ABSTRACT
Most of the DNA of mammalian cells is sequestered in the cell nucleus, a structure which is a defining feature of eukaryotes, distinguishing them from prokaryotes such as bacteria. In the cell nucleus, a vast amount of DNA is arranged in a spatially organized fashion defining the domains of individual chromosomes and providing a framework for replication, transcription, and repair. Just before cell division, DNA and the associated proteins which are together called chromatin, become ‘‘condensed’’ and can be stained and viewed under the light microscope as rod-like structures called chromosomes. The human genome consists of 23 pairs of chromosomes (including one pair of sex chromosomes) giving a total of 46 for every diploid cell. The hierarchical structure of chromatin has been discussed in Chapter 9.
The condensed form of the chromatid in the metaphase chromosome represents the ultimate level of chromatin organization and, in fact, exists in this state for only a very small proportion of the life of a dividing cell. For most of the time the nucleus is in the interphase mode, with the 30 nm chromatin filaments which form approximately 50,000 loop domains attached at their bases to the inner portions of the nuclear matrix, in a relaxed conformation (Figure 9.1). The transition to the metaphase chromatid requires that the 60 kbp loops wind into 18 radial loops to form miniband units that in turn are continuously wound and stacked along a central axis to form each chromatid (Figure 10.1).
