Supercoils: Reference Images



There are two types of supercoiling:

Plectonemically supercoiled DNA
    1. Plectonemic supercoils (“unrestrained”). The DNA in the above electron micrograph is said to be plectonemically supercoiled.


Characteristics of plectonemically supercoiled DNA (“unrestrained”)

    a. frequently branched
    b. Wr/DTw is constant and about 3
    c. length is independent of s and is ~40% ofthe couture length.
    d. the superhelix diameter varies with s at “physiological levels” of s, the superhelixaxis is extremely sensitive to small changesin supercoiling.
    e. the number of supercoils is directly proportionalto |s| and is Å 0.9DLk

    Two other (theoretical, not (yet) verified experimentally) properties:
    f. supercoiling increases the local concentration of two DNA sites in a DNA molecule by ~100x
    g. branching of the superhelix is enthalpically unfavourable but is driven by entropy.
torroid

    2.Toroidal supercoils (“restrained” by proteins)

Characteristics of Toroidal supercoils:

    a. compact (ratio is ~10x).
    b. writhe and twist may vary, e.g., they’re not “linked”.
    c. length becomes shorter with more turns.
    d. usually involved with proteins – often wrapping.

NOTE THAT the plectonemic & torroidal molecules are drawn to scale – that is, the torroidal supercoiling is much more efficient at compacting the DNA.

Inside a bacterial cell, both types of supercoiling are present.

We have done experiments to measure unrestrained supercoiling in plasmid DNA (small circles of about 3000 bp), and have found that very close to half of the supercoils are constraned by proteins in vivo. A cartoon of a the distribution of supercoiling in a 3000 bp plasmid is shown below. If you were to strip off all of the proteins, and measure the supercoiling, there would be 20 supercoils in this plasmid. Most DNA isolated from E.coli has roughly this level of supercoiling.