What is DNA?
DNA stands for deoxyribonucleic acid, the substance that makes up chromosomes and carries the genetic code that determines the characteristics of all living things. In humans, it determines such characteristics as eye color, hair texture, skin pigmentation, and the propensity to develop, or be immune from, disease.
A DNA molecule comprises two long, interlocking chains. The links of the chains are called bases, and the sequence of the bases is different in each person's DNA, except in the cases of identical twins. The bases are too small to be seen even with the most powerful microscope, so it is not possible to compare one sequence to another visually. However, in the early 1980s, a renowned British geneticist, Dr. Alec Jeffreys of the University of Leicester, invented a radioactive probe that did the same thing: It made it possible to capture a DNA fingerprint on film.
The Jeffreys process used an enzyme as a sort of chemical scissors to cut the DNA chain at specific points, known as restrictions sites. A restriction site is a specific sequence of six bases, and there are tens of thousands of restriction sites in each chromosome. Their distribution is unique for each person, except for, as noted, identical twins.
After the DNA is cut, the resulting fragments of the chain may be subjected to a technique known as electrophoresis, in which electric current propels the fragments through a gel. Because short fragments move faster than long ones, electrophoresis sorts the fragments according to length.
The distribution of the varying lengths of fragments in a sample, similar to the distribution of whorls and loops in a fingerprint, is said to be polymorphic, which means that it varies significantly from person to person. Although the fragments are much larger than bases, they are nonetheless invisible, like a fingerprint before dusting.
Once the fragments are sorted by electrophoresis, Jeffreys discovered that a series of identical radioactive probes could be introduced. The probes, which Jeffreys patented, bind only to a specific sequence of bases. Because there are thousands of fragments of varying lengths in every sample, the specific sequence to which the radioactive probes could bind inevitably were found in some DNA fragments but not in others.
Photographic film could then be exposed to the sample, and a distinctive pattern would appear, resembling the bar code used to price items in a supermarket. The pattern showed the distribution of the fragments, making it possible to compare the pattern in one sample to that in another.
As with fingerprints, the pattern would be the same if the samples came from the same person, or an identical twin, but different if the samples came from different persons. The chance of two unrelated persons having the same pattern deduced by the Jeffrey's probes was one in several billion.
The technique, known as RFLP, for restriction fragment length polymorphism, was extremely useful and veritably infallible in determining the parentage because of children, who inherit half of their DNA from each parent, but of limited use in forensic applications. The reason was that RFLP analysis required high molecular weight DNA, or genetic substance that had not lost mass through deterioration due to moisture, bacteria, or heat. RFLP worked perfectly with fresh blood samples, normally available for paternity testing. Trace evidence recovered at crime scenes or from crime victims was seldom pristine, however, often not suitable for RFLP analysis.
A few years after Jeffreys developed RFLP, the Cetus Corporation of California developed and patented a different technique of DNA analysis capable of obtaining a result from a degraded sample and, thus, more amenable to forensic application. Instead of cutting DNA into fragments, the technique induced a chemical chain reaction, which isolated a specific sequence of genetic information and, literally, made copies of that sequence.
Known as PCR, for polymerase chain reaction, the Cetus technique initially was far less exacting than RFLP. At the time it was patented in the mid 1980s, it was capable of categorizing DNA into only 21 different types. The probability of a random match ranged from one in seven for the largest category to one in 100,000 for the smallest. Over the next decade, however, improvements in PCR raised its discernment significantly. By the mid 1990s, the chance of a random match with PCR was one in billions.
This foregoing was written by Rob Warden, executive director of the Center on Wrongful Convictions, and posted here March 22, 2002. Permission is granted to reprint, quote, or post on other web sites with appropriate attribution.
