In early use of DNA fingerprinting, the technique was challenged, however, over concerns about sample contamination, faulty preparation procedures, and erroneous interpretation of the results. Efforts were made to improve its reliability, and today the technique has been refined through the use of more-specific and more-sensitive probes and better blotting membranes. It has been recognized that DNA fingerprinting, similar to other DNA-analysis techniques, is limited by the quality of the sample obtained. DNA samples that are degraded or collected postmortem typically produce less-reliable results than do samples that are obtained from a living individual.
If only a small amount of DNA is available for fingerprinting, a polymerase chain reaction may be used to create thousands of copies of a DNA segment. PCR is an automated procedure in which certain oligonucleotide primers are used to repeatedly duplicate specific segments of DNA. Once an adequate amount of DNA has been produced, the exact sequence of nucleotide pairs in a segment of DNA can be determined using one of several biomolecular sequencing methods. Automated equipment has greatly increased the speed of DNA sequencing and has made available many new practical applications, including pinpointing segments of genes that cause genetic diseases, mapping the human genome, engineering drought-resistant plants, and producing biological drugs from genetically altered bacteria.