9 DNA Replication and Nuclear Architecture

Abstract

Eukaryotic DNA replication takes place within a specialized subcellular environment: the cell nucleus. A characteristic double membrane separates this environment from the cytosol, allowing the intranuclear concentrations of proteins to be regulated by the trafficking of specific proteins through nuclear pores. Inside the nucleus, an intricate architecture reflects the structural and functional organization of the genome (Misteli 2005). Individual strands of DNA are condensed into chromatin fibers that are in constant, yet spatially restricted, motion (Marshall 2002). Given the large size of chromosomes and nuclei in mammals, each interphase chromosome generally occupies a distinct region or territory of the nucleus, despite the continuous movement of chromatin. This positioning is due to the fact that specialized chromosomal domains such as centromeres and telomeres assume distinct positions within the nucleus, being held in place by protein-mediated tethering (Taddei et al. 2004a). These anchorage sites link specific domains to the nuclear envelope or to nucleoli, while other domains are free to explore the nucleoplasm within the limits imposed by contiguity with the long chromosomal fiber. Unfolding of the fiber due to active transcription can also increase mobility and the radius of their constrained movement (Chambeyron and Bickmore 2004).

In budding yeast, the excision of a nucleosomal ring from the chromosome allows it to diffuse freely throughout the nucleoplasm, demonstrating that once chromatin is released from its chromosomal context, it moves even more freely than nontethered regions of a chromosome (Gartenberg et al. 2004). Remarkably, the excised domain can itself become anchored to the...