One place science and society frequently interact is within the courtroom. Seldom has that interaction been more dramatic than in the past few days, with the exoneration of Guy Paul Morin, who had served 18 months in jail for a murder he didn’t commit, and with the start of the murder trial of a certain well-known ex-football-player. In both instances, a relatively new technology called DNA fingerprinting has been in the limelight.
DNA, deoxyribonucleic acid, is the complex substance that contains all the genetic information that makes us, not only human, but distinct individuals. It exists in the nucleus of almost every human cell as a huge, coiled molecule that, fully stretched out, would approach two metres in length. Its consists of two matching strands twisted together and joined, ladder-like, by connected pairs of chemicals called bases: adenine (A), cystosine (C), guanine (G) and thymine (T). These four bases make up the alphabet of the genetic code. An A can link only to a T and a G can link only to a C, making each side of the ladder complementary.
Each strand of the ladder consists of about three billion of these base “rungs,” which means there are as many letters in a human’s genetic code as there are in a thousand 600-page books. Most of these, however, are meaningless; only about two percent are genes, sequences that contain the code for the production of the proteins that make up the life-form we call “human.”
Unless you’re an identical twin, your genetic code is unique to you. In fact, it IS you. And that’s what’s at the core of DNA fingerprinting.
In 1984 British geneticist Sir Alec Jeffreys and colleagues at the University of Leicester came up with a method of recording genetic sequences called “restriction fragment length polymorphism analysis”–RFLP for short. Within two years it had been used to free a man wrongly accused of raping and killing two teenaged girls in the English Midlands, and to point the way to the real murderer, Colin Pitchfork, the first man convicted on the basis of DNA evidence. Since that first case, DNA testing has been employed in more than 24,000 American criminal cases and in more than 1,000 Canadian ones.
The RFLP process begins with a forensic technician testing the crime-scene sample (usually bood or semen) to find out how much DNA is present and how degraded it is. (DNA is quite fragile and can be quickly destroyed by sunlight, high temperatures, excessive humidity or other environmental factors.)
If there is enough high-quality DNA in the sample, it is digested by a restriction enzyme, a chemical which is able to snip out from the long strands of DNA specific combinations of the four-letter code. The resulting fragments of DNA are suspended in a gel derived from seaweed, then exposed to an electric current. The current causes small fragments to move through the gel faster than large fragments, separating them into different-sized (but unfortunately invisible) bands.
Blotting with a nylon membrane removes the DNA from the gel while keeping the fragments in their relative positions. Next, segments of known DNA called probes, impregnated with a radioactive material, are added to the membrane; they bond to the precise type of DNA fragment being tested and transfer their radioactivity. Finally, the membrane is placed over standard X-ray film. Over a period of 14 days, the (very slight) radioactivity exposes the film. For a complete DNA fingerprint, four or five radiactive probes must be applied, one after the other, stretching the time needed for RFLP analysis to as much as 10 weeks.
The result looks something like a supermarket bar code, whose lines and bars can be compared against other samples.
RFLP analysis is slow, painstaking, and requires a fairly large sample. A newer form of analysis, PCR, can be conducted with as few as, say, 50 white blood cells, found in an almost invisible speck of blood, compared to the thousands or even millions needed for RFLP. As well, PCR analysis can be conducted in days.
PCR can use a smaller sample because it doubles and redoubles a single strand of DNA about 30 times, producing more than a billion copies. And rather than looking for repeating segments in the vast amount of meaningless DNA, the PCR test focuses on six specific genes.
Every gene has at least two alternative forms, called alleles: the hair-shape gene, for instance, might have a curly-hair allele and a straight-hair allele. An individual receives one allele from his mother and one from his father. For six different genes, there are just 21 possible combinations of alleles, which PCR detects. This makes it quicker, but also less precise: it’s far more likely that two individuals share the same combination of alleles than that they share the same combinations of randomly repeating DNA segments.
It’s estimated that an RFLP fingerprint might occur only in one person in every 100,000 to 100 million, while a PCR fingerprint might occur in one person in a thousand. That’s why a DNA fingerprint match is not conclusive for the prosecution; it must be coupled with other, more traditional forms of evidence, such as bloody gloves found in the suspect’s backyard or a strong motive. However, the lack of a DNA fingerprint match IS conclusive for the defense, as Guy Paul Morin found out when PCR analysis of 10-year-old stains proved that he is neither a rapist nor a murderer.
While there is still some argument over just how accurate DNA fingerprinting is, such controversy will probably fade with time, as better techniques are developed. It’s interesting to note that ink fingerprinting went through an almost identical debate when it was first introduced.
All of which means you’re going to be hearing a lot more about DNA fingerprinting, even when you’re no longer hearing about O.J….which, as far as I’m concerned, can’t happen soon enough.
