It is straightforward to understand how such differences could be established, since oogenesis and spermatogenesis occur in physically distinct locations, where different patterns of gene expression can produce the epigenetic differences. But, after fertilization, these parent-of-origin-specific epigenetic marks are maintained across many rounds of cell division. So, a cell in, say, your liver, will exhibit different epigenetic states on maternally and paternally alleles, despite the fact that they have occupied the same cellular environment throughout development.
|Alleles at an imprinted locus maintain substantial epigenetic differences, despite occupying the same environment across many cell divisions.|
There an enzyme, Dnmt1, that specifically targets the unmethylated cytosine at a hemimethylated CpG and methylates it. So, after the action of this enzyme, the methylated state has been restored in each of the daughter cells. The combination of the hemimethylase (or maintenance methyltransferase) activity of Dnmt1 and the semiconservative replication of DNA set up a system in which the epigenetic state of an allele can be set once, and it will be propagated across multiple cell divisions.
|DNA replication of a fully methylated CpG site results in two hemimethylated copies. The hemimethylase Dnmt1 then restores these hemimethylated sites to their fully methylated form.|
Yoder, J., Soman, N., Verdine, G., & Bestor, T. (1997). DNA (cytosine-5)-methyltransferases in mouse cells and tissues. Studies with a mechanism-based probe. Journal of Molecular Biology, 270 (3), 385-395 DOI: 10.1006/jmbi.1997.1125