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Chromatin fibers with different protamine and histone compositions.

Previous studies have established that chromatin fibers which contain histones have variable diameters in different biological materials. In this paper we show that chromatin fibers are also present in spermatozoa and spermatids of bivalve molluscs in which some or all histones have been replaced by protamines. In all cases thick chromatin fibers are formed, with a diameter in the range 25-50 nm, depending on the species. We conclude that the formation of chromatin fibers is determined by the partial neutralization of the DNA charges with any histone or protamine, rather than due to a precise association of nucleosomes. This conclusion is in agreement with current theoretical work which shows that the diameter of complexes of DNA with counterions is determined by an overall balance of the interaction forces present.