The absorption of UV light by DNA may cause carcinogenic changes. This occurs because chemical reactions, susceptible to deteriorate the genetic code, are triggered by the excess energy deposited by the UV absorption. Up to now, it was not known how this excess energy was distributed among the bases of the DNA. It was only postulated that each base absorbs a photon individually. By using ultrafast laser pulses, we have now shown that this assumption is not valid (J. Amer. Chem. Soc., 127, 2005, 17130). Our experimental results are instead explained by a collective behaviour of the bases: a certain number of them, being strongly coupled, absorb simultaneously the UV photon. The excitation is subsequently transferred to other parts of the double helix, losing energy at each step. This relaxation phenomenon (intraband scattering cf. Figure) takes place in 10-13s. This is the first time that such a collective behaviour in DNA is brought to attention, and this was only made possible by using physical chemistry methods. These findings may make it possible to understand better why the UV-induced damages appear at specific sequences of the DNA helix rather than at others.