Figure 5.

Midblastula transition (MBT) controlled by titration of regulators against DNA. (A) During the early cleavage cycles, Xenopus embryos divide without growth, resulting in an exponentially decreasing cell size and an exponentially increasing ratio of DNA to cytoplasm. (B) Titration of constant-concentration histones against the exponentially increasing quantity of DNA sets the timing of zygotic genome activation (ZGA) by inhibiting transcription below the threshold DNA concentration. When DNA concentration reaches a critical threshold, inhibition is relieved and transcription becomes activated (Amodeo et al. 2015). (C) Titration of key replication factors against DNA sets the timing of S-phase lengthening. When DNA concentration becomes sufficiently high, the available replication factors are no longer able to efficiently trigger S phase, resulting in a longer cell cycle (Collart et al. 2013).