Bidirectional resection of DNA double-strand breaks by Mre11 and Exo1

Specific DNA double-strand breaks are made during meiosis by Spo11, which remains bound to the DNA ends. Mre11 is a nuclease that can act exonucleolytically at DNA ends and endonucleolytically at internal sites. Previous studies have defined a role for the endonuclease, but not exonuclease, activity in DNA repair. Matthew Neale and colleagues show that Mre11 first makes a nick 300 bases from the end of the 5' strand, after which Mre11 degrades the DNA towards the break. Meanwhile, a second nuclease, Exo1, degrades the same strand in the opposite direction. This demonstrates that exonucleases can be loaded when the DNA end that is usually required for their initial binding is blocked.