= Holliday junction
{c}
A <Holliday junction> is a four-way <DNA> branch in which strands exchange pairing partners between homologous duplexes. It can occur as an intermediate in <homologous recombination>. The molecular configuration is distinct from a final <chromosomal crossover>, which is classified by the exchange of flanking chromosome arms. <Robin Holliday>'s 1964 model connected strand exchange, hybrid <DNA> and <gene conversion>.<A mechanism for gene conversion in fungi by Robin Holliday>[Section1, pages284–286 and Figure1]{ref}
For a schematic example, label homologous regions by distant markers $L\cdots R$ and $l\cdots r$. Alternative strand-pair cleavage and rejoining routes at a junction can preserve the parental combinations $L\cdots R$, $l\cdots r$, or produce the exchanged combinations $L\cdots r$, $l\cdots R$. Local mismatches and their repair can change an allele independently of that distant-marker outcome. The historical diagram illustrates possibilities; it does not establish equal frequencies for these outcomes in every cell.<A mechanism for gene conversion in fungi by Robin Holliday>[Figure1, alternatives4a/4b and accompanying outside-marker discussion]{ref}
Later work sought physical evidence. <H. Potter> and <D. Dressler> isolated colicinE1 plasmid <DNA>, linearized it with the <restriction enzyme> EcoRI, and examined it by <electron microscopy>. They found crossed structures containing two genome lengths, joined at homologous regions. Such forms occurred in wild-type and tested RecB-C hosts but were absent in their recombination-deficient RecA hosts. They interpreted these controlled observations as intermediates of the Holliday model.<On the mechanism of genetic recombination: electron microscopic observation of recombination intermediates by H. Potter and D. Dressler>[Abstract and original micrographs, pages3000–3004]{ref}
The combination of shape, DNA content and a genetic control is stronger evidence than an X-shaped image alone. A <Holliday junction> also need not be an obligatory step in every <noncrossover recombination> pathway; the outcome and the route producing it remain separate questions.
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