Radial loops and helical coils coexist in metaphase chromosomes.

Histone-depleted chromosomes have revealed a scaffold and loop architecture of metaphase chromosomes. In its simplest form this arrangement contradicts many classical observations suggesting chromosomes have a helical substructure. We have obtained preparations that allow the visualization of several levels of chromosome structure. These images suggest that metaphase packing is achieved by the compaction through helical coiling of a 200-300 nm fiber that is in turn composed of radial loops. These observations imply that any scaffold elements associated with radial loops are not distributed as previously proposed but must follow a complex and more extensive path within the metaphase chromatid.