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The DNA double-strand break repair complex Mre11-Rad50-Nbs1 (MRN) detects and nucleolytically processes DNA ends, activates the ATM kinase, and tethers DNA at break sites. How MRN can act both as nuclease and scaffold protein is not well understood. The cryo-EM structure of MRN from Chaetomium thermophilum reveals a 2:2:1 complex with a single Nbs1 wrapping around the autoinhibited Mre11 nuclease dimer. MRN has two DNA-binding modes, one ATP-dependent mode for loading onto DNA ends and one ATP-independent mode through Mre11's C terminus, suggesting how it may interact with DSBs and intact DNA. MRNs two 60-nm-long coiled-coil domains form a linear rod structure, the apex of which is assembled by the two joined zinc-hook motifs. Apices from two MRN complexes can further dimerize, forming 120-nm spanning MRN-MRN structures. Our results illustrate the architecture of MRN and suggest how it mechanistically integrates catalytic and tethering functions. [Display omitted] • Cryo-EM structure of full-length, eukaryotic Mre11-Rad50-Nbs1 bound to ATPγS • A single Nbs1 polypeptide wraps around and stabilizes the Mre11 dimer • ATP-dependent and -independent binding modes for DNA ends and internal DNA • Rad50 coiled-coil apex tetramerization forms MRN-MRN tethering structures Rotheneder et al. determine the structure of eukaryotic Mre11-Rad50-Nbs1, reveal the basis for Nbs1 interaction with the Mre11 dimer, uncover ATP-dependent and -independent DNA-binding modes for DNA ends and internal DNA, and reveal that Rad50 coiled-coil apex tetramers form DNA-tethering structures in DNA double-strand break repair. [ABSTRACT FROM AUTHOR]