these transposons then make the cancer worse. Methylation, according to this argument, serves to suppress the effect of selfish D N A . 6
L I N E - 1 is generally about 1,400 'letters' long. Alu is generally S E L F - I N T E R E S T 1 3 1
at least 180 'letters' long. There are, however, sequences even shorter than Alu that also accumulate in vast, repetitive stutters. It is perhaps too far-fetched to call these shorter sequences parasites, but they proliferate in roughly the same manner - that is, they are there because they contain a sequence that is good at getting itself duplicated. It is one of these short sequences that has a practical use in forensic and other sciences. Meet the 'hypervariable minisatellite'.
This neat little sequence is found on all the chromosomes; it crops up at more than one thousand locations in the genome. In every case the sequence consists of a single 'phrase', usually about twenty
'letters' long, repeated over and over again many times. The 'word'
can vary according to the location and the individual, but it usually contains the same central 'letters': GGGCAGGAXG (where X can be any 'letter'). The significance
Here is an example of a repetition of a minisatellite: hxckswapmeaboutlopl-hxckswapmeaboutlopl- hxckswapmeaboutlopl-hxckswapmeaboutlopl-
hxckswapmeaboutlopl-hxckswapmeaboutiopl-
hxckswapmeaboutlopl-hxckswapmeaboutlopl-
hxckswapmeaboutlopl-hxckswapmeaboutlopl. Ten repeats in this case. Elsewhere, at each of one thousand locations, there might be fifty or five repeats of the same phrase. Following instructions, the cell starts swapping the phrases with the equivalent series on the other copy of the same chromosome. But in doing so it makes fairly frequent mistakes, adding or subtracting to the number of repeats.
In this way each series of repeats gradually changes length, fast enough so that it is different in every individual, but slowly enough so that people mostly have the same repeat lengths as their parents.
Since there are thousands of series, the result is a unique set of numbers for each individual.
1 3 2 G E N O M E
Alec Jeffreys and his technician Vicky Wilson stumbled on minisatellites in 1984, largely by accident. They were studying how genes evolve by comparing the human gene for the muscle protein myoglobin with its equivalent from seals when they noticed a stretch of repetitious D N A in the middle of the gene. Because each minisatellite shares the same core sequence of twelve letters, but because the number of repeats can vary so much, it is a relatively simple matter to fish out this minisatellite array and compare the size of the array in different individuals. It turns out that the repeat number is so variable that everybody has a unique genetic fingerprint: a string of black marks looking just like a bar code. Jeffreys immediately spotted the significance of what he had found. Neglecting the myoglobin genes that were the target of his study, he started investigating what could be done with unique genetic fingerprints. Because strangers have such different genetic fingerprints, immigration authorities were immediately interested in testing the claims of would-be immigrants that they were close relatives of people already in the country.
Genetic fingerprinting proved that they were generally telling the truth, which eased much misery. But a more dramatic use was to follow soon after.7
On 2 August 1986, a young schoolgirl's body was found in a thorn thicket close to the village of Narborough, in Leicestershire.
Dawn Ashworth, aged fifteen, had been raped and strangled. A week later, the/police arrested a young hospital porter, Richard Buckland, who confessed to the murder. There the matter would have rested.
Buckland would have gone to prison, convicted of the killing. However, the police were anxious to clear up an unsolved case, of a girl named Lynda Mann, also fifteen, also from Narborough, also raped, strangled and left in an open field, but nearly three years before.
The murders were so similar it seemed implausible that they had not been committed by the same man. But Buckland refused to confess to Mann's murder.
Word of Alec Jeffreys's fingerprinting breakthrough had reached the police via the newspapers, and since he worked in Leicester, less than ten miles from Narborough, the local police contacted S E L F - I N T E R E S T 1 3 3
Jeffreys and asked him if he could confirm the guilt of Buckland in the Mann case. He agreed to try. The police supplied him with semen taken from both girls' bodies and a sample of Buckland's blood.
Jeffreys had little difficulty finding various minisatellites in each sample. After more than a week's work the genetic fingerprints were ready. The two semen samples were identical and must have come from the same man. Case closed. But what Jeffreys saw next astonished him. The blood sample had a radically different fingerprint from the semen samples: Buckland was not the murderer.
The Leicestershire police protested heatedly that this was an absurd conclusion and that Jeffreys must have got it wrong. Jeffreys repeated the test and so did the Home Office forensic laboratory, with exactly the same result. Reluctantly, the baffled police withdrew the case against Buckland. For the first time in history a man was exonerated on the basis of his D N A sequences.
But nagging doubts remained. Buckland had, after all, confessed and policemen would find genetic fingerprinting a lot more convincing if it could convict the guilty as well as acquit the innocent. So, five months after Ashworth's death, the police set out to test the blood of 5,500 men in the Narborough area to look for a genetic fingerprint that matched that of the murdering rapist's sperm. No sample matched.
Then a man who worked in a Leicester bakery named Ian Kelly happened to remark to his colleagues that he had taken the blood test even though he lived nowhere near Narborough. He had been asked to do so by another worker in the bakery, Colin Pitchfork, who did live in Narborough. Pitchfork claimed to Kelly that the police were trying to frame him. One of Kelly's colleagues repeated the tale to the police, who arrested Pitchfork. Pitchfork quickly confessed to killing both girls, but this time the confession proved true: the D N A fingerprint of his blood matched that of the semen found on both bodies. He was sentenced on 23 January 1988 to life in prison.
Genetic fingerprinting immediately became one of forensic 1 3 4 G E N O M E
