Extended Data Figure 2: DSB formation appears normal in SPO11-Flag and SPO11-PrA strains.

a, Southern blots probed for chromosome III. High molecular weight chromosomal DNA was purified 6 h after transfer to sporulation medium from meiotic rad50S cultures carrying the indicated SPO11 alleles (in spo11-yf the catalytic tyrosine 135 is mutated to phenylalanine), then separated on pulsed-field electrophoresis gels. Samples from a rad50S spo11-HA strain are shown for comparison; haemagglutinin-tagged Spo11 has reduced DSB frequency. Each lane represents an independent culture (SPO11⁺ samples from the same cultures were run on both gels). PrA, protein A. b, Quantification of blots in panel a and separate blots (not shown) probed for chromosomes VII or VIII. Break frequencies are per cent of DNA in lane (mean ± s.d. of 3–4 cultures). Numbers in parentheses indicate values from each tagged strain relative to SPO11⁺ for the same chromosome. Relative DSB frequencies at the bottom are averages across the three chromosomes assayed.