ZGA

ZGA, or zygotic genome activation, is a key stage in early embryonic development when the embryo begins to transcribe its own genome after fertilization. Before ZGA, development is driven by maternal mRNAs and proteins stored in the oocyte. ZGA marks the transition to zygotic control of development and is typically accompanied by widespread changes in chromatin structure and the degradation of many maternal transcripts.

Timing of ZGA is species-specific. In mammals, the activation of the zygotic genome occurs at different early stages: in mice, transcription from the zygotic genome begins at the two-cell stage, whereas in humans it typically starts around the four- to eight-cell stage. Other organisms exhibit different schedules, reflecting variations in developmental tempo and regulatory networks.

Mechanisms and regulation of ZGA involve the activation of transcription factors and large-scale remodeling of chromatin to enable genome-wide transcription. The process is governed by a network of maternal factors that set the stage, followed by zygotic factors that drive transcription. Transcription factors from the DUX family have been implicated as initiators of ZGA in humans and mice, with broader regulatory networks coordinating DNA accessibility, histone modifications, and the degradation of maternal transcripts.

Relevance and implications of ZGA span developmental biology, reproductive medicine, and evolutionary biology. Proper timing and execution of ZGA are crucial for embryo viability and normal development. Disruptions in ZGA can lead to developmental arrest and have implications for fertility treatments and studies of early human development.