DNA Profile Analysis
Summarizes whole text, results, conclusions, recommendations (under 200 words)
The purpose of this lab is to try to identify the person guilty of a burglary by looking into a crime scene and investigate which of the two suspects is the burglar. By using a method called “electrophoresis” we will try to identify the guilty suspect by matching the suspect’s DNA samples to the one found on the crime scene.
Deoxyribonucleic acid, DNA, is a nucleic acid, a macromolecule that stores genetic information; it is composed of 2 antiparallel sugar-phosphate chains and nitrogenous bases pairs connected together by hydrogen bonds. The ordering of the bases (Adenine, Thymine, Cytosine and Guanine) is called a DNA sequence and is what makes each organism so unique. The order differs from one organism to another; it determines the organism characteristics such as color, shape and size. (Campbell et al. 2018)
On average, 99.9% of our DNA is identical and ac acts like a ’code’ that “…directs the building and maintaining of an organism through the production of specific RNA and protein molecules…” (Ferguson, Cronin, 2017), with the exception of twins. The remaining 0.1% of the DNA is non-coding, there are about 3 million base pairs that are different from one individual to another. Scientists once thought that non-coding DNA was useless; however, in the early 1980’s, they discovered unique short repetitive sequences of non-coding DNA called microsatellite. This unique DNA sequence acts like a ‘fingerprint’ and makes it possible for forensic scientists to “…identify a guilty individual with a high-degree of uncertainty…” (Campbell et al. 2018) by looking at variations in the length of genetic markers using a process called gel “Electrophoresis”. (“What Is A DNA Fingerprint?” 2017)
If one of the suspect’s DNA fragments distance matches the exact same distance as the DNA fragments of the crime scene, then it means that he is guilty.
The lab technicians prepared the samples beforehand for running using a restriction enzyme and a blue dye with negative charge. We loaded the DNA samples into wells and proceeded with electrophoresis (taken care by lab technician). The gel was exposed to an electric current during 30-45 minutes; resulting in the migration of the DNA fragments from the negative end to the positive end of the gel. Lab technician stopped electrophoresis and proceeded with staining (10 minutes) and de-staining (30-45 minutes) of the bands of DNA. Once the de-staining was complete, the instructor placed the gels on light-boxes so we are able to visualize the DNA and interpret results. (Ferguson and Cronin 2017)
We did this experiment in group of 4 people with the help of the course instructor and lab technician. We followed the instructions found in the Biology 201, Introductory Biology laboratory manual, section 5: Deoxyribonucleic Acid; pages 5.3 and 5.4. I feel like the steps of this experiment were followed precisely as the sample DNA separated into multiple bands and the blue staining on the band demonstrated that the electrophoresis procedure “worked”. Furthermore, our results were conclusive.
Gel Electrophoresis in DNA Fingerprinting
Graph 1. Relationship Between DNA Fragment Size and Distance
The DNA fragments moved towards the end of the tray as predicted. Using a ruler, we measured the distance from each sample DNA movement. The known DNA fragments size and distance travelled in the gel from Sample 1 (Lambda) were used to graph the relationship between DNA fragment size and distance by plotting the size on the y-axis versus the migration distance on the x-axis. The data points yielded a straight line. I was able to estimate the size of the DNA fragments of sample 2-3-4 by measuring the distance travelled in gel from the 3 DNA samples and matching it with the corresponding size from Lambda sample. When all the data (Table 1-4) were plotted together on the graph line, it was evident that Sample 2 DNA matched the one found on the crime scene as the 4 data points on graph line corresponded.
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Moreover, the graph line (Graph 1) showed that as x increases, y decreases. The relationship between the DNA fragment size and the distance travelled in the gel is negative. Larger DNA fragments have a smaller distance whereas smaller DNA fragments have a greater one. Fragments who were the same size, had the same distance. Furthermore, each sample had a different pattern and distribution of data.
Although my experiment results seem accurate, one main possible source of error could have been technique as loading was not handled by the lab technician. We could’ve pipetted wrong; the DNA would’ve not made it into wells or placed wrongly as the gel is porous. Another source of error could have been contamination of the DNA samples by touching the tip of the micro-pipette. These errors can be minimized with proper preparation and handling. Furthermore, they could affect how valid and accurate the results are as there would be errors in reading the bands. (Schröder et al. 2008), (“Gel Electrophoresis – University Of Colorado Boulder” 2019)
The results of the experiment supported my hypothesis that if one of the suspect’s DNA fragments distance matches the exact same distance as each DNA fragment migrated the same distance. It took the exact same time to do so; therefore, proving that they would have the same size; both the crime scene and suspect 2 samples migrated the exact same amount. Furthermore, the experiment validated that the “…smallest fragments move the fastest and fragments of the same size move at the same speed…”. (Ferguson and Cronin 2017) This movement of fragments creates a ‘DNA fingerprint’ which is unique to each individual and can help confirm with certainty the identity of an individual from a DNA sample.
- Campbell, Neil A, Lisa A Urry, Michael L Cain, Steven A Wasserman, Peter V Minorsky, Jane B Reece, and Robert B Jackson et al. 2018. Biology (Canadian Edition). 2nd ed. Don Mills, Ontario: Pearson Education Canada.
- “Gel Electrophoresis – University Of Colorado Boulder”. 2019. Studylib.Net. https://studylib.net/doc/18326904/gel-electrophoresis—university-of-colorado-boulder.
- M. Ferguson, Ian, and Richard T. Cronin. 2018. “Lab 5. Deoxyribonucleic Acid”. Biology 201, Introductory Biology, Laboratory Manual, 5.3,5.4.
- Schröder, Simone, Hui Zhang, Edward S. Yeung, Lothar Jänsch, Claus Zabel, and Hermann Wätzig. 2008. “Quantitative Gel Electrophoresis: Sources Of Variation”. Journal Of Proteome Research 7 (3): 1226-1234. doi:10.1021/pr700589s.
- “What Is A DNA Fingerprint?”. 2017. Yourgenome.Org. https://www.yourgenome.org/facts/what-is-a-dna-fingerprint.
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