To understand chromatin, it is helpful to first explore chromosomes, structures within the nucleus that are made up of DNA, the hereditary material. You may remember that in prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are linear structures.
Every eukaryotic species has a specific number of chromosomes in the nucleus of each cell. For example, in humans, the chromosome number is 46, while in fruit flies, it is eight. Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide.
When the cell is in the growth and maintenance phases of its life cycle, proteins attach to chromosomes, and they resemble an unwound, jumbled bunch of threads. We call these unwound protein-chromosome complexes chromatin (Figure 18.104.22.168.4. Chromatin describes the material that makes up the chromosomes both when condensed and decondensed.
(A) This image shows various levels of chromatin’s organization, DNA tightly coiled into two thick cylinders and DNA coiled around proteins called histones. (b) Paired chromosomes, colored as pairs. Notice individuals of each pair are the same size to one another. (credit b: modification of work by NIH; scale-bar data from Matt Russell)