Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transmission
DNA: Structure of Chromosomes and Genes
Eukaryotic chromosomes consist of chromatin fibers

We have already used the term "chromosome" to refer to a nucleic acid molecule that serves as the repository of Genetic information in a virus, prokaryote, or Introduction/5.html">Eukaryotic Cell. Originally, however, the word "chromosome" (meaning "colored body") was used in a different sense to describe densely stained entities in eukaryotic nuclei that could be observed under a Light Microscope after staining the Cells. Eukaryotic Chromosomes, in the original sense of the word, appear as sharply defined structures only immediately before and during mitosis—The process of nuclear division in somatic cells (Fig. 27-22). In resting, non-dividing Eukaryotic cells, the chromosomal material, known as Chromatin, appears diffuse and randomly distributed throughout The Nucleus. However, as The Cell prepares to divide, the chromatin condenses and assembles into a species-specific number of distinct, well-defined chromosomes (Fig. 27-22).

Chromatin has been isolated from nuclei and analyzed. It consists of extremely fine fibers containing ~60% protein, ~35% DNA, and typically ~5% RNA (Section 2.7). The chromatin fibers within a chromosome are coiled to form numerous loops and convolutions (Fig. 27-21). The DNA in chromatin is tightly bound to Proteins called Histones, whose function is to package and organize the DNA into structural units known as nucleosomes. Chromatin also contains a variety of non-histone proteins. Unlike eukaryotic chromosomes, bacterial chromosomes lack histones and contain only a small amount of proteins that facilitate DNA looping and Condensation (packing).