Aspects of the primary DNA sequence or chromatin structure in different types of band could influence their replication time. Also differences in replication time could influence some characteristics of chromosome bands, e.g. base composition or chromatin structure. Sites of transcription at the G1/S boundary may seed the assembly of the first replication factories in early S phase and hence the most transcriptionally active regions of the genome, and theregionswiththehighestconcentrationsofgenes,would tend to be the ones to be replicated first.