There has been much speculation about the relationship between replication timing and gene expression in eukaryotes. In an Advanced Online Publication in Nature Genetics, Dirk Schübeler and colleagues describe how they used microarray hybridization experiments to construct a replication timing map for the entire Drosophila melanogaster genome (Nature Genetics, DOI:10.1038/ng1005, 30 September, 2002).

Schübeler et al. labeled Drosophila cell cultures with bromodeoxyuridine triphosphate (BrdU) and used flow cytometry to sort cells into different cell-cycle fractions. To compare material from early or late stage S phase, they immunoprecipitated labeled DNA with anti-BrdU antibodies and PCR-amplified the associated DNA fragments, before hybridization to DNA microarrays. They found that neighboring genomic regions were replicated at similar times during S phase and processed this data to generate a genome-wide profile of replication. Schübeler et al. observed that pericentromeric heterochromatin replicates late in S phase and also found a high correlation between gene...