When a cell is not dividing, its DNA is loosely organized into a structure known as chromatin, which appears as a shaded area in the nucleus when viewed through a microscope. A cell's DNA is never found as a naked molecule. Chromatin is made up of DNA and several kinds of proteins, RNAs, and even lipid molecules. Part of the chromatin is in long fibers called euchromatin, which is where genes are expressed (transcribed into messenger RNA then translated into a protein product - see below). The rest is compacted into relatively inert heterochromatin . As a cell prepares to divide, transcription stops, and the chromatin dramatically condenses and assembles into species-specific structures called chromosomes.

If the DNA in all 23 human chromosomes were stretched out and linked end to end, it would create a string over 2 meters long. Yet, all this DNA, plus the associated proteins, must fit into a nucleus that is only 1/1,000,000 of a meter in diameter. This remarkable degree of compaction is accomplished by supercoiling the DNA. Imagine twisting an already coiled telephone cord; it first becomes twisted upon itself, and then the twists themselves become twisted. This is what happens to form chromatin. Short regions (~200 base pairs) of the DNA are wrapped around a set of globular proteins called histones (Figure 2-A), then the DNA/histone complex - the nucleosome (2-B)- is further coiled to form chromatin. As a cell begins to divide, the chromatin loops back on itself and organizes into structures of increasing complexity until it forms a chromosome (2-C) with the characteristic X-like appearance (at least in multi-cellular organisms; bacteria have circular chromosomes).