Science Questions

TV crime programmes often show DNA samples taken from suspects by swabbing the inside of their mouth. Human mouths are filled with bacteria, so the swab must collect non-human DNA as well as DNA from our food. How can human DNA be separated out from this? Don, Castro Valley, California

DNA samples are contaminated with bacteria, and other contaminants, but to analyse the sample we amplify human DNA sequences and use a ‘probe’, a little piece of genetic material that only latches on to specific sequences of DNA. We use probes that only recognise human DNA sequences, which are then bound to the sample, so the only patterns you see relate to human DNA in the sample.

May 2007