Genes VII

When chromatin is examined in the electron microscope, two types of fibers are seen: the 10 nm fiber and 30 nm fiber. They are described by the approximate diameter of the thread (that of the 30 nm fiber actually varies from ~25 V30 nm).

The 10 nm fiber is essentially a continuous string of nucleosomes. Sometimes, indeed, it runs continuously into a more stretched-out region in which nucleosomes are seen as a string of beads, as indicated in the example of Figure 19.17. The 10 nm fibril structure is obtained under conditions of low ionic strength and does not require the presence of histone H1. This means that it is a function strictly of the nucleosomes themselves. It may be visualized essentially as a continuous series of nucleosomes, as in Figure 19.18. It is not clear whether such a structure exists in vivo or is simply a consequence of unfolding during extraction in vitro.

When chromatin is visualized in conditions of greater ionic strength the 30 nm fiber is obtained. An example is given in Figure 19.19. The fiber can be seen to have an underlying coiled structure. It has ~6 nucleosomes for every turn, which corresponds to a packing ratio of 40 (that is, each µm along the axis of the fiber contains 40 µm of DNA). The presence of H1 is required. This fiber is the basic constituent of both interphase chromatin and mitotic chromosomes.

The most likely arrangement for packing nucleosomes into the fiber is a solenoid, illustrated in Figure 19.20. The nucleosomes turn in a helical array, with an angle of ~60 X between the faces of adjacent nucleosomes (for review see Felsenfeld and McGhee, 1986).

The 30 nm and 10 nm fibers can be reversibly converted by changing the ionic strength. This suggests that the linear array of nucleosomes in the 10 nm fiber is coiled into the 30 nm structure at higher ionic strength and in the presence of H1.

Although the presence of H1 is necessary for the formation of the 30 nm fiber, information about its location is conflicting. Its relative ease of extraction from chromatin seems to argue that it is present on the outside of the superhelical fiber axis. But diffraction data, and the fact that it is harder to find in 30 nm fibers than in 10 nm fibers that retain it, would argue for an interior location.

How do we get from the 30 nm fiber to the specific structures displayed in mitotic chromosomes? And is there any further specificity in the arrangement of interphase chromatin; do particular regions of 30 nm fibers bear a fixed relationship to one another or is their arrangement random?