We present density functional theory (DFT) and time-dependent DFT (TD-DFT) study of the structures and electronic spectra of small CdSe nanocluster-adenine complexes CdnSen-adenine (n = 3, 6, 10, 13). We examine the changes in the geometries and excitation spectra of the nanoclusters induced by DNA base-binding. By comparing the results calculated for the bare (CdnSen), hydrogen -passivated (CdnSenH2n), as well as the corresponding adenine (Ade)-bound clusters (CdnSen-Ade, CdnSenH2n-Ade, CdnSenH2n-2-Ade), we find that binding with Ade slightly blue-shifts (Up to 0.18 eV) the electronic excitations of bare nanoclusters but strongly red-shifts (< 1.2 eV) those of hydrogen-passivated nanoclusters. Natural bond orbital analysis shows that the LUMO of CdnSenH2n-Ade is a pi* orbital located oil the purine ring.