The present invention provides a method for determining nucleic acid sequences of a template nucleic acid that requires no prior knowledge of the nucleic acid sequence present in the template nucleic acid. The method is based on combining information about the mass of a fragment, the mass of any one nucleotide and the combinations thereof, and the sequence specificity of a nucleotide cutter, either enzymatic or chemical cutter, to determine a sequence of a nucleic acid fragment. This method allows for de novo detection of sequences in a target nucleic acid without requiring any prior sequence information. This method is called Partial Sequencing by Fragmentation (PSBF) and it works by fragmenting a target into oligo- or polynucleotides whose masses or lengths are uniquely associated with known sequences. The identities of these sequences are determined solely by the specific fragmentation method used, and are always independent of the target. PSBF can be implemented using electrophoresis, mass spectrometry or any other method that can be used to distinguish the size of the cut nucleic acid sequence fragments.