DNA Fingerprinting

DNA fingerprinting has been used extensively in biology and forensic investigations. We have used this technology to do everything from identifying global human migration patterns over the past million years to convicting criminals based on DNA left at the scene of a crime. In this lab, you will be introduced to one technique commonly utilized in such investigations.

But how is it possible to differentiate humans that share 99.9% of their DNA? This means that all the variation in humans across the globe is caused by only a 0.1% difference in their DNA! And it is that 0.1% of the DNA that we can use to differentiate among humans.

If a crime were committed and the criminal’s DNA was left at the scene, we can test that sample against several suspects and potentially find a match.  First, a sample must be taken of the suspect (Fig. 1a). Next, the DNA must be isolated from the cells from the sample (Fig. 1b). Since humans share so much DNA in common, we isolate the DNA segments that are known to vary greatly from human to human (Fig. 1c). Current technology can not easily detect single pieces of DNA. We use our knowledge of how DNA replicates to artificially replicate the DNA segments millions of times in a process known as Polymerase Chain Reaction, PCR (Fig. 1d). This allows us to easily visualize those segments of DNA via gel electrophoresis (Fig. 1e). In this process, the DNA segments are loaded into pockets of a gel. Electric current is run through the gel, causing the different segments of DNA to move at varying rates. Larger segments move less distance than smaller segments, creating a unique series of bands. Comparing these bands allows us to easily identify DNA matches between evidence at the scene of a crime and potential suspects. 

Of the 0.1% of humans DNA that varies from human to human, the most variable parts of DNA are known as introns (Fig. 2). In eukaryotic organisms, transcription occurs in which DNA codes for the production of messenger RNA (mRNA). Following transcription, mRNA gets further processing within the nucleus of the cell. Segments of the original mRNA are removed, known as introns. The remaining segments of mRNA, exons, are reattached before leaving the nuclear pore. The mRNA at this point is known as mature mRNA. The processed mRNA travels to a ribosome and codes for the synthesis of a protein. 

While mutations rates occurs at random along the DNA, mutations tend to accumulate in intron regions of the DNA, because they do not affect the eventual proteins. When mutations occur in exon regions, they have the potential to change the eventual proteins created which can lower evolutionary fitness, and are therefore less common than intron mutations.