loop, which hybridizes to the noninvading a single strand, thus forming a small “ Aa ” heteroduplex region, and serves as a template to restore the missing bases on that strand. Filling in of gaps by polymerase activity and joining of DNA ends by ligase result in a peculiar struc-ture that looks like two single-stranded crossovers. Note that this structure also contains the single heteroduplex that we need. The single-stranded “crossovers” are called Holliday structures , after Robin Holliday, who ±rst proposed them in the 1960s. Physical (as opposed to genetic) evidence for Holliday structures has been obtained independently. They are unstable and must be resolved in one of two ways. Simply put, they can each be resolved by either “vertical” or “horizontal” breakage and reunion of single strands, as shown in the ±gure. One resolution results in a double-stranded reciprocal crossover (shown at left) and the other a noncrossover (right). Hence the association with crossovers is explained. Note that if the hetero-duplex formed via a double-strand break on the other chromatid, we could explain the 3:5 and 2:6 ratios, too. Overall, note again the use of molecular mechanisms
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