DNA 101

“Wherever he steps, whatever he touches, whatever he leaves, even unconsciously, will serve as a silent witness against him. Not only his fingerprints or his footprints, but his hair, the fibres from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or collects. All of these and more, bear mute witness against him. This is evidence that does not forget. Physical evidence cannot be wrong, it cannot perjure itself, it cannot be wholly absent. Only human failure to find it, study and understand it, can diminish its value.” –  Edmond Locard (1877 – 1966) on his principle of transfer of evidence.

In true crime cases, we talk a lot about DNA. But what exactly is it?

DNA is a molecule composed of two strands, made up of subunits called ‘base pairs.’ These strands contain all the information needed to create life on the planet and dictate someone’s characteristics, for example the fur of an animal, or someone’s eye colour. Even trees have DNA, although it’s a slightly different type. In humans, on average one in every 1,000 base pairs is different, and these differences are what makes each person unique.

DNA can be found in a lot of areas in the human body, including the white blood cells, the liver, skin, and nerves. Humans are made up of trillions of cells and it’s relatively easy to extract cells from humans. Everyone has a different DNA profile except identical twins, but even twins will have unique fingerprints.

Sources of DNA include blood, saliva, hair, teeth, bone, urine, semen, and skin cells. DNA can even be collected from items touched or worn by somebody, such as a hat, gloves, or other items of clothing. Even weapons, hairbrushes, glasses, cups, masks, and tools can reveal traces of DNA. This is known as touch DNA, as every time you touch something with your bare hands, you leave skin cells behind. Unfortunately, not all labs are able to process touch DNA. DNA can be destroyed by heat and humidity, and it is best kept frozen to enable long term storage.

Mitochondronial DNA is a specific type which is inherited from the mother’s side. You share this type of DNA with your siblings, mother, grandmother, and maternal relatives. This can be useful in police investigations for excluding suspects, and in cases where there isn’t much of a sample available to extract.

DNA profiling was first developed in Leicester and was first used by the UK police there in 1986, to prove that suspect Richard Buckland was innocent of the rape and murder of a teenager, even though he had given a false confession. In the US, the first case where DNA testing was used was in 1987, to convict burglar and rapist Tommy Lee Andrews.

It is sometimes hard to believe that DNA has only been in use in criminal investigations for less than 40 years. But it has proved invaluable countless times since then; after all, as Edmond Locard said, whatever a person leaves behind them unknowingly will serve as an incontrovertible silent witness.