In the first hours after fertilization, maternal factors residing in the oocyte cytoplasm dictate early development. But soon, the zygote’s genes start to take over. This maternal-to-zygotic transition involves massive epigenetic reprogramming, from the overall structure of the chromatin to the complete resetting of methylation on the genome. (Note: Most of the information depicted below is based on studies of mouse embryos; there are some differences in the timing of these events in human embryos.)
© KIMBERLY BATTISTA
Chromatin changes
In sperm, chromatin is very compact; the overall accessibility of the chromatin in the oocyte, which is still undergoing meiosis, is unclear. Shortly after fertilization, chromatin in both pronuclei undergoes major restructuring, taking on an open configuration before reestablishing local and global organizational features.
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