8 Chromosomal organisation in the eukaryotic nucleus
The average human cell has around two metres of DNA within its nucleus. In the interphase nucleus, in which transcription and replication are going on, this DNA is packaged into nucleosomes that are variably compacted, through association with H1, into larger 30nm fibres. In fact, the average nucleus most likely contains DNA with a continuum of chromatin configurations, ranging from highly open 10 nm fibres, through to 30 nm fibres and fibres that are even more tightly packed together, called chromonema fibres (Figure 39). The formation of these even larger fibres is probably facilitated by both the H1 linker histone and the tails of the core histones.
The nucleus is a dynamic site of activity and during each round of the cell cycle the entire genome must be duplicated and reassembled into chromatin. In addition to copying of the DNA, the cell must also ensure that the newly synthesized DNA is assembled in a chromatin structure that reproduces that of the original parent strands.
Having examined the structure of the DNA fibres themselves, we will now briefly examine the higher-order structures that the DNA is packed into within the nucleus, how these structures further compact the DNA and their role in gene regulation.