6 D
DNA
Pro
ofiling
What is DNA? deoxyribonuccleic acid, the body’s meeans by whicch it can DNA is d
replicatee all importtant proteins. It is mad
de up of tw
wo long chains o
of sugar and phosphate ggroups, whicch have sidee chains of four nitrogen‐co
ontaining nucleotide basses – adeniine (A), e (G) and thyymine (T) – millions of them, in cytosinee (C), guanine
fact. he two chain
ns are held ttogether in the famous double Th
helix byy hydrogen bonds between basees in each chain. However, only speccific pairs off the bases will bond to t each other: C to G and A tto T. he DNA chaains contain our genes and make up our Th
chromossomes, and are unique for each ind
dividual, sincce they are a co
ombination of o our paren
nt’s DNA. Only O
identicaal twins will havee the same D
DNA. ngerprintingg” DNA “fin
It is the sequence of o base pairrs that is in fact uniquee for an ome projectss have been undertaken to fully individuaal. The geno
sequencce different sspecies DNA
A base pairs. In practicee, this is an extreemely time‐cconsuming process p
whicch is impracttical for real applications, succh as forensiic evidence ggathering. he actual gen
netic informaation occupies only abou
ut 5% of Th
the DNA
A strands, thee rest in bettween is mad
de up of seq
quences of repeaating base paairs. It is in fact these rrepeating un
nits that are used
d to develop the DNA finggerprint. Th
he techniquees involves treating the D
DNA‐contain
ning sample with special enzymes (reestriction fragmen
nt enzymes) which chop the chain up
p into piecess of repeatin
ng base pairss. The fragm
ments are then sorrted on the basis of th
heir size by electrophoresis, and th
he pattern developed d
by special reagentss. The fragm
ment patterrn – position
n and intenssity ‐ is uniq
que (or veryy nearly so) for each individuaal. A fingerprint
The proccess of producing a DNA
DNA finggerprinting iss a laboratorry proceduree that requirees a number of steps: 1. Isolation – DNA m
must be reco
overed from the cells or tissues of th
he body. On
nly a small am
mount of tissue ‐ like blood, haair, or skin ‐ is needed. FFor example, the amountt of DNA found at the root of one hair is ussually sufficieent 2. Cuttin
ng, sizing and
d sorting ‐ sp
pecial enzym
mes called restriction enzzymes are ussed to cut the DNA at specific places; thee DNA piecces are sortted according to size by a sievin
ng techniqu
ue called phoresis; the pieces are p
passed through a gel mad
de from seaw
weed agarose (a jelly‐likee product electrop
made frrom seaweed); this tech
hnique is th
he biotechno
ology equivaalent of scre
eening sand through progresssively finer m
mesh screenss to determin
ne particle siizes. 3. Transsfer to nylon
n – the distriibution of DNA pieces iss transferred
d to a nylon sheet by plaacing the sheet on
n the gel and soaking theem overnightt. 4. Probin
ng – adding radioactive or coloured probes to th
he nylon sheeet producess a pattern ccalled the DNA finggerprint; eacch probe typically sticks iin only one o
or two speciffic places on
n the nylon sheet; the final DNA
A fingerprintt is built by u
using several probes (5‐1
10 or more) simultaneou
usly. It resem
mbles the bar codees used by grrocery store scanners. 6. DNA Profiling
Advances in DNA fingerprinting technology The first DNA fingerprint technique, developed in the 1980s by a British biologist, Alec Jeffreys, is known as Restriction Fragment Length Polymorphism (RFLP). As a new technology, it had various limitations, including sensitivity – it needed a relatively large amount of DNA – and speed – it took several days to complete. Over the next decade, improvements and changes were made, and the current technique is known as Polymerase Chain Reaction (PCR)‐Short Tandem Repeat (STR) which overcomes both problems. The PCR step creates replicate DNA to improve the sensitivity, and the STR focuses on only specific repeating unit sections of the DNA, speeding up the process. Problems with DNA evidence First and foremost, a DNA “fingerprint” obtained by the newest techniques is not unique: the probability of two non‐related individuals having the same DNA fingerprint is about 1 in 1 billion. Depending on the number of repeating unit sections examined by the STR process, the probability of two matching can be substantially less as seen in the Raymond Easton case (see below). Secondly, the PCR amplification process not only increases the amount of evidence DNA but also any other DNA that may have somehow contaminated the sample. Therefore, there is an even greater need than normal for absolute care in collection and processing of DNA evidence. Thirdly, it is both the position and density of the bands that comprise the information that can help to identify a suspect. Therefore, it requires subjective judgement of the analyst that two DNA profiles match or don’t. Finally, there is a public perception problem regarding this technology, where it is seen to be all‐powerful and absolutely unquestionable. Juries may be swayed by flawed DNA evidence simply because they don’t understand the first three points. What does this mean? A DNA match (or non‐match) is a powerful but not overwhelming piece of evidence. It should not automatically counteract other evidence to the contrary. Important cases using DNA fingerprinting Colin Pitchfork (UK, 1986) Two schoolgirls who were murdered in the small town of Narborough in Leicestershire in 1983 and 1986 sparked a murder hunt that was only to be resolved by a intelligence‐led screen, eventually leading to the conviction of a local man ‐ Colin Pitchfork. In 1983, a 15‐year‐old schoolgirl was found raped and murdered. A semen sample taken from Lynda Mann’s body was found to belong to a person with type A blood group and an enzyme profile, which matched 10 per cent of the adult male population. At that time, with no other leads or forensic evidence, the murder hunt was eventually wound down. Three years later, Dawn Ashworth, also 15, was found strangled and sexually assaulted in the same town. Police were convinced the same assailant had committed both murders. Semen samples recovered from Dawn’s body revealed her attacker had the same blood type as Lynda’s murderer. Introduction To Forensic Science IS6.2 6. DNA Profiling
The prime suspect was a local boy, who after questioning revealed previously unreleased details about Dawn Ashworth’s body. Further questioning led to his confession but he denied any involvement in the first murder – that of Lynda Mann. Convinced that he had committed both crimes, officers from Leicestershire Constabulary contacted Professor Sir Alec Jeffreys at Leicester University who had developed a technique for creating DNA profiles. Dr Jeffreys ‐ along with Dr Peter Gill and Dr Dave Werrett of the Forensic Science Service (FSS) ‐ had jointly published the first paper on applying DNA profiling to forensic science. Significantly, in 1985, they were the first to demonstrate that DNA could be obtained from crime stains, which proved vital in this case. Dr Gill said: "I was responsible for developing all of the DNA extraction techniques and demonstrating that it was possible after all to obtain DNA profiles from old stains. The biggest achievement was developing the preferential extraction method to separate sperm from vaginal cells – without this method it would have been difficult to use DNA in rape cases." Using this technique Dr Jeffreys compared semen samples from both murders against a blood sample from the suspect, which conclusively proved that both girls were killed by the same man, but not the suspect. The police then contacted the FSS to verify Dr Jeffrey’s results and decide which direction to take the investigation. Peter Gill said: "Since the technique had not been used in criminal casework before, the FSS were asked by the police to confirm Dr Jeffrey’s conclusions. Accordingly, we carried out further tests and indeed demonstrated that the prime suspect could be excluded." This suspect became the first person in the world to be exonerated of murder through the use of DNA profiling. Professor Alec Jeffreys said " I have no doubt whatsoever that he would have been found guilty had it not been for DNA evidence. That was a remarkable occurrence." The police then decided to undertake the world’s first DNA intelligence‐led screen. All adult males in three villages – a total of 5,000 men ‐ were asked to volunteer and provide blood or saliva samples. Blood grouping was performed and DNA profiling carried out by the FSS on the 10 per cent of men who had the same blood type as the killer. The murderer almost escaped again by getting a friend to give blood in his name. However, this friend was later overheard talking about the switch and that he’d given his sample masquerading as Colin Pitchfork. A local baker, Colin Pitchfork was arrested and his DNA profile matched with the semen from both murders. In 1988 he was sentenced to life for the two murders. from http://www.forensic.gov.uk/forensic_t/inside/news/list_casefiles.php?case=1 Tommy Lee Andrews (USA, 1987) Andrews’s trial was the first in the United States to use DNA fingerprint evidence. He was accused of into the home of a 27‐year‐old Orlando woman, raping and stabbing her on May 9, 1986. The woman identified Andrews during the trial as her attacker. He claimed that he had never left his apartment the night the woman was attacked. Laboratory tests carried out involved three DNA samples: one from the rapist's semen, one from the victim's blood and one from blood taken from Andrews after his arrest. A scientist from the testing laboratory and a research biologist from the internationally‐
renowned Massachusetts Institute of Technology testified that the DNA in the semen and in Mr. Andrews' blood matched. Andrews’ defence attorney argued that the tests were unreliable and that there were not enough checks and balances to prevent shoddy lab work. The jury disagreed – Andrews was found guilty. Jose Castro (USA, 1989) When police arrived at the Bronx apartment of Jeffrey Otero in February 1987, they discovered a scene of terrible carnage. Vilma Ponce, Otero's seven months pregnant common‐law wife, lay on the living room floor, nude from the waist down. She was perforated by more than sixty knife wounds. In Introduction To Forensic Science IS6.3 6. DNA Profiling
the bathroom, police found the body of her two‐year‐old daughter, Natasha, also repeatedly stabbed. Police interviewed Jose Castro, the janitor of a neighbouring building who fit Otero's description of the suspect. The detective noticed what he thought might be a dried bloodstain on Castro's watch and asked if he could retain it for examination. Shortly thereafter, Castro was arrested and charged with the double murder. The dried blood on Jose Castro's watch and how it was handled led to the first notable courtroom challenge to DNA typing. Police turned the watch, along with blood samples from Castro and the two victims, over to the Lifecodes Corporation. Scientists analysed the dried blood and during the 15 week long pre‐trial evidentiary hearing, testified that the DNA from the stain matched that of Vilma Ponce, and that the frequency of her patterns in the Hispanic population were 1:189,200,000. The defence undertook a thorough examination of the genetic analyses and mounted the first extended (and eventually successful) effort to have DNA evidence excluded. What also occurred in the Castro case that contributed to this turn of events was an unprecedented out‐of‐court meeting between two defence and two prosecution scientific witnesses after they had testified. These scientists all agreed that Lifecodes had failed to use generally accepted scientific techniques in reaching their results matching the blood found on Castro's watch with that of Vilma Ponce. The quality of the data they produced was poor and they did not even follow their own procedures for interpreting the data. One key player in this drama was Eric Lander, an academician who received his doctorate in mathematics from Oxford University and now directed a genetics research institute at the Massachusetts Institute of Technology. Lander is a powerful personality. Even his friends admit that Lander is arrogant, just as his enemies concede that he is brilliant. As a result of the testimony of Lander et al, the judge ruled that the inclusionary tests suggesting that Ponce was the source of the blood stain were inadmissible, while allowing the exclusionary evidence that the blood did not come from Castro. After almost one hundred cases where DNA evidence met little or no resistance and never was ruled inadmissible, the defence obtained their first victory. Later that year, in what was to be the anti‐climax to the case, Castro confessed to the murders, admitting that the blood on his watch came from Vilma Ponce, and pled guilty. Raymond Easton (UK, 1999) A DNA sample from a burglary thought to be from the perpetrator was submitted for testing and then comparison to the national DNA database in Britain. A match was found to Raymond Easton, who lived more than 300 kilometres from the where the crime was committed. Nevertheless, he was arrested. However, there was a problem: Easton suffered from Parkinson’s disease, and at that time was physically unable to even dress himself, let alone drive a car or break into a house. Clearly, he could have not committed the burglary, and had never visited the house. There was a problem with the DNA match. This time it was not a consequence of poor procedures as had been the situation in the Castro case: the match was real. The problem lay with the testing procedure itself: at that time, British authorities only used a six‐point comparison, for reasons for speed and cost. This brings the probability of two people having the same fingerprint down to 1 in 37 million, less than the population of Britain. As a consequence, the test was redone with a ten‐point comparison, which takes the probability up into the billions, and Easton’s “new” DNA fingerprint did not match. This led to a permanent change (to ten‐point comparisons) in the testing procedures in Britain. Further reading If you want to read some more, there is a well‐written and researched article about the use of DNA fingerprinting over more than a decade at http://www.city‐journal.org/html/10_1_dna_testing.html Introduction To Forensic Science IS6.4