Interphase chromosomes, chromatin fibers

Fig. 46, Folded fiber model of the chromosome (Du Praw, 1965a, 1965b, 1966). a) Each interphase chromatin fiber consists of a DNA molecule held in the form of a secondary helix by means of its protein envelope b) replication of the fiber takes place successively from both ends toward the middle c) after replication the daughter fibers are folded into a condensed, compact chromosome. It is postulated that this folding is brought about by contractile protein molecules in the membrane d) scheme of chromatid stracture.

During the cell cycle, chromosome structures shuttle between de-condensed interphase and condensed mitosis states. Dynamic changes also occur at the lower levels of architectures, i.e., at the chromatin and nucleosome levels. Upon gene activation and inactivation, folding and unfolding of the nucleosome structure and the chromatin fibers occur at limited loci of the genome. Namely, the structures of the chromosome are dynamic and mobile. Nevertheless, there are basic structural units that remain stable and constitute the fundamental chromosome architecture. 

Chromatin compaction - Wrapping DNA about histone cores to form nucleosomes (see here) accomplishes part of the compaction necessary to fit the long eukaryotic DNA into the nucleus. However, much of the chromatin in the nucleus is even more highly compacted. The next stage in compaction involves folding the beaded fiber into a thicker fiber like that shown in Figure 28.12. These fibers may be further folded on themselves to make the thicker chromatin fibers visible in both metaphase chromosomes and the nuclei of nondividing (interphase) cells. 

Figure 1. Hierarchical model of chromosome structure, (a) In interphase cells, DNA is packed in a nucleus as forming nucleosome and chromatin, (b) DNA forms nucleosome structure together with core histone octamer, which is then folded up into 30nm fiber with a help of linker histone HI. This 30 nm fiber is further folded into 80 nm fiber and 300 nm loop structures in a nucleus. In mitosis, chromosome is highly condensed. Proteins which are involved in each folding step are indicated above and non-protein factors are indicated below, (c) The amino acid sequences of histone tails (H2A, H2B, H3 and H4) are shown to indicate acetylation, methylation and phosphorylation sites. (See Colour Plate 1.)...

Two major techniques have been used to study the dimension of chromosome fibers by electron microscopy. The first is the surface spreading method, in which the cells are spread on a water-air interphase, and all the intracellular components are dispersed. But the chromatin and the spindle remain close together and can be picked up on a grid, which can then be prepared for electron microscopic examination. The second technique is the thin-sectioning method. After fixation the tissue is embedded in plastic and cut less than 1000 A thick. Refer to specialized texts for descriptions of these techniques [119]. The important finding is that with the first method, the dimension of the fibers diameter is estimated to be 200-300 A, whereas it is only 80-180 A with the second. 

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