89 3.2 The nature of chromosomes coils in the next level of condensation is the same as the diameter of the chromosome during cell division, often about 700 nm. What produces these supercoils? One clue comes from observing mitotic metaphase chromo-somes from which the histone proteins have been re-moved chem- ically. After such treatment, the chromo-somes have a densely staining central core of nonhistone protein called a scaffold, as shown in Figure 3-21 and in the electron micrograph on the ﬁrst page of this chapter. Projecting laterally from this protein scaffold are loops of DNA. At high magniﬁcations, it is clear from electron micrographs that each DNA loop begins and ends at the scaffold. The central scaffold in metaphase chromosomes is largely composed of the enzyme topoisomerase II. This enzyme has the ability to pass a strand of DNA through another cut strand. Presumably, this central scaffold ma-nipulates the vast skein of DNA during replication, pre-venting many possible problems that could hinder the
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