A method for producing a single stranded DNA (ssDNA) molecule of a defined length and sequence is disclosed. This method enables the preparation of, inter alia, probes of greater length than can be chemically synthesized. The method starts with a double stranded molecule, such as genomic, double stranded DNA (dsDNA) from any organism. A fragment of the starting molecule (dsDNA) is amplified by specific primers engineered to introduce cleavage sites on either side of the desired sequence. Cleavage steps on the amplified, engineered fragment are combined with a phosphate removal step, thereby creating a construct that can be digested with an exonuclease without damage to the desired ssDNA. Probes, which hybridize with large gaps between the ends of the probes, are also disclosed.