DNA is a chemical that occurs inside every cell of a person’s body. The DNA is contained in 22 pairs of structures known as chromosomes, shaped like an X, plus an extra pair – the sex chromosomes – which determine whether someone is male or female. In this final pair, women have two X chromosomes, but men have one X and one Y chromosome. Each chromosome consists of two long strings of chemical letters, twisted together in the famous shape of the double-helix. The chemical letters occur in pairs as rungs on this twisted chemical ladder. The four chemical letters of the genetic code spell out instructions to the cell about how to make the proteins that allow the human body to grow and function normally. The parts of the DNA sequence that contain the instructions for making proteins are known as genes.
DNA is useful to identify an individual because everyone’s genetic code is thought to be unique, unless they have an identical twin. The string of chemical letters in a person’s DNA can therefore act like a unique bar code to identify them. Because a person inherits half their DNA from their mother and half from their father, it can also be used to identify their relatives. Close relatives have a DNA sequence that is more alike than distant relatives or than someone who is unrelated.
Biological identifiers such as DNA, fingerprints, iris scans and digital photographs are known as ‘biometrics’. In recent years there has been a lot of interest in developing biometrics to track and identify individuals as they enter or leave different countries or as they use public or private services, such as banks, computers, workplaces or hospitals.
Unlike iris scans and photographs, DNA and fingerprints can be left wherever a person goes: for example, on a glass or cup that they have been drinking from. This means that they can be used to track individuals – i.e. to find out whether they have been at a particular place, such as a crime scene or meeting place – where there might not be a scanner or a camera.