text: The organization of the DNA that is achieved by the nucleosome cannot fully explain the packaging of DNA observed in the cell nucleus. Further compaction of chromatin into the cell nucleus is necessary, but it is not yet well understood.  The current understanding is that repeating nucleosomes with intervening "linker" DNA form a 10-nm-fiber, described as "beads on a string", and have a packing ratio of about five to ten. A chain of nucleosomes can be arranged in a 30 nm fiber, a compacted structure with a packing ratio of ~50 and whose formation is dependent on the presence of the H1 histone.
  A crystal structure of a tetranucleosome has been presented and used to build up a proposed structure of the 30 nm fiber as a two-start helix.
  There is still a certain amount of contention regarding this model, as it is incompatible with recent electron microscopy data.  Beyond this, the structure of chromatin is poorly understood, but it is classically suggested that the 30 nm fiber is arranged into loops along a central protein scaffold to form transcriptionally active euchromatin. Further compaction leads to transcriptionally inactive heterochromatin.