Page 385 - Carrahers_Polymer_Chemistry,_Eighth_Edition

348 Carraher’s Polymer Chemistry

30 nm

Histone core

DNA linker and “wrapper”

FIGURE 10.13. Illustration of regularly spaced nucleosomes consisting of histone protein bound to super-
coiled DNA with DNA links between the histone bound units forming a 30 nm higher-order fi ber.

containing sequence-specific (nonhistone containing) DNA-binding proteins. The name “30 nm

fibers” occurs because the overall shape is of a fiber with a 30 nm thickness. The additional modes
of compaction are just beginning to be understood but may involve scaffold assisting. Thus, certain
DNA regions are separated by loops of DNA with about 20,000–100,000 base pairs, with each loop
possibly containing sets of related genes.
The scaffold contains several proteins, especially H1 in the core and topoisomerase II. Both
appear important to the compaction of the chromosome. In fact, the relationship between topoi-
somerase II and chromosome folding is so vital that inhibitors of this enzyme can kill rapidly divid-
ing cells and several drugs used in the treatment of cancer are topoisomerase II inhibitors.
The central theme concerning the major secondary structures found in nature is also illus-
trated with the two major polysaccharides derived from sucrose, that is, cellulose and a major
component of starch—amylose. Glucose exists in one of two forms—an alpha and a beta form
where the terms alpha and beta refer to the geometry of the oxygen connecting the glucose ring
to the fructose ring.
Cellulose is a largely linear homosaccharide of the beta-d-glucose. Because of the geometry
of the beta linkage, individual cellulose chains are generally found to exist as sheets, the individ-
ual chains connected through hydrogen bonding. The sheet structure gives cellulose-containing
materials good mechanical strength allowing them to act as structural units in plants. Amylose, by
comparison, is a linear poly(alpha-d-glucose). Its usual conformation is as a helix with six units per
turn (Figure 10.14). Amylose is a major energy source occurring in plants as granules.
A number of repeating features occur in nature. Along with the ones noted before, another
depends on the buildup of structure from individual polymer chains to structures that are seen by
our human eye. One of these assemblies is illustrated in Figure 10.15 where individual alpha-helix

chains form slowly curving protofibril bundles. These bundles in turn form microfi bril structures
which in turn form macrofibril and finally the cortical cell of a single wool fiber. The bundling and

further bundling eventually gives the wool fiber. A similar sequence can be described in the forma-
tion of our hair as well as our muscles. The difference for the muscle is that the basic structure is not

the alpha helix but rather a sheet-like actin filament connected together by myosin filaments. In all

cases, flexibility is achieved through several features with one of the major modes being the simple

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