I was recently watching an episode of Brooklyn 99 where a forensic scientist came to teach the 99 team about forensics. The idea of science being involved in police work had completely slipped my mind. But I should know by now that science is deeply integrated into all career paths; in this case as a forensic scientist in the NYPD.
The first time DNA was used in an investigation was in 1986, only 34 years ago! And it has revolutionised crime investigations, making it easier to detect a suspect.
Usually on a crime scene, police investigators will collect blood, hair, saliva or other bodily fluid samples to be taken to a lab for testing. ‘Touch DNA’ can also be used, involving the DNA sequence of skin cells left on objects an individual has touched. Within these cells is the nucleus, which contains DNA coiled up into chromosomes.
Whilst collecting the samples, investigators have to be carful not to contaminate the sample. It is often easy to forget to change gloves when collecting different samples or to not wear a well fitting mask. Contaminating the sample will make it more difficult for scientists to match the DNA to one person as it is now a mixture of many people’s DNA.
As a precaution, elimination samples are also taken. These samples come directly from a victims family or the first responders on the scene, so that DNA from those individuals can be ruled out.
Additionally, biological evidence can degrade easily by heat or humidity. Therefore, they need to be stored in cold and dry environments to avoid this. If degradation occurs then a partial profile is created. This is a profile that contains half of the genetic information of a suspect, so it is less reliable and is more likely to indicate to more than one potential suspect. That is why, in criminal investigations, DNA evidence alone is not enough to convict someone.
After the DNA samples are collected, they must be processed by scientists to create a Suspect Profile. A series of steps are followed:
Extraction = releasing the DNA from the cell. This is done by adding a solvent that will dissolve the lipids in the plasma membrane and nuclear membrane. The DNA is then released.
Quantitation = determining how much DNA you have.
Amplification = multiple copies of the DNA are created using a polymerase chain reaction, explained below
Separation = separating amplified DNA for identification. Forensic scientists use capillary electrophoresis, which is explained below.
Analysis and Interpretation = comparing DNA samples to profiles.
Quality Assurance = reviewing analyst reports for technical accuracy.
Polymerase Chain Reaction (PCR)
PCR involves 3 main stages:
Denaturing = the double stranded DNA is heated to separate them into single strands
Annealing = temperature is lowered so that DNA primers can attach to single stranded DNA. DNA primers are small sets of nucleotides of DNA which can target a specific region for amplification.
Extending = temperature is raised so a new strand of DNA is made using DNA Polymerase, an enzyme.
Capillary Electrophoresis
Capillary Electrophoresis uses electricity and a capillary tube. The capillary tube has negatively charged walls so a positive buffer solution runs through it, allowing molecules to flow evenly through. Electricity runs from two different sides: the anode and the cathode. The anode has a positive charge, so attracts negative molecules and the cathode has a negative charge so attracts positive molecules. Therefore, using this technique DNA can be separated by charge. It can also be separated by size because smaller molecules will move faster than larger ones. A detector creates an electropherogram.
Analysing the DNA is where individuals begin to match the suspect. An electropherogram looks like this:
They compare these peaks to the database of DNA. There are usually National Archives, State Archives (for the US) and Local Archives. These archives will contain the DNA profiles of missing people, arrested convicts, unidentified human remains and family members of victims. Court orders can be issued to obtain the DNA samples from any suspects if there is no match on the database. The DNA profiles are only of a specific part of a DNA sequence, therefore, multiple profiles might match. However, the search is at least narrowed down whilst other factors will need to be considered to convict a suspect of a crime.
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