DNA fingerprinting

Genetic fingerprinting explained

About DNA

  • DNA is deoxyribonucleic acid, the molecules in cells that determine the genetic characteristics of all life. It takes the form of a double helix (two strands coiled together).
  • DNA was first discovered in the 19th century by Miescher, from pus on bandages.
  • By the 1940s scientists realised that DNA contained the code for life, and in 1953 Watson and Crick at Cambridge University discovered how information was stored in DNA and transferred to the next generation.
  • In 1984 Professor Alec Jeffreys discovered the variations in DNA, unique to each individual.

Genetic fingerprints

  • These exist in blood, bone, hair follicles, saliva, semen, skin and sweat.
  • They are the same in every cell and retain their distinctiveness throughout a person's life.
  • Human cells contain 23 chromosomes (packets of DNA) from the father and 23 from the mother.
  • Each DNA strand contains a unique sequence or code of genetic information. But while most of DNA shows only slight variation from one person to the next, certain areas, called 'minisatellites' (short sequences of chemical building blocks) show variation in the numbers of repeat units (or stutters) unique to each person.
  • DNA information can be recovered from human and animal remains as far back as Neanderthal man, and has been used to solve a number of high profile mysteries from the past, including the identification of Josef Mengele's skeleton and the identity of children of US President Thomas Jefferson's children by one of his slaves.
  • Apart from identification, paternity and immigration cases, the technique is also used in medical research including cancer and genetic conditions such as Huntington's disease.

Genetic fingerprinting involves...

  • the extraction of DNA,
  • using enzymes to cut it into fragments some of which will contain minisatellites
  • separating the fragments according to size
  • treating the fragments with a radioactive probe which identifies shared motifs and can be captured on X-ray film
  • The result will be a pattern of more than 30 stripes, resembling a 'bar code'

Genetic profiling

  • This involves testing minisatellites one at a time, producing a simpler image than genetic fingerprinting.
  • It gives a pattern unique to a particular person, and is therefore suitable for forensic cases.
  • In the words of Professor Sir Alec Jeffreys, "It does not solve crimes. It establishes whether sample X comes from person Y. It is then up to the court to interpret that in the context of other evidence in a criminal case."

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