Through thick and thin: the conundrum of chromatin fibre folding in vivo.

DNA in a human cell is approximately 2 m long but is compacted into micrometre-sized eukaryotic nuclei. To achieve this level of compaction, DNA is first wrapped up into nucleosomes, which are then thought to fold into fibres and loops. The basal unit of such folding is the 10 nm ‘beads on a string’ structure observed in all eukaryotic cells (Fig 1A; reviewed in [1]). The 10 nm fibre is subsequently folded into secondary and tertiary structures, the nature of which have been at the heart of a spirited debate for nearly 30 years [2].