Supercoiling of DNA

Figure 36-19. Supercoiling of DNA. A left-handed toroidal (solenoidal) supercoil, at left, will convert to a right-handed interwound supercoil, at right, when the cylindric core is removed. Such a transition is analogous to that which occurs when nucleosomes are disrupted by the high salt extraction of histones from chromatin.

Rodriguez, A.C. and Stock, D. (2002) Crystal structure of reverse gyrase insights into the positive supercoiling of DNA. EMBO J. 21, 418 26. 

Irinotecan is a topoisomerase-I inhibitory agent that prevents the initiation and elongation of RNA transcription, DNA replication, and supercoiling of DNA (44). Investi-... 

Fluoroquinolones are inhibitors of bacterial topoisomerase 11 (DNA gyrase). This enzyme reduces the supercoiling of DNA in order to allow separation of the two strands of DNA that are required for replication and transcription. In the first stage, topoisomerase II cleaves the two strands of DNA, after repairing it, and restores the supercoiling of DNA. ... 

FIGURE 24-11 Supercoiling of DNA. When the axis of the DNA double helix is coiled on itself, it forms a new helix (superhelix). The DNA superhelix is usually called a supercoil. 

Brown PO.. and N.R. Cozzarelli "A Sign Inversion Mechanism for Enzymatic Supercoiling of DNA." Science. 206. IIX - 1083 < t979]... 

Topoisomerase I catalyzes the relaxation of negatively supercoiled DNA no coreactants are needed. Topoisomerase II catalyzes the negative supercoiling of DNA, using ATP as a coreactant, and it is also able to relax positive supercoiling (Chap. 7). 

Onishi et al. (1993) developed an assay that monitors the relaxation of super-coiled plasmid DNA induced by DNA topoisomerase. Supercoiling of DNA is important in the formation of chromatin and in determining functions of DNA, including replication, transcription, recombination, and repair. 

Kimura K, Hirano T. ATP-dependent positive supercoiling of DNA by 13S condensin a biochemical implication for chromosome condensation. Cell 1997 90 625-634. 

FIGURE 43-3 From Cozzarelli NR DNAgyrase and the supercoiling of DNA. Science 207 953-960,1980. Reprinted with permission from AAAS. 

The text returns to the topic of the flow of genetic information and considers the detailed biochemical mechanisms underlying this complex process. Chapter 5 introduced you to DNA and RNA and outlined the storage, duplication, and expression of genetic information. In Chapter 6, the enzymes and techniques used to analyze, construct, and clone DNA were presented. You should review these chapters to prepare for studying Chapter 27. Pay particular attention to DNA structure, the supercoiling of DNA, and DNA polymerase I in Chapter 5 and to DNA ligase in Chapter 6. 

Another hypothesis suggests that the structure of DNA is altered as a result of histone-DNA association, and supercoiling of DNA occurs (Richards and Pardon, 1970). It was suggested that RNA-polymerase is not able to transcribe those regions which are supercoiled (Johns, 1972). 

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