The genome is highly compacted and concentrated, yet the timescales for the machinery of life are unexpectedly fast. For instance, the segregation of the intertwined sister chromatids in the anaphase of dividing cells occurs within a second. Topology controlling enzymes (topo II) are thought to play an essential role, but so far quantitative measurements of the effect on the viscoelasticity of DNA are lacking. We have studied the viscoelastic properties of solutions of lambda phage DNA immersed in buffer solution using video tracking of the Brownian motion of colloidal probe particles. The platform allows the measurement of the elastic storage and viscous loss moduli of minute samples of less than 50 microliters. We found that the viscoelastic response is critically dependent on the formation of entanglements among the DNA molecules with relaxation times on the order of seconds. We observed that topo II effectively removes these entanglements and converts the system from an elastic gel to a viscous fluid depending on the dissipation of energy (hydrolysis of ATP). Our work contributes to the understanding of energy dependent, non-equilibrium dynamics of biomolecules, which is a key feature of life. \par \begin{center}(séminaire exceptionnel)\end{center}