THE TSAR'S BONES: DNA techniques used to identify human remains

THE PROBLEM of accurately identifying long-dead corpses, which time has often reduced to a pile of bones, has been revolutionised with the techniques of DNA analysis.

British researchers are at the forefront of a branch of forensic science that can extract minute amounts of DNA from human bones. The DNA molecule - the chemical blueprint of life that we inherit from our parents - can be amplified and then analysed to determine probable relationships with known persons.

Last year British scientists helped to identify the bones of Josef Mengele, the Nazi SS doctor of Auschwitz who died in Brazil in 1979. By comparing DNA recovered from the bone with DNA extracted from a blood sample given by his living son, Rolf, scientists were able to say with more than 99 per cent certainty that the two were related.

In 1991, the same scientists - Professor Alec Jeffreys, of Leicester University, and Dr Erika Hagelberg, now at Cambridge University - demonstrated that the DNA extracted from the bones found in a Cardiff garden was highly likely to be that of the teenager Karen Price, whose body had remained buried for eight years after her murder in 1981.

Positive identification from buried bones relies on the chemical stability of DNA under certain circumstances. Dr Hagelberg and colleagues have been able to extract small amounts of DNA from human bones that are 5,000 years old.

Scientists can recover DNA from bone cells using the polymerase chain reaction (PCR) test, which amplifies fragments of DNA to usable quantities. The Home Office scientists who analysed the badly decayed bones from Ekaterinberg looked in particular for DNA from the thousands of tiny structures within cells, known as mitochondria, because so many copies are present, which is not the case with the DNA of the chromosomes.

Mitochondrial DNA is inherited from the mother, and was used to establish that three of the bodies in the grave were daughters of the Tsarina, and related to the Duke of Edinburgh through a common female ancestor. Mitochondrial DNA from living relatives of the Tsar confirmed the identity of his remains, although it appears that a mutation, possibly occurring during his lifetime, caused him to have two slightly different forms of mitochondrial DNA, a rare condition known as heteroplasmy.

Because mitochondrial DNA is inherited through the maternal line, it was not possible to use it to establish that the Tsar was related to the three daughters of the Tsarina. For this, scientists analysed DNA from the chromosomes to look for what are known as short tandem repeats. The pattern of these repeats are inherited from both parents.

As in all DNA analysis, there is a chance of getting a positive match as a result of chance alone. However, the Home Office scientists conservatively estimate that they are 98.5 per cent sure of a correct match.