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.

Figure 12 Schematic diagram illustrating the ordered binding of DNA to histone proteins. The double helix is wound around a complex of histone proteins (shown in purple), forming a series of structures called nucleosomes, which look rather like beads on a string. This compacts the chromatin into a strand which has a width of 11 nm. The complex winds further and then loops and condenses. The greatest degree of chromosome condensation is seen in cells that are about to divide.

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The diagram shows the dimensions of the chromosome components from the bare, unfolded DNA double helix of diameter 2 nanometres to the highly compacted mitotic chromosome. The double helix is wound around a histone-protein complex, forming a succession of nucleosomes - an extended chromatin strand of width about 11 nanometres, The strand is wound into a spiral structure of width 30 nanometres, which is looped and condensed into a structure 300 nanometres wide, characteristic of cells during interphase. Further spiralling and condensation occurs in cells preparing to divide, when each member of a pair of chromatids has a diameter of about 700 nanometres.

Figure 12 Schematic diagram illustrating the ordered binding of DNA to histone proteins. The double helix is wound around a complex of ...

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.