Holliday junction

A Holliday junction is a branched nucleic acid structure that contains four double-stranded arms joined together. These arms may adopt one of several conformations depending on buffer salt concentrations and the sequence of nucleobases closest to the junction. The structure is named after the molecular biologist Robin Holliday, who proposed its existence in 1964.

In biology, Holliday junctions are a key intermediate in many types of genetic recombination, as well as in double-strand break repair. These junctions usually have a symmetrical sequence and are thus mobile, meaning that the four individual arms may slide through the junction in a specific pattern that largely preserves base pairing. Additionally, four-arm junctions similar to Holliday junctions appear in some functional RNA molecules.

Immobile Holliday junctions, with asymmetrical sequences that lock the strands in a specific position, were artificially created by scientists to study their structure as a model for natural Holliday junctions. These junctions also later found use as basic structural building blocks in DNA nanotechnology, where multiple Holliday junctions can be combined into specific designed geometries that provide molecules with a high degree of structural rigidity.

Holliday junctions may exist in a variety of conformational isomers with different patterns of coaxial stacking between the four double-helical arms. Coaxial stacking is the tendency of nucleic acid blunt ends to bind to each other, by interactions between the exposed bases. There are three possible stacking conformers: an unstacked form and two stacked forms. The unstacked form dominates in the absence of divalent cations such as Mg²⁺, because of electrostatic repulsion between the negatively charged backbones of the strands. In the presence of at least about 0.1 mM Mg²⁺, the electrostatic repulsion is counteracted and the stacked structures predominate. As of 2000, it was not known with certainty whether the electrostatic shielding was the result of site-specific binding of cations to the junction, or the presence of a diffuse collection of the ions in solution.

...
Wikipedia