Association of bw D with other blocks of heterochromatin occurs only during the G1 phase of the cell cycle, leading us to suspect that heterochromatic cohesion is a regulated process that plays a functional role in nuclear architecture and gene expression. It does not appear that binding of these particular proteins to repeats is essential for an organism, because closely related species of Drosophila that produce GAGA factor and Prod protein lack the corresponding heterochromatic repeats altogether. Rather, we think that DNA-binding proteins that evolved for euchromatic roles were acquired later by expanding repeats, which require these proteins for packaging into mitotic chromosomes. The short evolutionary lifetime of satellite blocks seems inconsistent with functional conservation. Nevertheless, satellite blocks are conspicuously present at centromeres of essentially all animals and plants.
Satellites efficiently form heterochromatin, and heterochromatin silences genes.
Perhaps heterochromatin also silences replication origins, and as a result, the only way uninterrupted blocks of heterochromatin can be replicated is from replication forks approaching from the outside. Thus large blocks of heterochromatin would contain the latest replicating regions, and so organisms would have harnessed satellites to maintain regions of late replication to facilitate centromere function. We have been pursuing these possibilities experimentally. Understanding the relationship between centromeres and heterochromatin will require a better knowledge of the fundamental components of centromeres. In Drosophila and human cells, H3-like proteins from yeast and worms localize preferentially to heterochromatin around centromeres, consistent with their deposition as nucleosomes on newly replicated DNA within heterochromatin.