Final answer:
The main difference between the original Holliday model and the double-strand break model is that in the Holliday model, recombination begins with single-stranded breaks, while the double-strand break model begins with double-strand breaks.
Step-by-step explanation:
A key difference between the original Holliday model and the double-strand break model is the way the DNA strands are initially broken. The Holliday model of recombination was one of the earliest models proposed to explain the molecular mechanism of recombination. It describes a process starting with a single-stranded break (nick) in each of the DNA molecules, leading to strand invasion and the formation of a cross-stranded structure known as a Holliday junction. Branch migration and resolution would then lead to the characteristic recombination outcomes.
In contrast, the double-strand break repair model, which is a more recent understanding of homologous recombination, begins with a double-strand break. This model emphasizes the critical role of proteins and enzymes that process the broken ends to create overhangs that can then form H-bonds with a homologous DNA sequence. The endonuclease Sae2, the helicase Sgs1, and the MRX or MRN complex, among others, are implicated in this model for the repair of double-stranded breaks through homologous recombination.