The DNA database at the Forensic Science Service laboratory

All that is needed to extract DNA is one cell - a speck of blood, or a miniscule fragment of skin that clings to a strand of hair.

This week, Prime Minister Tony Blair announced plans to store the DNA fingerprint of every criminal in the UK in an overhaul of the justice system.

Some �109m over three years was allocated for the tests in the Comprehensive Spending Review.

Until now, the �40 cost of taking and processing each sample has limited the tests to only those suspected of serious crimes.

Currently, police have 940,000 DNA samples in the national database.

By 2004, the database should hold more than three million samples - equivalent to "almost the whole criminal class of the UK", Downing Street has claimed.

Long-term storage

Professor Ray Waters, a Swansea University DNA expert, says DNA samples can be stored at -80C in a test tube about the size of the top joint of a thumb.

DNA tests are used in the US to solve old crimes

"DNA is a pretty robust molecule. Spontaneous changes can occur over time, but we're talking thousands of years."

DNA fingerprinting involves isolating the molecule from a cell, which is then treated with enzymes to break it into pieces. These are separated using an electric current, and the pattern of the separated fragments is used to compare samples.

Although increasingly accurate, DNA testing is not a foolproof tool to solve a crime.

"The technology is so sophisticated now, there is a one in one billion chance of making a mistake," Professor Waters says.

The tests may fail to make an accurate match if the sample was contaminated at the scene of the crime, he says.

Current DNA tests show that, for example, sperm in a rape case is "exceptionally unlikely" to have come from a source other than the perpetrator. Which is why, in criminal trials, forensic experts talk about the probability that a sample came from the defendant.

When OJ Simpson went on trial for the murder of his ex-wife, Nicole Brown Simpson, and her friend Ronald Goldman, forensic experts for the prosecution said there was only a one in 170 million chance that blood at the scene was not his.

Simpson was acquitted of the 1994 murder in a criminal trial, but was later found liable for the deaths in a civil trial.

Numbers game

DNA can either be taken directly from a suspect with a mouth swab or from the scene of a crime.

Not all of the samples taken will be stored. Under existing laws set out in the Police and Criminal Evidence Act, DNA samples must be destroyed if the suspect is acquitted or the charge discontinued.

Professor Ian Shaw, who runs a forensic science course at the University of Central Lancashire, has said there are dangers in expanding the database.

Although the chances are remote, "the more samples you get, the greater the chance that you find two samples that look the same but are not actually from the same person".

An investigation could be complicated if, for example, the prime suspect has a twin. The pair will have either identical or very similar genetic profiles.