English: The top panel illustrates the chromatin structure of the mature sperm immediately following fertilization (highly protaminated with some retension of paternally derived histones). From the following cell is seen the protamine to histone transition where maternally derived histones replace protamines resulting in the decondensation of the sperm head. The middle panel illustrates the various stages of early embryonic development. The bottom panel shows the methylation changes that occur over time in the maternal and paternal pronucleus, where the paternal pronucleus undergoes active demethylation and the maternal DNA is demethylated passively in a replication dependent manner. The approximate chronology of major events in the early embryo is outlined along the bottom of the figure and correlates to the illustrations of embryos above. (text from article)