An amplified genome that may have resulted from recombination within bidirectionally replicating DNA

Temperature shift-down of permissive mouse cells transformed by a temperature-sensitive (ts) polyomavirus (Py) genome has been shown to induce the accumulation of free copies of the viral DNA. We report here on an unusual product from such induction. The structure of this product is that expected from the occurrence of recombination between growing points in a bidirectionally replicating Py-mouse DNA molecule. This observation may be relevant to the mechanism of gene amplification in mammalian cells.