Nucleosome, molecular structure

Dimensions of the DNA and its higher-order molecular structures, the nucleosome and the basic liber, are given in Table 2. It is assumed that the genome of the mammalian cell con-... 

T. Richmond and his team, see also ref. 34, have obtained a X-ray structure that provides a detailed picture of the core particle. This is a remarkable feat, because with a molecular weight of 206 000 Da, about half of it protein and half DNA, the nucleosome core structure is, so far, the largest DNA- protein complex to have been resolved at atomic... 

Ribosomes are too large for the application of the techniques that are used for determining the molecular structures of enzymes and smaller complexes, such as nucleosomes. Thus, atomic-level information is not available. 

The crystallization and structural determination of the histone octamer was first reported in 1984 [34], However, the overall dimensions of the 3.3 A structure [15] did not appear to fit within the known X-ray structures of the nucleosome core particle [12,13], In an elegant analysis [16], re-examination of the original phasing of the histone octamer data revealed misplacement of the heavy atom site by 2.7 A. The structure was resolved, after which it was possible to build molecular models of the individual histones into the 3.1 A resolution electron density map of the histone core of the nucleosome [17]. Figure 2 shows the first atomic resolution model of the core histone octamer. Several additional publications followed in which the histone octamer structure formed the basis for constructing models of the NCP [17-21],... 

Histones are very basic proteins with an isoelectric point between 10.31 and 11.27 for human complement. They are present in virtually all eukaryotes (with the exception of dinoflagellates [14]) where they are associated with most of the nuclear DNA. The DNA is wrapped around an octamer formed by the four core histones H2A, H2B, H3 and H4 to build a nucleosome. This particle is the fundamental repeating unit of chromatin [15]. A string of nudeosomes can fold into a higher order structure, the exact molecular nature of which is still not fully understood but clearly has a strong influence on gene expression. 

Otwinowski, Z. and Yu, H.T. (2006) Structural basis for CoREST-dependent demethylation of nucleosomes by the human LSDl histone demethylase. Molecular Cell, 23, 377-387. 

Following the S phase is the G2 phase, in which the cell prepares for mitosis and cytokinesis. Although few of the molecular events that occur in G2 phase have been identified, it is known that some proteins synthesized during this period are essential for cell division. It is believed that contractile proteins necessary for chromosome separation and cytokinesis accumulate and are accompanied by modifications in the structure of chromosomal proteins. The phosphorylation of histone HI, known to be involved in the packaging of nucleosomes, could cause the initiation of chromosome condensation that directs the cell into mitosis. 

Figure 3.18 The Watson-Crick double helical structure of DNA illustrated by the crystal structure of an oligonucleotide. (a) Skeletal model representation, in stereo (note the tilting of the base pairs in certain cases this is responsible for causing the DNA double helix to coil up, for example, into the nucleosome, a key component of the chromosome). (b) Space filling representation, in stereo, (c) Beevers molecular model. Figures kindly provided by Dr W. N. Hunter with permission.

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