diameter). Initially, the histones produce a complex with the DNA to form the first structural
unit of chromatin called thenucleosome. The nucleosome consists of DNA wrapped twice
around a core of eight histones to form a 10-nm fiber (Fig. 6.3). This fiber is further folded
to result in a chromatin fiber. Chromatin is dynamic in terms of its ability to coil and uncoil
Figure 6.3.Chromatin structure. The different levels of chromatin structure are shown. The basic
building block consists of the nucleosome–DNA wrapped around a core of histone and nonhistone
proteins. Nucleosomes are strung together by strands of DNA, which are densely packed to create
chromatin. Tight winding of chromatin creates the metaphase chromosome.
6.2. DNA PACKAGING INTO EUKARYOTIC CHROMOSOMES 139