The timing of zygotic genome activation.

After fertilization, the embryonic genome is inactive until transcription is initiated during the maternal-to-zygotic transition. How the onset of transcription is regulated in a precisely timed manner, however, is a long standing question in biology. Several mechanisms have been shown to contribute to the temporal regulation of genome activation but none of them can fully explain the general absence of transcription as well the gene specific onset that follows. Here we review the work that has been done toward elucidating the mechanisms underlying the temporal regulation of transcription in embryos.