Evidence for a folded structure of human chromosomes

摘要

Chromosomes isolated from human metaphase cells and resuspended in chromosome isolation buffer were observed to be unstable and to disintegrate spontaneously at 4°C. Intermediate structures between intact metaphase chromosomes and extended fibers were frequently obtained at short intervals after attempts at isolation. The disintegrating structures observed by light microscopy were of a loop-like configuration. A general model for the folding of a unit fiber in chromatids is suggested from studies of a large number of such disintegrating structures. We suggest that a chromatid is obtained by folding and supercoiling of the folded loop to achieve Condensation of the chromatid. This condensation of the unit fiber is, however, not sufficient to account for the packing of DNA in the chromatid. We conclude that the primary chromatin fiber (12.5 nm) must be condensed 200-fold within the unit fiber.