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Naturally Occurring Polymers—Animals 347

FIGURE 10.12 Description of a typical bacterial chromosome.

represents the “at rest” or “unstressed” coupled DNA. As I answer the telephone, I have a tendency
to twist it in one direction and after answering and hanging up the telephone for awhile it begins
forming additional coils. Thus, additional coiling tends to result in supercoiling. The second, and
more common form, involves the presence of less than normal coiling. This can be illustrated by
taking a rubber band, breaking one end and then attaching it about a stationary object. Begin brad-
ding the two ends until just before a bunching or formation of supercoiling through over coiling.
Then separate the two ends pulling them apart. The resulting strain produces supercoiling and illus-
trates supercoiling resulting in undercoiling or underwinding. Thus, underwinding occurs when
there are fewer helical turns than would be expected. Purified DNA is rarely relaxed.

Supercoiling with bacterial DNA gives a largely open, extended, and narrow, rather than com-
pacted, multibranched structure. By comparison, the DNA in eukaryotic cells is present in very
compacted packages. Supercoiling forms the basis for the basic folding pattern in eukaryotic cells
that eventually results in this very compacted structure. Subjection of chromosomes to treatments
that partially unfold them show a structure where the DNA is tightly wound about “beads of pro-
teins” forming a necklace-like arrangement where the protein beads represent precious stones
imbedded within the necklace fabric (Figure 10.13). This combination forms the nucleosome, the
fundamental unit of organization upon which higher-order packing occurs. The bead of each nucle-
osome contains eight histone proteins. Histone proteins are small basic proteins with molecular

weights between 11,000 and 21,000 and specified by names such as H1, H2, and so on. H1 is espe-
cially important and its structure varies to a good degree from species to species whereas some of
the other histones, such as H3 and H4, are very similar. Histones are rich in the amino acid residues
from arginine and lysine.
Wrapping of DNA about a nucleosome core compacts the DNA length about seven fold. The
overall compacting though is about 10,000 fold. Additional compacting of about 100 fold is gained

from formation of so-called 30 nm fibers. These fibers contain one H1 within the nucleosome core.
This organization does not occur over the entire chromosome but rather is punctuated by areas

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