Fig 7. Model for formation of unstable chromosomes in the telomere.

A Cdc13 defect leads to a defect in replication of one homolog. The defect leads to extensive ssDNA of the lagging strand template sister, and a complete leading strand template sister. The lagging strand template is nicked and acts as the parental primer strand, which invades an intact homolog in single copy sequence. Two outcomes are possible: replication to the end of the chromosome or re-initiation of BIR. BIR may again abort, in which case the parental primer 3’ may shorten, invade the homolog, and attempt replication again.