Chromosomal unit fibers in Drosophila

Chromosomal unit fibers consisting of long, regular fibers of about 0.40 micron diameter were obtained from disintegrated, isolated chromosomes of two Drosophila melanogaster cell lines. In one cell line with an essentially normal karyotype, three clearly defined size classes of 15,13, and 11 micron length were observed corresponding to the three larger chromosomes of Drosophila. In a cell line carrying an additional translocation between the two largest chromosomes a 19 micron fiber derived from the translocation chromosome was observed. Direct determinations of the DNA content per micron length of Drosophila unit fibers show that DNA is contracted by a factor of about 1400x in agreement with calculations based on the length of the unit fibers and the known DNA content of the individual Drosophila chromosomes. These findings support our previously proposed model for the unit fiber sub-structure of chromosomes as being derived by a hierarchy of coiling with the corresponding contraction ratios being 7 (100 A string of nucleosomes), 5 to 6 (250-300 A thick nucleohistone fiber), and about 40 (unit fiber), resulting in a total contraction of DNA in unit fibers in the order of 1400x.