The battle of the sexes after fertilization: behaviour of paternal and maternal chromosomes in the early mammalian embryo

In the early diploid mammalian embryo, a father's chromosomes don't mix with the mother's until some time after fertilization. This topological genome separation is preserved up to the four-cell embryo stage and then gradually disappears. Unlike maternal DNA, sperm DNA arrives in an almost crystalline structure, heavily modified with methylcytosines (MeCs), which keep genes inactive. Compartmentalization of the nucleus according to parental origin may make it easier for the cellular machinery of the fertilized egg to revive the paternal chromosomes and to control paternal gene expression. Active zygotic demethylation of the paternal genome by a putative demethylase in the egg is a striking example for the battle of the sexes at the genomic level and beyond the single-gene level. It has important implications for genomic imprinting, and the establishment of genetic totipotency in fertilized eggs and in somatic cells during mammalian cloning.