You are studying the prokaryotic transposon TN10 and have just figured out an elegant wayto determine whether Tn10 replicates during transposition or moves directly without intervening DNA replication. Your idea is based on the key difference between these two mechanisms: both parental strands of the Tn10 will move if transposition is nonreplicative, whereas only one parental strand will move if transposition is replicative. You plan to mark the individual strands by annealing strands from two different Tn10s. Both Tn10s contain a gene for tetracycline resistance and a gene for lactose metabolism (lacZ), but in one the lacZ gene is inactivated by a mutation. This difference provides a convenient way to follow the two Tn10s since lacZ+ bacterial colonies (with the appropriate substrate) turn blue, but lacZ- colonies remain white. You denature and reanneal a picture of the two transposon DNAs, which produces an equal mixture of heteroduplexes and homoduplexes. You introduce them into lacZ- bacteria, and spread the bacteria onto Petri dishes that contain tetracycline and the color generating substrate. Once inside a bacterium, the transposon will move (at very low frequency) into the bacterial genome, where it confers tetracycline resistance on the bacterium. The rare bacterium that gains a Tn10 survives the selective conditions and forms a colony. When you score a large number of such colonies, you find that roughly 25% are white, 25% are blue and 50% are mixed with one blue sector and one white sector.