Chromosomes supercoiling

The native form of chromatin in cells assumes a higher order stmcture called the 30-nm filament, which adopts a solenoidal stmcture where the 10-nm filament is arranged in a left-handed cod (Fig. 5). The negative supercoiling of the DNA is manifested by writhing the hehcal axis around the nucleosomes. Chromatin stmcture is an example of toroidal winding whereas eukaryotic chromosomes are linear, the chromatin stmctures, attached to a nuclear matrix, define separate closed-circular topological domains. 

Bacterial as well as eukaryotic chromosomes contain too much DNA to fit easily into a cell. Therefore, the DNA must be condensed (compacted) to fit into the cell or nucleus. This is accomplished by supercoiling the DNA into a highly condensed form. When relaxed circular DNA is twisted in the direction that the helix turns, the DNA becomes positively supercoiled, if it is twisted in the opposite direction, it is called negatively supercoiled. Bacterial DNA is normally found in a negatively supercoiled state. Supercoiling reactions are catalyzed by topoisomerases. 

The condensin complex has been identified in the same scaffold fraction (Maeshima and Laemmli, 2003) and shown to be essential for the mitotic chromosome condensation (Hirano et al, 1997). The frog condensin complex exhibits ATP-dependent DNA-supercoiling activity (Kimura and Hirano, 1997). It consists of a heterodimer of SMC and a trimer of non-SMC proteins (Hirano et al, 1997). The SMC complex has a globular head domain and a coiled-coil tail region (Anderson et al., 2002 Melby et al, 1998 Yoshimura et al., 2002). In vertebrates, two types of condensin complex, condensin I and condensin II, exist they are composed of the same SMC subunits but with different non-SMC subunits (Ono et al, 2003). 

Y. Saitoh, and U. K. Laemmli, From the chromosomal loops and the scaffold to the classic bands of metaphase chromosomes. Cold Spring Harb. Symp. Quant. Biol. 58, 755-765 (1993). A. S. Belmont, and K. Bruce, Visualization of G1 chromosomes A folded, twisted, supercoiled chromonema model of interphase chromatid structure. J. Mol. Biol. 127, 287-302 (1994). 

Belmont, A.S. and Bruce, K. (1994) Visualization of G1 chromosomes a folded, twisted, supercoiled chromonema model of interphase chromatid structure. J. Cell. Biol. 127(2), 287-302. 

Electron microscopy shows that DNA consists of either linear or circular structures. The chromosomal DNA in bacteria is a closed circle, a result of covalent joining of the two ends of the double helix (Figure 10.11). Note the presence of supercoils, branch points, intersections, and the generally thin and open structure. The chromosomal DNA in eukaryotic cells, like ours, is believed to be linear. 

HGURE 24-35 Model for the effect of condensins on DNA super-coiling. Binding of condensins to a closed-circular DNA in the presence of topoisomerase I leads to the production of positive supercoils (+). Wrapping of the DNA about the condensin introduces positive supercoils because it wraps in the opposite sense to a solenoidal supercoil (see Fig. 24-24).The compensating negative supercoils (—) that appear elsewhere in the DNA are then relaxed by topoisomerase I. In the chromosome, it is the wrapping of the DNA about condensin that may contribute to DNA condensation. 

Isolated "naked" bacterial DNA, from which proteins have been removed, is supercoiled. DNA in the bacterial chromosome is also supercoiled. When naked DNA is nicked, its supercoiling is abolished. In contrast nicking the chromosomal DNA does not abolish its supercoiling. Explain. 

I I Electron micrographs of the E. coli chromosome sug-jj gcst a folded circular structure containing about 40-f 100 supercoiled loops (diagrammatically indicated in fig. 25.16). It is believed that the folded structure is held together by an RNA-protein core, although the manner in which this is done is not well understood. The structure is further stabilized because the core forms a complex with positively charged polyamines and certain basic proteins. D. E. Pettijohn and his co-workers have provided evidence of such a core. They first showed that the individual super-... 

The E. coli chromosome exists as a circular, folded, supercoiled duplex (a). This can be converted to a partially unfolded structure by brief treatment with RNase (c). There are 50-100 loops in the structure supercoiling may be selectively eliminated from individual loops by single-strand nicking of the DNA within the loop (b). 

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