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.
