What is the DNA database?
In the mid-1980s, British scientist Prof Sir Alec Jeffreys found a way of taking a sample of someone's DNA and converting the sample into a unique genetic fingerprint. He realised that these numeric profiles could become an even more powerful crime-fighting tool than fingerprints.
In 1995, the government launched the national DNA database to allow the police to store DNA profiles.
Today, the combined British database (the system is run differently in Scotland and separately for Northern Ireland) is the largest in the world, holding almost six million profiles, as of the autumn of 2009. About 30,000 profiles are added each month.
Where do the profiles come from?
There are two sources: crime scenes and suspects.
Forensic officers gather DNA samples by securing crime scenes and recovering material such as hair samples or blood found on clothing. Gathering this material is a painstaking business but has become a regular part of modern policing.
In the case of suspects, police officers swab the mouths of anyone arrested for a recordable offence. This is where the controversy lies because police are allowed to retain the database profile gained from the sample, even if the individual is later released without charge or acquitted in court.
Why are police allowed to keep these profiles?
When it was launched in 1995, the only DNA that could be stored belonged to convicted criminals. The law changed in 2004 to allow the database to hold the DNA of anyone arrested for a recordable offence and detained at a police station.
Innocent people who volunteer to give a DNA sample during a police inquiry, for instance to help narrow a police search in a major murder investigation, also have their details kept on record.
How does it compare to Scotland?
In Scotland, most DNA profiles must be destroyed if the individual is neither charged nor convicted, unless the suspect was prosecuted for a violent or sexual offence.
In those cases, the police can hold the DNA for three years and then apply for a two-year extension.
What's the situation with children and teenagers in England and Wales?
Children have been included on the database - but samples could only be taken from the under-10s (the age at which the law says someone can be held criminally responsible for their actions) with the permission of a parent. The Home Office has recently deleted the profiles of under-10s.
About a quarter of all those arrested are under 18-years-old so a great deal of the samples on the database are from this age group.
So does it help fight crime?
Yes - but it needs to be put in context. Less than 1% of crimes are solved with the help of DNA profiles.
DNA comes into its own in cases where a profile recovered from a crime scene later connects a suspect to the deed.
The most obvious examples are an attacker's DNA on the hilt of a knife, a burglar's hair snagged on a window or semen recovered from a rape victim.
DNA plays a role in solving a third of cases like these, where a crime scene sample is loaded onto the database and later linked to a suspect.
What are the figures?
The database provides some 3,500 matches to crime scenes every month. Between April 1998 and September 2009, DNA profiling provided matches to 410,000 crimes. Between 2007-08 police successfully gathered DNA profiles relating 83 killings, 184 rapes and a further 15,420 crimes they went on to solve.
And what about profiles from people who were never charged?
About one million profiles on the database have been taken from people who are neither charged nor linked to a crime for technical reasons.
Officials estimate that as of 2005 there were 200,000 profiles on the database which were from people who had not been convicted. But, they add, that 200,000 led to some 6,000 criminals being linked to 14,000 offences - including 114 murders and 116 rapes.
So, DNA profiling has proven beyond doubt that it can have an important role in some cases. The debate then is really about the ethics and legality of holding such a large number of profiles belonging to people never convicted of an offence.
So what are the issues?
Let's start with race. Experts know what young black men are disproportionately more likely to be arrested. It's thought that 40% of all black men in the UK are on the system, compared with 9% of white men.
Professor Sir Alec Jeffreys has criticised the way his invention has been used, likening the database to creating a "presumption of likely future guilt".
All of this came to a head in a damning 2008 European Court of Human Rights judgement. It found the England and Wales system manifestly discriminatory and unfair. In fact, the Strasbourg judges said that their ruling was influenced by the fact that Scotland had found a way of operating a fairer system.
So what is the Home Office now planning to do?
Adult suspects who are let go will see their DNA held for six years. Under-18s convicted of a minor offence will have their DNA deleted after five years.
Under-18s who are arrested for a serious crime will see their DNA retained for three years, or six if they are 16 or 17 years old.
Anyone arrested for a terrorism-related offence may find their DNA retained indefinitely, subject to a case-by-case review.
The Home Office had originally proposed much longer time limits. The Conservatives say the measures are not good enough and they will move to the Scottish model if they win next year's general election.