Filing Date: 2005-10-27.-- Priority Data ES P200402605 (2004-10-28)The invention relates to a biosensor which is obtained by immobilising peptide nucleic acid (PNA) molecules onto gold surfaces and to the uses thereof in the detection and characterisation of nucleic acid molecules (DNA or RNA) of biosanitary, environmental and dietary interest. The method of constructing said biosensors is based on the fact that PNA molecules can be immobilised in a controlled manner with one end thiolated onto monocrystalline or polycrystalline gold surfaces, such as to produce self-assembled monolayers (SAMs). The inventive biosensor can bind specifically to complementary DNA with a high sensitivity and a specificity that enables discrimination between DNA molecules which are differentiated in a single mutation or polymorphism and which do not require the pre-marking of the target molecule being analysed.