Figure 1

Unequal SCE assay. A. The his3 substrate for measurement of unequal SCE. The his3-Δ3' construct is marked with a tail, and the his3-Δ5' construct is marked with an arrowhead. The shaded region indicates the regions shared by the two deletion constructs. Expanded region under the linear map represents the palindromic insertion. B. DSB repair by gene conversion. A DSB is formed when the replication fork has stalled at the secondary structure. Although a secondary structure can form on both the lagging and the leading strand, the discontinuous nature of DNA synthesis is likely to facilitate formation of greater amounts of secondary structures on the lagging strand than on the leading strand. Shown here is the repair of a DSB formed on the lagging strand via unequal SCE, using the sister chromatid as a template. The unequal SCE events generate a wild-type HIS3 gene. DSBs can also be repaired by equal SCE. However, equal SCE will not give rise to a wild-type HIS3 gene. DSBs are likely to form via the endonuclease activity of a structure-specific nuclease.