Demethylation of the zygotic paternal genome

In mammals, both parental genomes undergo dramatic epigenetic changes after fertilization to form the diploid somatic genome. Here we show that the paternal genome in the mouse is significantly and actively demethylated within 6–8 hours of fertilization, before the onset of DNA replication, whereas the maternal genome is demethylated after several cleavage divisions. This active demethylation of the paternal genome may be associated with epigenetic remodelling of sperm chroma-tin, in order to establish parent-specific developmental programmes during early embryogenesis.