How is so much DNA packed into the space of a cell nucleus that’s only around two to ten microns (0.002-0.01 millimetres) wide? Each double helix strand is wrapped around proteins called histones to form structures called nucleosomes, which under the microscope have the appearance of beads on a string. These nucleosomes coil up, further compressing the DNA molecule into compact fibres. The fibres are then tightly folded to produce the 250-nanometre-wide fibres that make up chromosomes.
This arrangement is adjustable, so portions of DNA strands can effectively be opened up when the molecule needs to be ‘read’ during transcription or replication. Since these structural changes are reversible, the DNA reverts to its compact form when these processes are complete.