Practice question

During replication fork progression, parental nucleosomes ahead of helicase are disrupted and histones must be reassigned to daughter strands. Classic labeling with cycloheximide blocking new histone synthesis and isotope tracking demonstrate that H3-H4 tetramers remain intact, not splitting into dimers, and segregate randomly between leading and lagging duplexes. Approximately equal numbers of old and new tetramers assemble behind fork with interspersed distribution. This random dispersal preserves epigenetic information in mosaic fashion, requiring copying of modifications by reader-writer enzymes like PRC2 and Suv39h1 to maintain heterochromatin memory across divisions.