A DNA strand can be made up of thousands of nucleotides joined by a backbone of phosphate sugars. Chromosomes typically are constructed of two strands of DNA twisted together in the shape of a double helix. Probes can be employed, for example, to bind to the genetic material of microbes for purposes of detection, identification, or, in some cases, inactivation. DNA fingerprints illuminate the genetic differences between and among individuals and are used in fields ranging from agricultural research to law enforcement. The extent of the variation depends on the target region of DNA being compared.
Some regions appear to be common to all forms of life, while others are highly variable.
This technique was made possible by the Nobel Prize-winning development of molecular cloning techniques for replicating genetic material. PCR has been instrumental in major breakthroughs in diagnostic kit development, forensic medicine, and detection of genes associated with inborn errors of metabolism. DNA is made of only four nucleotides, in varying sequences, so it can be modified more easily than can proteins, which are difficult to alter using standard chemical means. Protein engineering was the basis for a Nobel prize in 1994.