Crick here proposed a general model for the structure of chromosomes in higher eukaryotic cells. His model was designed to account for discoveries made during the 1960s that most of the DNA in higher organisms did not code for proteins but carried out genetic control functions, and that chromosomal DNA was most often associated with proteins, mainly histones, rather than in its pure double-helical form. Crick's model posited that chromosomal DNA consisted of "bands" of highly folded, unpaired single-stranded segments of double-stranded DNA, which carried out control functions. In between these stretches of "globular" DNA were much shorter stretches of "fibrous" DNA, similarly unpaired and single-stranded but extended and not folded, which coded for proteins.
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