Nucleosome model:
This model proposed by Woodcock (1973) suggests that the chromatid is a string of beads made up of repeating units. These beads called nucleosomes are oblate particles with a diameter of 110A°. They have a core of histones 40A° high and 80A° wide, around which is found DNA in a super helical structure.
Finch and Clug (1976) have proposed an arrangement of chromatin in the chromosome model of nucleosomes. The coiled DNA according to them is in the string of nucleosomes. The nucleosome string is then coiled into 300A wide solenoid, which in turn coils to form a supersolenoid with a diameter of 4000A0. This super solenoid structure is the unit fibre (chromonema seen in light microscope) whose coils constitute the chromosomes.