Short Tandem Repeats (STRs)
& Genetic Identity Field Trip

Background Information

DNA and the information that it contains has long been a source of inspiration for scientists and the public; from the time when DNA was first discovered to be the biochemical responsible for the transmission of traits, through the understanding of its chemical language, and into the current era of complete genome sequencing. For as much as we have learned about DNA, it remains for many a stunning combination of information treasure trove and Pandora's box.

When CBS first aired the television show CSI in 2000, it exposed a broad television audience to the ideas that the tools of biology and chemistry could be used for solving mysteries and for bringing about justice. Among the technologies in this high-tech police toolbox is the use of DNA for genetic identification. Real-world forensic DNA typng was in its infancy in the 1980s and early 1990s, and by the time the show CSI came on the scene, the technology was coming in to a robust, and rigorously tested, maturity.

The STR & Genetic Identity Field Trip covers some of the general concepts of how people are identified and distinguished from one another based upon the As, Cs, Ts, and Gs that are contained within their DNA.

In 1984, Sir Alec Jeffreys discovered that repetitive regions of non-coding DNA were conserved within a person's DNA, and that they were passed from parents to offspring in a Mendelian way, like genes. He worked out a method for identifiying these DNA regions, called Variable Nucleotide Repeat Regions (VNTRs).

The above diagram is an example of what is meant by a tandem repeat region in DNA. The top strand represents one copy of one person's DNA (say, the maternal copy). The bottom strand represents the second (paternal) copy of that person's DNA. (A single VNTR repeat is anywhere from 10 to 60 DNA base pairs (bp) in length. A single STR repeat is between 2 and 5 bp in length.)
The person's DNA in the above diagram has inherited 7 and 8 repeats of the tandem repeat region from his or her parents in a heterozygous manner.

Dr. Jeffreys' technique for examining a person's VNTR repeats involved using restriction enzymes to chop up a person's DNA, running the resulting fragments through gel electrophoresis to separate the different-sized pieces into bands, and then using radioactive DNA probes to see the bands of interest on the gel when exposed to film. Because the patterns of bands that were seen differed between individuals, the technique became known as DNA fingerprinting.

In 1987, authorities in Leicestershire, England asked Dr. Jeffreys to use his VNTR DNA fingerprinting technique to help them prove that a suspect had committed two separate murders. The result was surprising. It showed that the suspect was not guilty, but tha some other man had been involved in both killings. More detective work led to the apprehension of the real culprit, who was later shown to be the killer using the VNTR technique. This case was the first time that DNA had been used to both exonerate an innocent person and to convict a guilty person of murder. Since that first case, the technology used to do DNA fingerprinting has changed drastically, though the underlying principles remain the same.

• Today, DNA fingerprinting makes use of the polymerase chain reaction (PCR)technique which is roughly 1,000 times more sensitive than what Dr. Jeffreys was able to do.
• Short Tandem Repeat (STR) regions that have a length of 2 to 5 base pairs per repeat have replaced the longer Variable Nucleotide Tandem Repeats (VNTRs), in part because the shorter length of the DNA regions make them easier to amplify using PCR.
• The process of separating the PCR-amplified fragments is still done using electrophoresis, but employs a capilary electrophoresis technique, like that used in DNA sequencing, that is capable of separating two fragments of DNA that are only one base pair different in size. The bands that result are visualized by using fluorescent molecules that are incorporated into the PCR primers used to generate them, so radioactivity is no longer used.

Students who participate in the BTC Institute's Short Tandem Repeat & Genetic Identity Biotechnology Field Trip will set up a PCR, amplifying DNA fragments of different length. They will then separate those fragments using gel electrophoresis, and analyze the results to determine heterozygosity or homozygosity of a polymorphic DNA region of varying length. Additionally, participants will analyze the results of real STR DNA fingerprint data using the statistics of population genetics, gaining an appreciation that determining human identity from DNA combines the concepts of science and math (biotechnology and statistics). Students are encouraged to bring along scientific calculators.

If you have any questions about this field trip, please give us a call, or bring your questions along and we can discuss them during the lab. Thank you for your interest in BTCI's Biotechnology Field Trips program.