The higher order structure of the centromere.

The architecture of the centromere region of mouse chromosomes has been studied in cells grown in the presence of 5-azacytidine. This drug interferes with normal condensation producing elongated centromere regions. It has been found that this effect is reversible in the presence of the drug, allowing the observation of the repackaging of the extended centromere into a structure exhibiting native centromere morphology. Light microscopy as well as transmission and scanning electron microscopy of this condensation process suggests that the native centromere is formed by the helical folding of a subfiber with an approximate diameter of 100 nm. This fiber is in turn composed of loops of the 30-nm fiber class. The boundary between successive gyres of the subfiber are obscured at the completion of condensation resulting in the formation of a homogenous 250- to 300-nm fiber that is the native centromere. These observations provide evidence for an additional level of chromatin organization within the metaphase chromosome.