Euchromatin indicates active chromatin—that is, chromatin that is stretched out so that the genetic information in the DNA can be read and transcribed. It is prominent in metabolically active cells such as neurons and liver cells. Heterochromatin predominates in metabolically inactive cells such as small circulating lymphocytes and sperm or in cells that produce one major product such as plasma cells.
The smallest units of chromatin structure are macromolecular complexes of DNA and histones called nucleosomes.
Nucleosomes are found in both euchromatin and heterochromatin and in chromosomes. These 10-nm-diameter particles represent the first level of chromatin folding and are formed by the coiling of the DNA molecule around a protein core. This step shortens the DNA molecule by approximately sevenfold relative to the unfolded DNA molecule. The core of the nucleosome consists of eight histone molecules (called an octamer). Two loops of DNA (approximately 146 nucleotide pairs) are wrapped around the core octomer. The DNA extends between each particle as a 2-nm filament that joins adjacent nucleosomes. When chromatin is extracted from the nucleus, the nucleosomal substructure of chromatin is visible in transmission electron microscopy (TEM) and is often described as “beads on a string” (Fig. 3.2a).
In the next step, a long strand of nucleosomes is coiled to produce a 30-nm chromatin fbril. Six nucleosomes form one turn in the coil of the chromatin fibril, which is approximately 40-fold shorter than unfolded DNA. Long stretches of 30-nm chromatin fibrils are further organized into loop domains (containing 15,000 to 100,000 base pairs), which are anchored into a chromosome scaffold or nuclear matrix composed of nonhistone proteins. In heterochromatin, the chromatin fibers are tightly packed and folded on each other; in euchromatin, the chromatin fibrils are more loosely arranged.
In dividing cells, chromatin is condensed and organized into discrete bodies called chromosomes.
During mitotic division, chromatin fbers formed from chromatin loop domains attached to a flexible protein scaffold undergo condensation to form chromosomes [Gr., colored bodies]. Each chromosome is formed by two chromatids that are joined together at a point called the centromere (Fig. 3.2b). The double nature of the chromosome is produced in the preceding synthetic (S) phase of the cell cycle that maintains telomere length. For example, in cells that (see page 87), during which DNA is replicated in anticipation have been transformed into malignant cells, an enzyme of the next mitotic division. called telomerase is present that adds repeated nucleotide
FIGURE 3.1 • Electron micrographs of nuclei from two different cell types. The large electron micrograph shows the nucleus of a nerve cell. Two nucleoli are included in the plane of section. The nucleus of this active cell, exclusive of the nucleoli, comprises almost entirely extended chromatin or euchromatin. 10,000. Inset. The smaller nucleus belongs to a circulating lymphocyte (the entire cell is shown in the micrograph). It is a relatively inactive cell. Note the paucity of cytoplasm and cytoplasmic organelles. The chromatin in the nucleus is largely condensed (heterochromatin). The lighter areas represent euchromatin. 13,000.
The area located at each end of the chromosome is called sequences to the telomere ends. Recently, expression of this the telomere. Telomeres shorten with each cell division. Re-enzyme has been shown to extend the lifespan of cells. cent studies indicate that telomere length is an important in-With the exception of the mature gametes, the egg and dicator of the lifespan of the cell. To survive indefinitely sperm, human cells contain 46 chromosomes organized as 23 (become “immortalized”), cells must activate a mechanism homologous pairs (each chromosome in the pair has the chromatin fiber with loops of chromatin fibril