Crick here evaluated research results by two of his coauthors, Leth Bak and Jesper Zeuthen, used in an article the three wrote on the structure and folding of DNA and its associated proteins in human chromosomes during cell division (mitosis). In their article they postulated that chromatids, the one-half of a chromosome that splits off from the other half during cell division, were organized in a hierarchy of helices: the chromatid is a folded and coiled super-solenoid (a long, regular, hollow cylindrical structure), also called the unit fiber, which in turn is formed by a coiled solenoid of smaller diameter, which in turn is formed by coiling the string of nucleosomes, bead-like complexes of DNA and protein.. See Leth Bak, Jesper Zeuthen, and Crick, "Higher-Order Structure of Human Mitotic Chromosomes," Proceedings of the National Academy of Sciences, vol. 74 (April 1977), pp. 1595-99.
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