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Reciprocal Exchange leads to branched structures. (a) A single exchange leads to a Holliday junction. (b) Two exchanges lead to the DX molecule. (c) Exchange at every possible position generates the PX molecule. Note that the exchanges in (b) are between strands of the opposite polarity, but those in (c) are between strands of the same polarity. The exchange in (a) is shown to be between strands of opposite polarity, but the polarity of the strands does not matter in this case.
