During meiosis Spo11-induced twin strand breaks (DSBs) are processed into crossovers ensuring segregation of homologous chromosomes (homologs). ATPase. Processing pathways controlled by Mec1ATR kinase take over these functions only above a distinct DSB threshold resulting in progressive strengthening of homolog bias. We conclude that Tel1ATM/Pch2 and Mec1ATR DNA damage response pathways are sequentially activated during wild-type meiosis due to their distinct sensitivities to global DSB levels. Relative DSB order controls DSB repair pathway choice and ultimately recombination outcome. (hereafter) form at high Gentamycin sulfate (Gentacycol) frequency within a narrow zone of ～150 bp making this hotspot suited for analysis of homolog bias and DSB resection (Xu and Kleckner 1995 Kim et al. 2010 Parental homologs (“Mom” and “Dad”) as well as recombination intermediates and products are detected by Southern blot analysis (Figure 1A). DSBs and COs are detected on one-dimensional (1D) gels (Figure 2G) whereas joint molecules (JMs) are analyzed on 2D gels. JMs comprise dHJs and SEIs (Figure 1A). Importantly relative abundances of dHJs between sister chromatids (IS-dHJs) compared to those between homologs (IH-dHJs) can be used to infer recombination frequencies between sister chromatids and homologs (Kim Gentamycin sulfate (Gentacycol) et al. 2010 Here relative IS-dHJ and IH-dHJ abundances are expressed as “IS:IH ratios”. Figure 1 Gradual Establishment of Homolog Bias during Wild-Type Meiosis Figure 2 Incomplete Homolog Bias at Low Global DSB Levels during Early Wild-Type IL9 antibody Meiosis To examine whether homolog bias is fully in place at the time when dHJs first appear recombination was monitored in synchronous meiotic WT cultures at 33°C (B?rner et al. 2004 At 33°C DSBs and JMs appear and disappear in the expected order followed by accumulation of CO products (Figure 1C). Both IH-dHJs and IS-dHJs typically become detectable at t～3 h reach peak levels at ～5 h and have essentially disappeared by ～9 h (Figure 1B D). When JMs are abundant (t = 4 Gentamycin sulfate (Gentacycol) h to 8 h) prevalence of IH-dHJs over IS-dHJs indicates robust homolog bias (Figure 1A B). Surprisingly IS-dHJs are more abundant relative to IH-dHJs when JMs are first detected compared to later time points (Figure 1B). Quantitative comparison confirms higher IS:IH ratios of 1 1:1.5 at the time of initial dHJ detection (“early”) followed by a decrease to 1 1:4 as dHJs reach peak levels (“max”; Figure 1F). IS:IH ratios remain ～unchanged thereafter (Figure 1F G; for reproducibility see Figure S2). Kinetic analysis confirms that IS-dHJs reach half maximum levels before IH-dHJs whereas both species disappear concurrently. Moreover IS-dHJs accumulate prior to IH-dHJs in a strain background that blocks turnover of all dHJs (was carried out in a meiotic culture also assayed for Rad51 foci Gentamycin sulfate (Gentacycol) as indicator of nucleus-wide DSB abundance. Zip1 set up into synaptonemal complicated (SC) was utilized to monitor meiotic development. (B?rner et al. 2004 Four classes of nuclei with specific localization patterns of Rad51 aswell as Zip1 represent known phases of meiotic development (Shape 2A). In (leptotene) nuclei little if any linear Zip1 staining and ≤ fifty percent maximum degrees of Rad51 foci (we.e. 4-16) indicate that just a few DSBs possess shaped. nuclei contain higher global DSB amounts (≥17 Rad51 foci) aswell as more intensive linear Zip1 constructions in a few (zygotene) nuclei (～40%). nuclei show fewer (4-16) Rad51 foci as well as intensive linear Zip1 constructions recommending that in these past due zygotene or pachytene nuclei recombination offers progressed at night DSB stage for the most part loci. Lastly in (pachytene) nuclei exhibiting intensive linear Zip1 localization few if any recombination occasions remain in the DSB stage (≤ 3 Rad51 foci). Rad51 positive cells (Rad51+ composed of a with early instances (t = 1.66-2.33 h; Shape 2D) accompanied by the maximum at t = 2.66 h and appearance of and nuclei thereafter (t = Gentamycin sulfate (Gentacycol) 3 h). Significantly high great quantity of nuclei at early instances claim that cells with low global DSB great quantity persist for a considerable time ahead of appearance of mass DSBs. Physical evaluation of recombination in the same meiotic tradition reveals impressive coincidence between Rad51+ nuclei and DSBs at (Shape 2E G). Many cells form DSBs in during additional.