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Imprinted genes appear to be embedded in large DNA domains with different replication patterns. These patterns may provide a structural imprint for parental identity. Replication timing patterns for the chromosomal regions containing the imprinted genes Igf2, Igf2r, H19 and Snrpn in the mouse were examined in order to find the relationship between timing patterns and parental-specific transcription. The two homologous alleles replicate asynchronously at all of these sites, and it is always the paternal allele that is early-replicating.

Replicons, with sizes ranging from 50 to 300 kilobases, constitute the animal cell genome. Individual origin of the replication characterizes each of the replicons. The replicons are the starting point of DNA production. The cis-acting elements regulate the origin and functions of the replicons in animal versus cell-type specificity which is also found in the replication. The replication direction assay determines the organization and control of endogenous tissue-specific gene replication. Bidirectional DNA synthesis reverses replication directions at several regions.