SARs and metaphase chromosome structure

Introduction Various structural studies have demonstrated that the chromosomal architecture of the nucleus is highly ordered and suggest that it might be held in place by some sort of nuclear scaffold. This organization must permit dynamic movements of chromatin during activities such as enhancer looping, recombination and chromosome condensation. The mechanisms whereby nuclear order and chromatin dynamics are achieved remain enigmatic. These biological processes are presumably governed by cis elements that are distributed at intervals along the genome. Activities mediated by such cis elements presumably include tethering chromatin to some kind of nuclear scaffold, serving as signalling or initiator stations from which open, closed or heterochromatic chromatin states spread, and limiting the spreading of these chromatin states. Different types of cis elements could be involved in these activities, although it is also possible that one type of cis element could govern more than one type of activity. Scaffold associated regions (SARs; also called matrix attachment regions [MARs]) are the only candidate cis elements identified to date that are implicated in facilitating dynamic changes in chromatin states and in the structural organization of chromosomes.