Diagram summarizing the chromatin distribution of proteins analyzed in this study. For simplicity, replication proteins that were not analyzed are not depicted in this diagram. Diagram is not drawn to scale. Light blue spheres represent nucleosomes and dark blue lines represent the DNA component of the chromatin fibers. We found that the relative distribution of ORC and GINS proteins changed in relation to DNA replication activity. (A) ORC proteins were found on chromatin before the binding of GINS and before DNA replication began. Some ORC complexes were found in close proximity on these fibers; they appeared as continuous streaks in digital photomicrographs. (B) GINS proteins were loaded onto chromatin after ORCs but before DNA replication began. (C) The number of ORC complexes per replication unit is not known, but it is presumed to be one; an ORC complex is depicted in the center of the replicon. The ORC complex that is shown associated with chromatin at the right edge of the replication track represents a dormant origin passively replicated later from a nearby active origin. The red dotted line represents newly synthesized DNA, yellow rings represent PCNA. (D) As replication proceeds, nucleosomes are assembled on the newly replicated DNA and ORC proteins are no longer visible on the fibers. GINS and PCNA also dissociate from sites were replication has been completed.