Vertebrate cells possess two different condensin complexes, known as condensin I and condensin II, that play a fundamental role in chromosome assembly and segregation during mitosis. Each complex contains a pair of structural maintenance of chromosomes (SMC) ATPases, a kleisin subunit and two HEAT-repeat subunits. Here we use recombinant human condensin subunits to determine their geometry within each complex. We show that both condensin I and condensin II have a pseudo-symmetrical structure, in which the N-terminal half of kleisin links the first HEAT subunit to SMC2, whereas its C-terminal half links the second HEAT subunit to SMC4. No direct interactions are detectable between the SMC dimer and the HEAT subunits, indicating that the kleisin subunit acts as the linchpin in holocomplex assembly. ATP has little, if any, effects on the assembly and integrity of condensin. Cleavage pattern of SMC2 by limited proteolysis is changed upon its binding to ATP or DNA. Our results shed new light on the architecture and dynamics of this highly elaborate machinery designed for chromosome assembly.