integrates into the host genome. Over the last 25 years the mechanism of reverse transcription has been studied in great detail and these studies have led to the model shown in Fig. I. The result of reverse transcription is a linear double-stranded DNA molecule with a long terminal repeat (LTR) at each extremity. Synthesis of each DNA strand by the virus-encoded reverse transcriptase requires one template switch, also called a jump: the first jump is needed for synthesis of a minus DNA strand complementary to the viral RNA, the second jump for plus DNA strand synthesis.


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