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Temporal control of DNA replication has been implicated in epigenetic regulation of gene expression on the basis of observations that certain tissue-specific genes replicate earlier in expressing than non-expressing cells. Here, we show evidence that several leukocyte-specific genes replicate early in lymphocytes regardless of their transcription and also in fibroblasts, where these genes are never normally expressed. Instead, the heritable silencing of some genes (Rag-1, TdT, CD8alpha and lambda5) and their spatial recruitment to heterochromatin domains within the nucleus of lymphocytes resulted in a markedly delayed resolution of sister chromatids into doublet signals discernable by 3D fluorescence in situ hybridization (FISH). Integration of transgenes within heterochromatin (in cis) did, however, confer late replication and this was reversed after variegated transgene expression. These findings emphasise that chromosomal location is important for defining the replication timing of genes and show that retarded sister-chromatid resolution is a novel feature of inactive chromatin.

Original publication




Journal article


Nat Cell Biol

Publication Date





668 - 674


Animals, CD8 Antigens, Cell Cycle, Cell Line, Cell Nucleus, Cerebrospinal Fluid Proteins, Chromatids, DNA Nucleotidylexotransferase, DNA Replication, Fibroblasts, Flow Cytometry, Gene Expression Regulation, Gene Silencing, Heterochromatin, Homeodomain Proteins, Humans, Lymphocytes, Mice, Transgenes