Final answer:
The initiation of recombination differs in the Holliday model and the double-strand-break model, involving differences in breakage mechanisms and recombination steps.
Step-by-step explanation:
The main differences in the initiation of recombination proposed by the Holliday model and the double-strand-break (DSB) model involve the mechanism of breakage and the steps involved in recombination. In the Holliday model, recombination is initiated by the formation of a DNA double-strand break, followed by the formation of a Holliday junction and branch migration.
On the other hand, the DSB model proposes that recombination is initiated by the formation of a DNA double-strand break, which is processed by nucleases to generate 3' single-stranded DNA ends, which then invade a homologous DNA sequence for repair.