When a suspect is arrested for a record-able offence a mouth swab is taken, usually when the person has arrived at a police station and the custody sergeant has authorised detention.

A sterile kit is used, and the sample is transferred to a forensic science laboratory. A huge machine will process a tray of about 100 samples, in separate glass vessels, at a time.

Chemicals are added that turn the DNA on the swab into a unique bar-code. This is then stored electronically in a national DNA database in the Midlands run by the National Policing Improvement Agency.

Every new sample put into the database is automatically checked against those already there. If there is a match the force that took the initial swab will be alerted by e-mail.

All DNA that is taken from crime scenes, as opposed to people, is also stored nationally, and each new sample will also be automatically checked against the human DNA that has been stored to see whether there is a match.

No names are kept on the database; there are just electronic codes, so names cannot be entered to see if there is a match to someone specific. Whenever a mouth swab is taken, fingerprints will be taken as well. These days that is done by an electronic scanner, similar to those used for tourists who are entering America.

Each finger is placed against a glass screen and it records the print before storing it in another database. Fingerprints will always be taken no matter how many times someone is arrested as the prints may change over time because of scarring.

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