A retrovirus is a virus that works backward relative to the usual DNA transcription process. Instead of DNA-makes-RNA-makes-protein, the normal path from genetic information to living application, a retrovirus uses its RNA genome to make DNA, the double-stranded molecule. That trick, plus a few others, allows it not only to invade a cell but also to enter the cell nucleus and patch a DNA version of itself into the cell’s DNA, becoming a permanent part of the cell’s genome. Whenever the cell or its descendants replicate, that alien stretch is copied too. If the retrovirus happens to infect
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