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Friday, July 30, 1999 Published at 17:08 GMT 18:08 UK


Sci/Tech

Q & A: DNA, the building blocks of life




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Forensic science expert, Professor Ian Shaw, answers some important questions about DNA:

What is DNA?

DNA is an enormous, complex molecule found in the nuclei of all cells in the body. It carries the genetic code and is responsible for passing information on from one generation to another.

Individual sections of the DNA molecule code for specific proteins - these sections are called genes. Each piece (or strand) of DNA carries many genes which might code for something as simple as eye colour or as complex as an individual's susceptibility to a particular disease.

The genes and their order on DNA are thought to be unique to an individual person. This is logical if you remember that a person's physical and biochemical makeup is unique and both are coded for by DNA. It is for this reason that DNA can be regarded as a "fingerprint" for an individual.

Why is it so important to forensic science?

DNA is so important because it can be extracted from samples of tissues or perhaps body fluids.

The extracted DNA can be investigated to look at its specific makeup. The total makeup is unique to an individual and can be matched up with the person from whom it originated.

Having said this, in forensic investigations fragments - not the entire strand - of DNA are investigated, which significantly reduces the uniqueness of the fingerprint. However, it is the most discriminating technique available to the forensic scientist currently.

How are DNA samples taken?

DNA is present in the nuclei of all cells. The DNA isolation procedure is relatively simple and only very tiny quantities of DNA are required for comparison. The DNA present in a few cells is sufficient for an analysis.

As scientists get better at isolating DNA they need smaller and smaller samples. Now, in theory, the DNA from a single cell is all that is necessary.

For this reason the DNA from a few cells from the inside of the cheek which might be found in saliva are sufficient, as are the skin cells left behind when someone touches something.

Clearly the possibilities are almost endless - we leave cells containing DNA everywhere we go, so, in theory, a forensic scientist could prove that we had been there.

How small can a sample be?

The sample can be very small. The DNA from a single cell is, in theory, sufficient. However, the bigger the sample the better the result so forensic scientists like to work with 20 or so sells in order to produce a reliable result.

How are samples matched up?

The DNA fingerprinting technique involves separating fragments of DNA under the influence of an electric current (electrophoresis). The isolated DNA is treated with enzymes to fragment it.

These fragments form a pattern of bands, which form the fingerprint, which is compared.

If we have a suspect for a particular crime we can isolate a small amount of their DNA (eg. from the cells of a plucked hair) and compare this with the DNA isolated from cells left at the scene of a crime (eg. sperm in a rape case).

If the DNA pattern (ie. order of genes) is the same for both samples, the sperm is likely to have originated from the accused. If they do not match, the sperm did not originate from the accused.

Because the use of DNA is an excellent means of identification and it is relatively simple to isolate and compare, it is commonly used in criminal cases.

How would profiles most likely be stored for a national database?

Profiles are most likely to be stored in a computer retrieval system so they can be called up and compared with samples taken from a current criminal case.

How easy is it for juries to understand DNA evidence?

It is easy for a jury to understand that patterns are compared and if they coincide, that DNA samples are identical.

If just a little of the background and process is explained to them they will readily understand the significance of a finding in the context of a criminal case.

Perhaps the most difficult thing for them to understand is the probabilityof whether two matched samples came from the same person.

Is DNA evidence foolproof? Can it ever be wrong?

No scientist would ever say anything is foolproof. In the case of DNA fingerprinting only a relatively small portion of the DNA from a cell is compared.

There is the possibility that more than one person might have the same pattern for such a fragment.

(If it was possible to look at the pattern generated from an entire strand of DNA, agreement between two samples would be absolutely infallible.)

In order to use such data reliably in criminal cases, forensic scientists include in their deliberations the probability that a certain pattern might occur. The lower the probability the greater its value as evidence. Of course, identical twins will have identical DNA.

What is the future likely to hold for DNA evidence?

In my opinion DNA evidence will continue to increase in usefulness and importance. As we build up a better database its use in eliminating suspects and tracing criminals will increase.

DNA fingerprinting is among the best examples of the application of complex, state-of-the-art science to the service of mankind.


Professor Ian Shaw is based at the Centre for Toxicology at the University of Central Lancashire. He is subject leader for forensic science.

UCLan runs a joint honours BSc programme in forensic science.



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