To an exponentially growing cell culture, 3HT is added to mark the DNA. After an hour, DNA damage is set, not repairable, and lethal to the cell. If apoptosis would occur, the DNA would be degraded fast, due to active cellular lysis. If further incorporation of 3HT to the nucleic acids occurs, either repair or replication of the DNA is active. This can be distinguished after another hour, by adding HU in this example. If replication is active, HU would hinder further 3HT incorporation; if repair is active, 3HT incorporation continues.
