Fig. 6.

Precision and accuracy in the timing of transcription. (A) Examples of differences in the timing of transcriptional activation of a gene between wild-type (wt) and perturbed/mutant (mut) early Drosophila embryos. (B) Transcriptional synchrony in a population of cells can be determined by measuring the time when transcription is activated in a given fraction of cells, e.g. t_wt and t_A are the times when 50% of wt and mut-A cells are active, respectively. Accuracy in the timing of transcription can refer to changes in the average value of this time, as demonstrated between mut-A and wt embryos. (C) Precision in timing can be measured in absolute time, or relative to a developmental event, or relative to other cells within an organism. Synchrony of the population of cells can be determined by measuring the time between initial activation (when the signal surpasses a threshold, thd) and t₅₀ (dt_i). Here, activation in mut-B is less synchronized than in the wt (dt_B is larger than dt_wt) and therefore the timing of activation is less precise. Since wt and mut-B embryos have the same t₅₀ value, there is no change in the accuracy of the timing of activation. It should be noted that temporal dynamics can affect transcript levels in cells/nuclei. For example, earlier transcription activation can result in the accumulation of more transcripts over time.