Sunday, April 17, 2011

Development of Forensic DNA Typing

Starting in the 1980s, methods were developed that made DNA sequencing much easier and faster than it had been. These methods could be automated, enabling a very large amount of sequence data to be collected in many laboratories throughout the world. With these methodologies in place in the larger research centers, the U.S. government and a private company made commitments to pursue sequencing the entire human genome. This task, one of the more impressive in the history of biological science, was completed in the late 1990s.

As sequence data accumulated it became clear that a large fraction of human DNA was not involved in specifying protein structure; that is, it did not seem to have any obvious function. It has been called “junk DNA” for this reason. Much, but not all, of this nonfunctional DNA has repetitive sequences. There are several different types of repeat-sequence DNA, but the most important for purposes of human identification, and thus forensic purposes, is a kind that has head-to-tail sequences repeated one after the other along the DNA strand. Many regions in the human DNA have this type of repeated sequence structure, known as tandem repeats. The reason tandem-repeated DNA is useful for human identification is that there is a lot of variation among different people in the number of times the sequence is repeated.

Jeffreys was studying regions of DNA that have the head-to-tail repeat sequences when he developed the first forensic DNA typing. Under certain lab conditions many of these regions can be examined simultaneously using a technique called restriction fragment length polymorphism (RFLP). RFLP will be fully described in chapter 3, but briefly, the technique generates a series of parallel light and dark lines on X-ray film. It looks something like a bar code. Every individual has a different DNA “bar code,” and this observation forms the basis of forensic DNA typing—that is, DNA typing for human identification.

Since the 1980s, when the earliest versions of forensic DNA typing were developed, several new technologies have been developed and implemented. The most important breakthrough was using a technique called polymerase chain reaction (PCR) to make multiple copies of small quantities of DNA in a specimen for subsequent typing.