1. Why Do Cheek Cells Make A Convenient Source For Obtaining DNA Samples?

In this laboratory, two experiments were performed; the first one was performed to determine whose fingerprint was found at a crime scene. For this experiment, four-10 mL DNA samples were placed in a well of agarose gel, using a micropipette; the sample was also covered in a TBE buffer solution. The four samples were then put in the electrophoresis apparatus to be run through electrophoresis. After 20 minutes, the samples were ready and the results were documented; it was found that suspect “B” was the culprit.
The second experiment was performed to isolate and extract DNA from epithelial cells. 1 mL of 8% sodium chloride solution was put into a test tube. 10 mL of tap water was put into a cup and one individual from each lab group put
the water in their mouth, swirled it around for 30 seconds to wash cheek cells, and then deposited the cheek cells and water into the same cup; the contents of this cup were then poured into the test tube filled with the sodium chloride solution. Then 1 mL of 10% sodium lauryl sulfate was added to the test tube and the tube was gently mixed, releasing the DNA from the cheek cell membrane. 5 mL of 95% cold ethyl alcohol was added to the test tube to precipitate the DNA; the DNA was then collected using a glass extraction rod and winding the polymers around the probe. The strands of DNA were then placed in 2 mL of 95% ethyl alcohol and observed.
Four questions were posed for points of discussion in this lab.
The first question was “Why do cheek cells make a convenient source for obtaining DNA samples?”. Cheek cells are composed of epithelial cells, which are those cells that “cover the internal and external surfaces of the body and line hollow structures,”(Facts On File). These cells are flat and cover the surface of the inner cheek. This is suitable for harvesting DNA because of the presence of the Barr body, “A condensed clump of chromatin (DNA) which corresponds to an inactivated X chromosome,” (Facts On File). The Barr body is only found in females, so when extracting DNA from cheek cells to determine sex, the DNA will match a female if the Barr body is found. The only way that a male could have a Barr body is if a genetic mutation occurred; in that case, the extraction of DNA from cheek cells would be
unsuitable.
The next question posed was “Why were the cells processed in the manner as noted above?”.
When the DNA was released from the cheek cells, 1 mL of 10% sodium lauryl sulfate was added to the test tube. Sodium lauryl sulfate works as a detergent that removes the cheek cell from its cell membrane, allowing the DNA to be released into the test tube. After the DNA was released into the test tube, 5 mL of 95% cold ethyl alcohol was added so it formed a layer over the cheek cells and so it could start precipitation. Any clouding that formed between the cheek cells and ethyl alcohol was a polymer of DNA; this clouding occurred because DNA is insoluble in alcohol. DNA can then be “spooled” onto the glass rod because the polymers of DNA are now visible. Therefore the DNA precipitates where the cheek cells and ethyl alcohol
meet.
The final question asked, “What processes must be employed in a forensic setting to ‘fingerprint’ this DNA for identification purposes?”. DNA typing is a very valuable tool in the field of forensics today. The AFLP (amplified fragment length polymorphism) technique is a way that forensic scientists today can find the DNA sequence of any organism. This method of DNA fingerprinting is based on the polymerase chain reaction- PCR (Nature Protocols. 2.6. p 1387). PCR is used to analyze DNA and is much less time consuming than other methods of analysis. PCR “takes very small amounts of DNA from biological evidence and making millions of copies of it,” (Turman 3). This allows the DNA analyst to generate a DNA profile according to that sample; this profile can then be submitted to CODIS, which is a database maintained by the FBI that stores fingerprints and other criminal evidence.
The AFLP process is as follows: restriction digestion by restriction enzymes, adaptor ligation, preamplification, selective amplification, and gel electrophoresis. This process has many advantages over other DNA marking systems, including simple sequence repeats and single nucleotide polymorphisms. But the most important advantage is AFLP requires no prior DNA sequence information or profile. (Nature Protocols. 2.6. p 1387). However, the AFLP system does have limitations; some of the sequences and fingerprints could share some of the same amino acids. This means that the AFLP method is not able to compare two genomes to each other, because they might share some of the same DNA sequences and the AFLP cannot recognize which one is which. (Nature Protocols. 2.6. p 1387). With a population so vast and one that spans many genotypes, there are variety of DNA sequences in many different individuals. One must be certain that the DNA sample matches that of a suspect, because otherwise one could be falsely accusing another.