Molecular recognition occurs at all levels of living matter but the mechanisms are not understood in physical terms. One striking example is that of DNA whose properties are intimately related to the specific molecular interactions of four nucleosides, based on hydrogen-bonds and size complementarities. We have directly measured the interaction between two of them, adenosine and thymidine, using a surface force apparatus. In these experiments, lipids functionalised with nucleosides were synthesised, and used to coat the surfaces between which forces were measured. The interactions of complementary molecules were compared to those, markedly different, for which the complementarity was hindered by a small modification of one of the molecules. The distance range of the specific forces, deduced from this comparison, was surprisingly long. The adhesion energy of the surfaces covered by these nucleosides were highly specific. Binding energies obtained from these measurements were in good agreement with values from the literature. The results also show that without the size effect existing in DNA, H-bonds alone can generate the specificity. An unusual behaviour, attributed to the sticky and fluid character of the layers, was pointed out. A long-range non specific interaction, also unexpected, was found. These features observed on surfaces coated with chemical functional groups may partly result from a collective behaviour. They illustrate the variety of physical effects one can obtain by playing on the chemistry of a surface.