Undeniable Identity – DNA Fingerprinting

Tommie Lee Andrews, Kirk Bloodsworth, OJ Simpson.  What do all of these supposed criminals have in common?  In all three of these people’s trials, significant evidence was offered through an interesting, and increasingly important method known as DNA Fingerprinting.  Though modern technology offers a wide variety of processes for this purpose, often involving the increasingly inexpensive process of DNA sequencing, the most popular process is known Restriction fragment length polymorphism analysis, or RFLP analysis.  What does this advanced process used both dubitably and effectively by some of the scariest people in our society entail, you ask?  Well, nothing a high school biology class can’t accomplish in their classroom.  We start off by gathering our sample in the form of cells from a swab on the inside of our cheeks.  This is an easy sample for us to gather, but in a police investigation, anything goes, be it blood, semen, hair, or most any sort of remains, as most tissue contains the segments of DNA that make up an individual’s fingerprint.

Once our simple scientific process of separating our DNA from the cells through the destruction of various other parts of the cell yields the DNA that we are to analyze, it is necessary to replicate it via a process known as Polymerase Chain Reaction.  However, before our DNA can undergo this process, we must select a specific location in our DNA to compare, known as a variable number tandem repeat, or VNTR.  These regions are located in the 95% of our DNA which does not serve the purpose of holding the instructions for the creation of our bodies, and consist of a single sequence repeated a varying number of times.  The VNTR chosen by us was D1S80, which has 29 different possible lengths.  Considering the fact that each person has two copies of every gene, one from each parent, this gives 435 different possibilities for the identity of a person by their combination of these two VNTRs.  This makes it highly unlikely for two people in our class to have the same genetic identity here, but this probability can be reduced much farther.  The FBI tests 13 different locations in subjects, creating 100 billion different possible identities.  With a world population of just 7 billion, this makes it nearly impossible to find another person with the same genetic identity, excepting an identical twin.

PCR itself is a process which involves the replication of a certain VNTR in a chromosome, as marked by primers made of small pieces of DNA, by an enzyme which constructs DNA out of separate nucleotides, called DNA polymerase.  The PCR Machine that we use cycles the mixture of these materials through a variety of temperatures, resulting in billions of copies of a VNTR.

Now that plentiful copies of the VNTR are available, these molecules are dyed and put through a process known as electrophoresis.  Electrophoresis is the exposure of a material in a gel to two different charges, allowing said materials to be separated based on charge and size.  All DNA being negatively charged, smaller VNTRs, with less repeats, proceed towards the positive diode more quickly than larger ones, allowing them to be compared to a ladder to determine their approximate base pair length.  If for example, a sample of DNA from a suspect in a murder case is compared with blood left at the crime scene are put through this process together, this final display will show visibly where the two bands (one for each gene from each parent) show up in the electrophoresis gel, displaying the blood to either match the suspect’s DNA sample, or not, perhaps proving the suspect guilty or innocent.  Not only is the potential of this great in that it offers scientific backing for court verdicts, but it also significantly speeds up court proceedings by avoiding more easily falsified forms of evidence, as DNA is quite difficult to fabricate in most crimes.

Students and witnesses of the OJ Simpson trial may recall that one of the important arguments brought up by the defense was the incompetence of the police crews that facilitated DNA fingerprinting.  Due to situations like this, national investigations require analysis of DNA by professional laboratories, unlike our little biology lab at school.  One of the most exciting developments in the field as of late has been the development of kits that would allow analysis of DNA information within 90 minutes at a crime scene.  Not only would this offer the chance to decrease contamination, but it would further increase the effectiveness of the technology, and help it accomplish the purpose it was created to: to enforce true justice on the right people.


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