4. Why do eukaryotes have special arrangement to accommodate DNA in nucleus? How are they arranged in prokaryotes?
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A tour of the cell
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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.