Crime scene forensics: How does it work?

From the painstaking work at the crime scene, forensic examination moves to the lab. Here, a scientist from the Metropolitan Police North West fingerprint lab talks us through a range of optical, physical and chemical techniques used to detect and enhance hidden marks.

Depending on whether the item is porous or non-porous, a range of treatments are used to obtain a visible representation of a finger mark which can then be photographed and sent for identification.

Finger marks linked to the scene are sent to the national fingerprint bureau at New Scotland Yard for identification against the national database. Here, a fingerprint expert explains how suspects can be matched to marks, and vice versa.

Footwear marks are the second most common evidence type left at a crime scene. The Forensic Science Service Lambeth has an online database that contains tread pattern and code identification gathered over the past 25 years, as one of its forensic scientists explains.

No two firearms are identical. Unique marks on the internal surfaces of the firearm are transferred onto the cartridge and the bullet during firing. These marks can provide enough information to allow them to be traced back to the firearm, as shown here by a scientist from LGC forensics in Leeds.

The generation of a DNA profile allows investigators to link an individual to a biological sample taken from a crime scene. Sample sources can include blood, saliva, semen, hair, bone, faeces, urine, teeth and tissue.

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The process of generating a DNA profile begins once an item that may contain biological evidence, such as blood or hair, is retrieved from the crime scene and sent to a forensic laboratory.
Cellular material can be collected by swabbing the item, either at the crime scene or at the lab. Some items may need a more complex initial examination such as a "presumptive test" which would be necessary on potential blood samples.
Once cellular material has been identified, a solution of single stranded DNA needs to be obtained from the sample. The parts of DNA not involved in encoding proteins, or "junk DNA", vary significantly from person to person and are therefore ideal for analysis.
A single strand is obtained by breaking-up the cells to release the DNA. A lysis solution is added to loosen the cells. The sample is then placed in a centrifuge which forces the proteins of the cellular material sink to the bottom, while the DNA strands remain in the top liquid. An alcohol solution is then added. DNA is not soluble and so it will clump together.
Following extraction, the DNA needs to be amplified or copied so that there is an optimal amount for analysis, the technique is called Polymerase Chain Reaction (PCR). Specific regions of the DNA are amplified so that its nucleotide sequence can be determined.
A primer is added to the sample to identify the target sequence. Two primers are needed per short tandem repeat (STR) of DNA and they bind to either side. The addition of a polymerase (or enzyme) causes the DNA between the primers to replicate, doubling in amount. The cycle of repeats results in the exponential growth of the target DNA sequence.
The amplified DNA then undergoes a technique to ascertain the number of repeats at a given loci (location), called electrophoresis, which separates the molecules according to length. The DNA carries a negative charge and is pulled through a polymer towards an anode when an electric field is applied. The smaller pieces migrate through the polymer more rapidly.
During amplification, the DNA is labelled by chemicals which identify fragments that end in a particular genetic letter; A, T, C or G. When a letter is spotted, fluorescent tags are added. The tags are read by a laser, giving a genetic sequence for that piece of DNA. The STRs are distinguished from each other by colour and length. The number of repeats are determined by comparison with the alleleic ladder - a set of DNA molecule sizes that correspond in length to known alleles at a given location.
The results from the analysis are plotted on an electropherogram. The profile is made up of peaks along a baseline. The human genome, the totality of our DNA, is gigantic, there are well over 10 million sites of variation. The chance that one DNA profile is the same as another using current technology would be about one part in 10 million, million, that's one part in 10 trillion.
   

Slideshows photographed and produced by Emma Lynch

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