Chromosomal Elements:- DNA Compaction Requires a Special Form of Supercoiling
Supercoiled DNA molecules are uniform in a number of respects. The supercoils are right-handed in a negatively supercoiled DNA molecule , and they tend to be extended and narrow rather than compacted, often with multiple branches (Fig. 1). At the super helical densities normally encountered in cells, the length of the supercoil axis, including branches, is about 40% of the length of the DNA. This type of supercoiling is referred to as plectonemic (from the Greek plektos, “twisted,” and nema, “thread”). This term can be ap plied to any structure with strands intertwined in some simple and regular way, and it is a good description of the general structure of supercoiled DNA in solution.
FIGURE 1 Plectonemic supercoiling. (a) Electron micrograph of plectonemically supercoiled plasmid DNA and (b) an interpretation of the observed structure. The purple lines show the axis of the supercoil; note the branching of the supercoil. (c) An idealized representation of this structure.
Plectonemic supercoiling, the form observed in isolated DNAs in the laboratory, does not produce sufficient compaction to package DNA in the cell. A second form of supercoiling, solenoidal (Fig. 2), can be adopted by an underwound DNA. Instead of the extended right-handed supercoils characteristic of the plectonemic form, solenoidal supercoiling involves tight left-handed turns, similar to the shape taken up by a garden hose neatly wrapped on a reel. Although their structures are dramatically different, plectonemic and solenoidal supercoiling are two forms of negative super coiling that can be taken up by the samesegment of underwound DNA. The two forms are readily interconvertible. Although the plectonemic form is more stable in solution, the solenoidal form can be stabilized by protein binding and is the form found in chromatin. It provides a much greater degree of compaction (Fig. 24–24b). Solenoidal supercoiling is the mechanism by which underwinding contributes to DNA compaction.
FIGURE 2 Plectonemic and solenoidal supercoiling. (a) Plectonemic supercoiling takes the form of extended right-handed coils. Solenoidal negative supercoiling takes the form of tight left-handed turns about an imaginary tubelike structure. The two forms are readily interconverted, although the solenoidal form is generally not ob served unless certain proteins are bound to the DNA. (b) Plectonemic (top) and solenoidal supercoiling of the same DNA molecule, drawn to scale. Solenoidal supercoiling provides a much greater degree of compaction.
