If biologically complex molecules as DNA were present in the extraterrestrial targets, their spectral signatures would be rather difficult to be unambiguously identified. As a matter of fact, the molecular array of a single nucleobasis will generate a tangled spectral signature. On the other hand, a part of it, e.g. bands due to the group of HNCO of guanine may have been detected but associated to smaller molecules, e.g. isocianic acid (HNCO). However, if comprised in a nucleobasis, its detection would be misinterpreted. Five key transitions were preliminarly selected for either purines and pyrimidines that should be observed together in the same target. If this happens, it may be that we are detecting the whole from its parts.