The chromosomes of eukaryotic cells possess many potential DNA replication origins, of which a subset is selected in response to the cellular environment, such as the developmental stage, to act as active replication start sites. The mechanism of origin selection is not yet fully understood. In this review, we summarize recent observations regarding replication origins and initiator proteins in various organisms. These studies suggest that the DNA‐binding specificities of the initiator proteins that bind to the replication origins and promote DNA (...) replication are primarily responsible for origin selection. We particularly focus on the importance of transcription factors in the origin selection process. We propose that transcription factors are general regulators of the formation of functional complexes on the chromosome, including the replication initiation complex. We discuss the possible mechanisms by which transcription factors influence the selection of particular origins. BioEssays 27:1107–1106, 2005. © 2005 Wiley Periodicals, Inc. (shrink)

Heterochromatin remains condensed throughout the cell cycle, is generally transcriptionally inert and is built and maintainedbygroupsoffactors witheachgroupmember sharing a similar function. In mammals, these groups include sequence-specific transcriptional repressors, functionalRNAandproteinsinvolvedinDNAandhistone methylation. Heterochromatin is cemented together via interactions within and between each protein group and ismaintainedbythecell’sreplicationmachinery.Itcanbe constitutive (permanent) or facultative (developmentally regulated) and be any size, from a gene promotor to a whole genome. By studying the formation of facultative heterochromatin, we have gained information about how heterochromatin is assembled. We have (...) discovered that there are many different architectural plans for the building of heterochromatin, leading to a seemingly never-ending variety of heterochromatic loci, with each built according to a general rule. (shrink)