Eukaryotic genomes are folded into loops and topologically associating domains, 
which contribute to chromatin structure, gene regulation, and gene 
recombination. These structures depend on cohesin, a ring-shaped DNA-entrapping 
adenosine triphosphatase (ATPase) complex that has been proposed to form loops 
by extrusion. Such an activity has been observed for condensin, which forms 
loops in mitosis, but not for cohesin. Using biochemical reconstitution, we 
found that single human cohesin complexes form DNA loops symmetrically at rates 
up to 2.1 kilo-base pairs per second. Loop formation and maintenance depend on 
cohesin's ATPase activity and on NIPBL-MAU2, but not on topological entrapment 
of DNA by cohesin. During loop formation, cohesin and NIPBL-MAU2 reside at the 
base of loops, which indicates that they generate loops by extrusion. Our 
results show that cohesin and NIPBL-MAU2 form an active holoenzyme that 
interacts with DNA either pseudo-topologically or non-topologically to extrude 
genomic interphase DNA into loops.