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DNA Profiling

A gel with one suspect sample vs. three suspect DNA profiles. The DNA profiles evidence A (e.g., blood sample from a crime scene) and suspect 2 are a match.

Four DNA profiles compared using gel electrophoresis. Each profile is shown as a vertical column of short horizontal lines that appear at different intervals. The first DNA profile is from the crime scene evidence, the remaining three belong to different suspects. One of the suspect profiles has lines in exactly the same pattern as the sample from the evidence, meaning that suspect's profile matches the evidence.

Every human has a unique DNA profile, similar to having unique fingerprints.

Unique DNA profiles

DNA profiling is a way of analyzing differences among individuals on the DNA level. This method is also called DNA fingerprinting, because similar to having unique fingerprints, we also have unique DNA profiles. Large amounts of our DNA are the same in every person. But some regions, called DNA satellites, consist of long stretches of small repeated sequences. The number of repeats in any of these sequences differs among individuals due to mutations accumulated through generations.

Visualizing DNA Profiles

To identify the DNA profile of an individual, we must look at the DNA in these particular regions. This is conducted by amplification using PCR (Polymerase Chain Reaction) and Gel Electrophoresis. PCR can amplify only the specific repeat regions by using specific primers. To perform DNA profiling, purified DNA is required. DNA can be isolated from most human cells.

Reducing risk of identical profiles by using 13 PCR primer sets

There is a chance that two individuals will share the same number of repeats at a repeated site. If we were to use only one primer set (amplify only one repeated region), the risk that an innocent individual shares the same number of repeats with the actual murderer would be too great. Therefore, a minimum of 13 tandem repeat sites are investigated while performing DNA profiling.

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Article
Tandem Repeated Regions