Practice question

Mid-blastula transition in Drosophila, also called maternal-to-zygotic transition, occurs at nuclear cycle 14 when embryo switches from maternal to zygotic genetic control. Early cycles are rapid, lacking gap phases and checkpoint regulation, dependent on maternally deposited mRNAs and proteins. At cycle 14, cell cycle lengthens dramatically, G2 phase appears, DNA replication checkpoint activates due to high nuclear-to-cytoplasmic ratio. Large-scale zygotic transcription initiates, maternal transcripts degraded via smaug and miR-309. Cellularization coincides with this transition, marking shift where developmental programs become governed by zygotic genome rather than maternal products.