Homolog locus pairing is a transient, diffusion-mediated process in meiotic prophase

The pairing of homologous chromosomes in meiosis I is essential for sexual reproduction, yet pairing dynamics at individual homologous loci remain largely uncharacterized. We track individual homologous locus pairs at various stages of meiosis. We observe mean squared change in displacements (MSCDs) that exhibit the t^(1/2) power-law scaling behavior characteristic of polymer diffusion followed by a plateau-from which we can infer that homologous loci are weakly tethered to each other. We develop a theory for Rouse polymers with ``homologous'' linkages, and show that the plateau behavior is consistent with a handful of randomly-spaced linkages per chromosome forming over the course of meiosis. Brownian dynamics simulations of our linked polymers quantitatively reproduce the search times and pairing lifetimes observed at each successive stage of meiosis. These results suggest that homolog pairing is driven purely by chromatin diffusion, and that the DSB-dependence of homolog pairing comes not from the tagged loci experiencing DSBs, but instead from distal homologous linkages along the chromosome.