Friday, August 14, 2009
DNA Fingerprinting Using the PCR Process
DNA Fingerprinting (a.k.a. DNA Profiling or DNA analysis) is a sub-category of Biotechnology that has several uses among scientists as well as other fields. A broad definition of Biotechnology is, “any use or alteration of organisms, cells, or biological molecules to achieve specific practical goals” (Audersirk, Audersirk, & Byers, 2007).
Deoxyribonucleic acid (DNA) is located in the nucleus of every cell that has a nucleus. Its appearance is similar to a twisted ladder or staircase, which is referred to as a double-helix. DNA is an extremely long polymer made from four nucleotides: Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). It is the sequence of A, G, T, and C that codes information for each gene.
In 1986 Kary B. Mullis developed the Polymerase Chain Reaction process (PCR), that produces Short Tandem Repeats (STR) which are relatively small fragments of DNA (Audersirk, Audersirk, & Byers, 2007). This means that very small amounts of DNA, found at a crime scene for instance, can be multiplied by the PCR process.
There are two main reasons why the PCR process was such a huge breakthrough. The previous system took nearly four-five weeks for results to return from the lab, but PCR could return results within twenty-four hours (Ragle, 2002). Another reason was that the previous process required almost perfect samples of DNA, and there has to be a large amount to test successfully; while the PCR process requires a relatively small amount of DNA and is successful with almost every sample (Ragle, 2002).
Once in the lab, the DNA sample needs to be amplified. To do this, the DNA double-helix needs to be separated first. Heating a solution of the DNA to a temperature of 90C separates the two strands. After the strands unwind and cool, they are put into a DNA Amplifier and an enzyme called polymerase makes two new DNA strands; which are exact duplicates of the original. It takes approximately 4 minutes per cycle; each cycle doubling the amount of DNA. This process can be repeated every 4 minutes, which comes to 30 cycles every 2 hours. This means that in 2 hours, the small sample has been amplified 2^30 or 1 billion times.
“In 1999, British and American law enforcement agencies agreed to use a set of 10 to 13 STR’s…that vary greatly among individuals. A perfect match of 10 STR’s in a suspects DNA and DNA found at a crime scene means that there is less than one chance in a trillion that the two DNA samples did not come from the same person” (Audersirk, Audersirk, & Byers, 2007). In this can be realized the power and significance of this system.
In 1990 the FBI formed a “working group” to come up with a national data base that would hold all DNA Profiles collected (Ragle, 2002). They named this new data base, Combined DNA Index System (CODIS), the genetic equivalent to the Automated Fingerprint Identification System (AFIS). By 1994, CODIS was operational, but it wasn’t until 1999-2000 that most labs nationwide started relying on it as the official data base for sharing among agencies (Genge, 2002). Once most labs began testing the same thirteen STR points (1999-2000), CODIS could then be used to cross reference DNA Profiles from all over the United States; a practice that is widely used today. As of December 2004, CODIS contained 2,132,470 DNA profiles; and as of June 2009, over 7,137,468 offender profiles; and has assisted in more than 91,800 investigations (Federal Bureau of Investigation, n.d.).
Audersirk, T., Audersirk, G., & Byers, B. E. (2007). Biology: Life on Earth with Physiology (8th ed.). Upper Saddle River, NJ: Pearson-Prentice Hall.
Federal Bureau of Investigation - Laboratory Services. (n.d.). In Federal Bureau of Investigation Homepage. Retrieved August 04, 2009, from http://www.fbi.gov/hq/lab/codis/clickmap.htm
Genge, N. E. (2002). The Forensic Casebook The Science of Crime Scene Investigation. New York, NY: Ballantine Books.
Ragle, L. (2002). Crime scene from fingerprints to DNA testing, an astonishing inside look at the real world of c.s.i. New York, NY: Avon Books.
Trimm, H. H. (2005). Forensics the easy way. Hauppauge, NY: Barron's.