Human mitotic chromosome structure: what happened to the 30-nm fibre?

The long-standing view of chromosome packaging is that 10-nm beads-on-a string chromatin fibres fold into higherorder 30-nm fibres, which further twist and coil to form highly condensed chromosomes. After four decades of intense pursuit of the structure and properties of 30-nm chromatin fibres, work of Nishino et al (2012) in this issue of The EMBO Journal demonstrates that regular 30-nm fibres are absent from human mitotic chromosomes. The emerging view is that chromosome-level condensation can be achieved through packaging of 10-nm fibres in a fractal manner. At the heart of a human chromosome is a single contiguous DNA molecule ranging in size from 50 to 250 million base pairs. This raises a question that has fascinated scientists for many years: how is chromosomal DNA condensed so that it all fits into the cell nucleus? The nucleosome, which consists of B147 bp of DNA wrapped 1.75 times around an octamer of core histone proteins, is the first level of chromosomal DNA packaging (Figure 1). Arrays of nucleosomes spaced at B10–60 bp intervals along chromosomal DNA make up the next level of organization and are known as 10-nm ‘beads-on-a string’ chromatin fibres (Figure 1). In vitro studies of short chromatin fragments isolated from nuclei, and more recently of reconstituted oligonucleosomes, have established unequivocally that 10-nm beads-on-a string chromatin fibres fold in the presence of cations into ‘higher-order’ structures that are B30nm in diameter (Hansen, 2002; Tremethick, 2007). The precise structure of the 30-nm fibre remains a point of contention, with the most commonly proposed models being a two-start helix and a one-start solenoid (Tremethick, 2007). Based on the huge body of in vitro data, it has long been assumed that the 30-nm chromatin fibre is a requisite intermediate in the packaging of 10-nm chromatin fibres into chromosomes. In what can be thought of as the continuous folding model of chromosome structure, 10-nm beads-on-a string chromatin fibres first form folded 30-nm fibres, which further twist and coil until chromosomal-level compaction is achieved (e.g., see Hansen, 2002). Despite the nearly universal belief in the importance of the 30-nm fibre as a structural module within chromosomes, there have been few direct investigations of this question in situ and in vivo. To address this deficiency, Nishino et al (2012) used the techniques of cryo-electron