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DNA Fingerprinting

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DNA fingerprinting or profiling is a technique that identifies the DNA from a certain individual without examining the entire three billion letters in the full human genome. It can be used to track down criminals, test the parentage of children, and to follow the evolution of species.

DNA - Deoxyribonucleic Acid

Deoxyribonucleic Acid is a molecule found in the nucleus living cells that stores the genome of an organism, or the blueprint required to build proteins. It has a double helix backbone which supports the four bases labelled G, C, A and T; the sequence of these bases is the genetic code which characterises an organism. Although 99.9% of human DNA is identical for different individuals, only identical twins have exactly the same pattern. So in principle, given a single cell, we can identify the individual it came from. DNA fingerprinting works by screening for certain DNA sequences which are found in some individuals, but not others.

DNA Fingerprinting

The fingerprinting process uses enzymes to cut out specific sequences of DNA; these are then arranged in order of length and tagged with radioactive probes. These emit X-rays, so when the sample is photographed they show up. This produces the 'fingerprint' - a series of black lines corresponding to the DNA sequences present.

Initially the DNA is removed from the sample cells by chemical methods, and the two strands of the double helix are separated. Restriction enzymes are then added, which each identify a particular sequence and cut it away. This produces a mixture of free lengths of DNA.

The next stage is to sort these sequences into length order. This is done using agarose gel electrophoresis. DNA is a charged molecule, so in an applied electric field it moves towards an electrode. The agarose gel slows down the larger molecules, but the shorter DNA strands move faster, so the process arranges the sequences in order. Acrylamide gel is sometimes used in a similar way for higher resolution ordering.

Once the DNA sequences are ordered, chemical probes are added to the sample; like the restriction enzymes, these each select a particular sequence, and bind to it. As they contain radioactive atoms, when the sample is exposed to photographic film, these sequences will show up. An alternative method uses chemiluminescent labelling, where enzymes emit light by a chemical process.

The whole process is repeated several times with different selections of enzymes, to build up a detailed fingerprint showing which DNA sequences are present. If a large number of sequences are used, and there is sufficient variation of the presence of these sequences among individuals within a population, then we have a unique profile of a single individual.

PCR - Polymerase Chain Reaction

PCR is a powerful technique which effectively 'amplifies' DNA. It can produce many copies of the DNA from a single cell, allowing the tiniest of samples to be tested. The process first heats the sample to unravel and split the double helix; primase and polymerase enzymes are then used to identify DNA sequences and produce a copy of them. The cycle is repeated a number of times, each time doubling the sequence samples.

PCR considerably increases the usefulness of DNA fingerprinting, however, it also increases the risk of contamination with DNA from laboratory staff or anyone who had handled a sample. This would be amplified along with the sample DNA.

There are many variations of this method and the technology is developing quickly. Chemiluminescent labelling is being used increasingly instead of radioactive tags due to the practical problems of using radioactive materials; and the process is now largely automated.

DNA Fingerprinting and Forensic Science

The technique was developed in England by the geneticist Alec Jeffreys in the mid-1980s. He was initially looking at ways to screen for hereditary diseases and genetic defects. However, he was asked by the local constabulary to help investigate a rape case, and he successfully identified the rapist from his DNA profile. Within a few years, DNA profiling was being used by forensic scientists around the world.

DNA fingerprinting allows a criminal to be identified from any blood, semen, or even hair or dead skin they may leave at the scene of the crime. A DNA fingerprint allows police to very quickly exclude or include a suspect in their investigation.

The accuracy of DNA testing is of crucial importance in many legal cases. Although no two unrelated people have been found with the same fingerprint, this cannot be ruled out. Geneticists usually talk about the probably of a certain fingerprint matching a randomly selected person, which can range from one in 10,000 or less to one in a billion or more. However, some researchers point out that certain patterns of DNA are more common in certain ethnic groups, and a match may be more likely than currently thought.

In an increasing number of serious cases police forces are trying to identify a criminal by testing a large number - sometimes tens of thousands of people living near the scene of the crime. This increases the risk of a miscarriage of justice if DNA testing is wrongly believed to be reliable. However, in general DNA testing is a reliable technique which has helped to convict many criminals and clear the name of many innocent suspects.

In many countries, all convicted criminals, or sometimes just those convicted of serious offences have their DNA profile taken and recorded along with their conventional fingerprints. It has been suggested carrying out a nationwide survey and keeping a database of the DNA of the entire population. This could be a very effective deterrent to potential criminals. Could they be sure of not leaving a single hair which would allow the police to identify them from a population of millions? It would also reveal any repeated profiles across the population. However, the idea raises many questions about civil liberties, and the fear that it could be abused by corrupt authorities.

Paternity or Maternity Testing

You inherit your DNA from your parents, some genes from each parent. Therefore DNA fingerprints can be used to identify your parents, grandparents or children. This has allowed people to identify lost relatives, and prove the parentage of children. It has changed the lives of thousands of single mothers, who can now force former partners who have abandoned their children to accept some responsibility.

An earlier version of DNA testing was used in Argentina following the overthrow of the military junta. After the 1975 coup, around 15,000 people - known as the 'disappeared' - were murdered by the military. Many children raised by military families were shown to have been taken from the many 'disappeared', and many were given to their grandparents.

Other Uses

DNA fingerprinting is also used by researchers on a larger scale to track the historic migrations of groups of people. It also works with other species as well as humans, and is now used extensively by biologists to examine the links between species showing how they evolved.

Your DNA contains a great deal of information about you, including your sex, details of your physical characteristics and any hereditary illness you may have. However, as a DNA fingerprint only indicates the presence or absence of certain DNA sequences, it may not contain any interesting information at all. It is simply a biological identity. However the principles explained here can also be used to try to identify certain genes within an individual's DNA, which allows a great many more applications.

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