Induced reversal of order of cell division and DNA replication in Tetrahymena.

The sequence of macronuclear DNA replication (S) and cell division (D) in two normal or synchronized cell cycles in (amicronucleate) Tetrahymena can be symbolized S₁, D₁, S₂, D₂. Using heat shock synchronized cells, the final heat shock (33.8 °C) has been extended from 20 min to 5 h. This leads to an altered sequence: S₁, S₂, D₁, D₂, characterized by reversal of D₁ and S₂. S₂ occurs during the extended shock, D₁ comes in standard time after the shock has been discontinued. Thus prolonged stay at elevated temperature can dissociate two cell cycles into a subcycle of DNA replication followed by a subcycle of cell division. S₁ and S₂ charge the cells with four times the amount of DNA in newly divided cells, and D₁ and D₂ partition this DNA to four cells. Synthesis of DNA between D₁ and D₂ is not required, and it occurs in only few cells.