Preserving histone modification patterns during DNA replication. After DNA replication, the two new daughter duplex DNAs that are produced during replication lack histones H2A-H2B, and the H3-H4 heterotetramers distribute among the two new strands (left). The parental (marked) H3-H4 heterotetramers bind quickly to both daughter strands, along with new (unmarked) H3-H4 heterotetramers (right). The old (marked) and new (unmarked) H2A-H2B heterodimers reassemble randomly on daughter strands with old and new H3-H4 heterotetramers. The epigenetic marks are subsequently spread to adjacent nucleosomes on the two daughter strands, preserving the histone modification pattern of the parental DNA.