The arrangement of DNA in the nucleus
You will recall that in eukaryotes, DNA is organised into chromosomes, each of which contains a single, very long double-stranded DNA molecule. These molecules are much longer than the diameter of the nucleus, and are packaged into the nucleus by winding around proteins known as histones, which help to coil the DNA, as shown in Figure 12. This complex of DNA and proteins is known as chromatin. The binding of DNA to histones is very ordered and the degree of packing varies. The DNA is most tightly condensed in mitotic chromosomes (as shown in Figure 12) just before cell division, when the highly condensed chromosomes can be easily seen under the light microscope. The rest of the time, individual chromosomes are not visible by light microscopy because they are far less condensed, allowing easy access to the enzymes that carry out DNA replication and transcription. Constant adjustment of DNA packaging and the spacing between nucleosomes is one way in which gene expression is controlled.
If you look at Figure 11a, you see that some areas of chromatin (particularly around the edges of the nucleus) have a dark, electron-dense appearance, while other areas of the nucleus appear paler. The chromatin in the dark areas is known as heterochromatin and is more highly condensed than the DNA in the paler areas which is called euchromatin. In general, heterochromatin contains far fewer genes and is far less 'transcriptionally active' than euchromatin.