Autorad is short for autoradiograph, the final result in a DNA analysis. An autorad resembles a bar code or a ladder and each different DNA sample will give a different pattern on an autorad. Therefore, an autorad can be an important piece of identification evidence in a forensic investigation. Autorads are also sometimes known as DNA fingerprints.
The procedure for creating an autorad has to be meticulously followed if the end result is to be of value in identification of a criminal. Thanks to advances in DNA technology, it is now possible to extract meaningful results from even tiny samples of DNA. The first step is to use specific chemical reagents to remove the DNA from the sample, be it blood , hair, or some other tissue. This depends on breaking down cell membranes and digesting proteins in the cell so the DNA can be released in a "clean" form ready for analysis.
DNA is a very long molecule and its analysis depends on chopping it into segments of a more manageable size. This is done using enzymes that produce a mixture of segments that are characteristic of the DNA in the sample. DNA from suspect A will give a mixture of segments of different lengths from the DNA of suspect B. The segments are then separated by first applying the mixture of segments to a slab of gel to which an electric current is applied. Shorter segments move faster through the gel than do longer segments. This creates the bar code pattern, but at this stage it is still invisible.
The pattern on the gel is transferred to a nylon membrane that is then exposed to radioisotopes. These attach to the DNA segments. The nylon membrane is now placed between two sheets of x-ray film and photographed. The segments now show up dark, through exposure to the film, and the familiar bar code pattern can be clearly seen. To make a comparison, DNA from a sample from the scene of the crime would be placed on the gel alongside DNA taken from a suspect, from their saliva or blood, for example, as well as with reference lab samples of DNA. The samples run in parallel. If the crime scene DNA is that of the suspect, then the two corresponding auto-rads will appear identical. If they do not, then this piece of evidence, at least, eliminates the suspect, although there may be other evidence linking the suspect to the crime.
see also DNA fingerprint; DNA isolation methods; DNA profiling.