What is DNA?

Have you ever wondered what is DNA? It is a term we are familiar with, but you may not know the actual science behind what DNA is. Called the genetic fingerprint, DNA stands for deoxyribonucleic acid. DNA is found in every cell in every human being, and each person possesses a different DNA profile. This revolutionary discovery allowed for everyone to be identified through their unique DNA, which in turn revolutionised crime solving.

The famous ‘double helix’ structure of DNA was discovered in 1953 by James D. Watson and Francis Crick, but it took another 30 years before scientists discovered that not only is a person’s DNA individually unique, but it is also found in every cell in that person. DNA can identify who we are and where we have been, as we leave traces of DNA wherever we go.  Our DNA is found in our fingernails, our saliva, our blood, our skin and even in the roots of our hair.

What is DNA?Credit: Wikimedia

DNA in Crime Solving

Because of its ability to effectively identify individual people through their unique genetic markers, DNA profiling has revolutionised how crimes are solved, and criminals caught.

When analysing samples to produce a DNA profile, a blood sample has a 90 per cent chance of providing a DNA profile. Saliva found on a cigarette butt has a 67 per cent chance. A stand of hair – 25 per cent. Sweat on the handle of a weapon – 17 per cent. With this sort of successful profiling percentage, evidence discovered at crime scenes can now be analysed and provide a DNA profile of alleged suspects, something that had been impossible to do before the discovery of DNA.

The first case of using DNA profiling in a criminal investigation involved an Englishman by the name of Colin Pitchfork, who had attacked, raped and killed to two teenage girls. In 1983 the first attack and murder took place in the Leicestershire village of Narborough. Four years later, he attacked and killed again. The only thing the police had to go on was the DNA profile that they had taken from semen samples, which showed that the same person had raped and killed both girls. The police then took around 4500 blood samples from men who lived in and around Narborough. A friend of Pitchfork took the blood test pretending to be him, to try and hide his DNA match, and because Pitchfork already had a criminal history. However, the police discovered the deception, and a blood sample from Pitchfork confirmed he was the culprit. He then confessed to his crimes and was sentenced to live in prison.

In another case in 1999, in the town of Wee Waa in northern New South Wales in Australia, 600 men volunteered their DNA for profiling after the rape of a 91-year-old woman. Knowing that he would eventually be caught with the DNA tests taking place, a local man came forward and confessed. He was later charged and convicted.

DNA profiling has also had some positive action for convicted criminals. DNA profiling has been able to be used in cases to overturn sentences and prove convicted criminals of their innocence. The first to have their conviction overturned because of DNA evidence was American Gary Dotson. He served a 50 year sentence for rape, but was released in 1989 when a DNA profile of the semen sample gathered from the victim did not match his DNA genetic profile. 

DNA and Identical Twins

A problem that arises with DNA profiling is identical twins. Because identical twins are formed from the same egg and sperm, they both have the same genetic DNA profiles. This can create both problems and benefits for police and crime solvers.

In 2003, Houston police screened a DNA profile that was discovered at a crime scene, and discovered that it was a match for a 21-year-old prisoner who was currently serving time. Police were baffled as to how the prisoner’s DNA could be at the scene of the crime, until they learnt that the prisoner had an identical twin brother. The brother was arrested and charged.

In another case of identical twins and DNA, there was a case in Michigan, were a victim was raped. DNA from a semen sample was matched to identical twin brothers. Neither of the brothers had an alibi, both of whom could have committed the crime, and neither of them were talking or cooperating with the police.  The DNA evidence told the police that one of the brothers was a rapist, just not which one. 

Testing for DNA

How do you test for DNA?

Firstly, samples need to be taken from an alleged suspect, or from the scene of a crime. An example of this could be that a swab of saliva is taken from the suspect and compared with a stand of hair that was discovered at a crime scene.

Scientists then cut up the DNA’s molecules by using a chemical enzyme, before placing the pieces on to a special jelly. This is then poured into a metal tray that is electrified at each end. The DNA is then soaked up through a special membrane that has been placed over the tray, before being subjected to a radioactive dye. The next step involves placing an x-ray film is put over the membrane to generate a reaction to the radioactive dye.

This reaction triggers a pattern on the x-ray film. When the film is processed, a series of dots and dashes are revealed. These dots and dashes are a map of the unique DNA profile, which is different with every person.

A DNA profile does not absolutely identify an individual, as in it is not 100 per cent guaranteed. Instead, what it does is make statistical estimates for how likely it is that two samples are from the same person. For example, during a trial where DNA profiling is being used as evidence, it might be stated that a blood sample found at the scene of a crime had only a one in 57 billion chance of coming from someone else other than the defendant.

In understanding what is DNA, scientists have been able to explore our genetic make-up, that is what makes us all individuals, while crime solving has been revolutionised with the ability to tie suspects to crime scenes through their DNA genetic profile.