Developmental roles for chromatin and chromosomal structure.

Chromosomal architecture is emerging as a key controlling influence in the developmental regulation of gene expression. Recent genetic experiments using Caenorhabditis elegans, Drosophila melanogaster, and the mouse have provided clear evidence for the functional differentiation of chromosomal structures during development. Chromosomes are visualized as highly specialized entities, within which the activity of particular domains is largely determined by defined structural proteins. At a more local level, the mechanisms regulating gene transcription during early embryogenesis in Xenopus and the mouse have been found to be dependent on the biochemical composition of individual nucleosomes. Thus, variation in the type and modification of chromosomal and chromatin structural proteins provides a dominant means of controlling the transcriptional activity of individual genes, individual chromosomal domains, and of entire chromosomes.