Unravelling the strands of DNA

The process of obtaining a DNA profile from a crime stain is a complex process and involves a number of steps.

Firstly the biological material has to be recovered from the item or the stain. This step is usually carried out by the crime scene examiner or forensic examiner. This step is arguably the most important step and is one that is often overlooked.

This biological material will be made up of different components such as nuclei, proteins and various cellular structures.

Within the nucleus of the cell is the DNA, which is the bit we want. So we have to extract the DNA from the cell. We can do this by adding a detergent to a solution containing these cells.

These detergents disrupt the cell walls and that of the nucleus. This exposes the DNA. The next step is to isolate the DNA away from the other components.

One way of doing this is to run the sample past a silica membrane. In a high saline solution, the DNA binds specifically to the silica membrane.

This membrane is then washed which removes the remaining components of the cell. This allows us to obtain a highly purified DNA sample.

However, a common feature of DNA samples is that we only have a very small amount to work with; too small to carry out any meaningful analysis. This was a big stumbling block, not just for forensic genetics, but medical genetics as well.

This changed in 1985, when an American biochemist, Kary Mullis, came up with a technique called Polymerase Chain Reaction or PCR. PCR gave us the ability to “amplify” DNA. For his work in this, Kary Mullis won a Nobel Prize for Chemistry.

DNA is a double stranded molecular structure. The first stage of PCR is to break the bonds between the two strands, resulting in two separate single stranded molecular structures. We can then add an enzyme which then binds to each single strand. This enzyme is called taq polymerase, which is like a builder enzyme.

DNA can be considered as a string of letter made up of G, C, T and A. As said before, DNA is a double stranded structure; therefore it is more accurate to say that it is made up of a string of paired letters, such as GC and AT.

G and C can only bind to each other as can only A and T. Therefore, the builder enzyme can only add a certain letter. So for example, if the enzyme is at the letter G, it can only add C and if it is at the letter A, it can only add T.

Once the enzyme has done this to the whole structure, each of the single stranded DNA has turned into a double stranded DNA.

Only this time, there are two of them. This step is called a cycle and one cycle can double the amount of DNA.

In standard DNA profiling, 28 cycles are used. So the DNA has been doubled up 28 times. If you try working it out yourself, i.e. 1, 2, 4, 8, 16, 32, 64, 128 and so on, you can see that this ends up being a very large number. This gives us sufficient amount of DNA in order to obtain a DNA profile.

It is possible to use even higher cycle numbers. For example, there is a specialist DNA profiling technique called Low Copy Number or LCN. This utilises 34 cycles and is capable of obtaining DNA profiles from very small amounts of DNA such as touch DNA.

However, this does not come without problems. PCR is often described as a biological photocopier, but if you copied a photocopy and did that several times, you would end up with a very poor quality image.

This can be said of PCR. If you amplify up too far, you can end up with a very poor and misleading result.

One of the most famous cases involving the use of LCN was the Omagh bombing trial. This case collapsed because doubts were raised about the usefulness of LCN.

This subsequently led to the use of LCN being temporarily suspended for forensic case work. However, an inquiry was carried out that showed that LCN was scientifically robust.

The flaw in the Omagh trial was down to poor recovery of the DNA in the first place.

While the use of PCR in DNA profiling has caused a paradigm shift in the way cases are investigated, it is only useful if the DNA has been properly recovered by the crime scene examiner in the first place. If this is not done properly at the first step, no amount of wonderful technology can overcome this.


Graham Williams is a senior lecturer in forensic science at the School of Applied Sciences, University of Huddersfield.