Practice question

During semi-conservative DNA replication parental histone octamer distribution must conserve epigenetic information. Pulse-chase labeling combined with density gradient shows H3-H4 tetramer (H3-H4)2 remains intact as unit, does not split into dimers, segregating randomly to one of two daughter duplexes behind fork. This allows recycling of parental marks like H3K9 methylation to reestablish heterochromatin on both daughters via reader-writer mechanisms. In contrast H2A-H2B dimers exchange rapidly, mixing old and new, H1 readily dissociates. Therefore H3-H4 tetramer conservation underlies epigenetic inheritance during and after replication.