Higher-order structure of mitotic chromosomes.

From observations on the partial disintegration of isolated mitotic chromosomes we propose that mitotic chromosomes are characterized by a rather simple organization based on the coiling of a large cylindrical structure, the unit fiber. The unit fiber is postulated to be formed by three levels of coiling being the string of nucleosomes, the 300 A solenoid coiling of the string of nucleosomes, and additional coiling of the 300 A solenoid to result in formation of the unit fiber as a super-solenoid structure with a diameter of 4000 A, a wall thickness of 300 A, and a hollow center of 3400 A diameter. The contraction ratio corresponding to each level of coiling in ascending order of size would be 7, 6, and about 32. To account for the final contraction of DNA within the chromatid we envisage a further level of coiling by a factor of about 5 to yield the final contraction of DNA in the chromatid by a factor of about 7,000. This model appears to explain the estimated mass/unit length of chromosomes from different species (man, mouse, and Drosophila) and the observed lengths of the isolated unit fibers are in good agreement with the values expected on the basis of the super-solenoid model.