Graph to illustrate the differential exposure time theorem.

<p>The graph is based on a simulation of 500 nuclei with a fixed S-phase length and cell cycle length randomly entering S-phase. The S-phases are then sorted and plotted in gray. The Y-axis has a length of 1 (all cells) and the X-axis has a length of one cell cycle (T_C). The slope of the line formed by the end points of the S-phases is therefore by definition 1/T_C. Thymidine analogues are incorporated into the nucleus during the S-phase (between the black lines). The fraction of labelled nuclei is thus determined by the length of the S-phase plus the time of exposure to the thymidine analogue divided by the total length of the cell cycle <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047719#pone.0047719-Sanders1" target="_blank">[7]</a>. When two different exposure times are applied, either by two different thymidine analogues or in two different animals, the difference in fraction of labelled nuclei (ΔF) is linearly related to the difference in exposure time (ΔT) (red triangle). Because the slope of this relation is equal to 1/T_C, the cell cycle length can be calculated (red box). Similarly, the length of the S-phase can be determined from the slope in the blue triangle and the calculated T_C (blue box).