Proper DNA collection can make or break a case. There are certain standards that investigators must follow when gathering evidence to maximize the viability of the samples. To first get samples from a person of interest, investigators use buccal (cheek cell) swabs, and in some cases, blood draws.
How is evidence stored?
All evidence is usually dry before being packaged, as moisture incurs bacterial growth, which would lead to severe damage and decay of the DNA. DNA is contained within cells, so any chemicals that might damage the cellular environment will also affect the DNA. Furthermore, stains or other fluids are often used as evidence; when not completely dried, these may rub off on packaging. The evidence must then be handled with non-powdered gloves, which are changed in between handling of new evidence. Oftentimes, samples from victims’ clothing need to be cut out, and scissors are properly disinfected before coming into contact with the item. Finally, the stains are never stored with plastic as plastic traps moisture.
When retrieving already dried evidence, one would use a disinfected scalpel or knife to scrape the evidence off, although a considerable amount of the sample will likely be lost.
Tissue samples – which may come from surgery or autopsy – must be “in sterile, plastic containers and should be frozen as soon as possible.” However, since plastic is damaging to DNA, the containers will not have tissue samples that need DNA testing.
How is DNA processed in the laboratory?
DNA is found within the nucleus in the cells, and so chemicals are first injected into the samples to break open the nuclear membrane and release DNA. Sometimes, extraction is also performed on the DNA samples in solution to isolate the DNA from any other cell components.
Following extraction, the DNA must be amplified. Samples usually contain very little DNA, so PCR (Polymerase Chain Reaction) is used to make millions of copies of the DNA in just a few hours. Fluorescent tags are added to the copies to make them easily identifiable.
Finally, the PCR products are distinguished from each other based on size using a technique called gel electrophoresis. Gel electrophoresis consists of a gel-like polymer through which samples must move — smaller samples here migrate faster through the gel than larger ones. Since the STRs (the key regions of DNA used by forensic personnel to make DNA matches) are of different sizes, isolation based on size allows for the isolation of respective STRs. The results of the gel electrophoresis are then entered into software to develop a DNA profile.
How are DNA matches made?
CODIS (Combined DNA Index System) is the national databank of DNA samples that law enforcement has access to. CODIS includes DNA profiles from victims, offenders, arrestees, “unidentified human remains,” and other crime scene evidence. While CODIS is overseen by the FBI, it is accessible by forensic laboratories at the local, state, and federal levels, so that reference samples can be collected, DNA profiles can be uploaded, and matches can be made from anywhere.
Within CODIS, evidence is divided into several “indices,” which categorize the DNA profile based on who it was collected from. These include the Convicted Offenders Index, the Victims Index, the Voluntary Donors Index, and the Crime Scene Index.
But how are the matches made? With over 99.7% of the human genome shared by all individuals, there are a few DNA regions that distinguish us. Since the part of the human genome that codes for proteins cannot differ significantly for each person without creating faulty or ineffective proteins, scientists look to the noncoding areas of DNA. In fact, most of our DNA (around 3 million DNA bases) does not code for anything, and within these regions one can find several short, repeating sequences of bases, called STRs (Short Tandem Repeats).
These DNA sequences repeat a different number of times in different individuals. Previously, the FBI had recognized 13 STRs that “naturally occur[ed] in people of different ethnic backgrounds” in the US. Since each STR can have a different number of repeats of the specific sequence, “[i]f any two samples have matching genotypes at all 13 CODIS loci, it is a virtual certainty that the two DNA samples came from the same individual.” Each STR “version” (known as an allele) has a certain frequency, so as a higher number of specific STRs match a person’s profile, the frequencies are multiplied. This implies that the probability of the DNA of the person being unrelated to the DNA of the sample decreases significantly, making the match more likely. Usually, around 8-10 STRs are examined by scientists to build a DNA profile, although examining more would yield more accurate ones.
However, the probability calculations must “account for the frequency with which each STR allele occurs in the individual’s ethnic group,” since similar populations will naturally tend to yield higher probabilities. The same goes for people directly related, such as siblings. Fortunately, in 2017, the FBI updated its database to recognize 20 STRs instead of 13, which will augment the sensitivity of these screenings and significantly reduce the risk of false positives. In recent years, these developments coupled with more precise DNA collection and storage guidelines have increased the reliability of the DNA profiling system.
Sources:
https://www.doj.state.wi.us/dfs/dna/dna-databank
https://www.rcmp-grc.gc.ca/en/forensics/use-dna-criminal-investigations
https://nij.ojp.gov/topics/articles/what-str-analysis
https://www.nature.com/scitable/topicpage/forensics-dna-fingerprinting-and-codis-736/
https://www.mesaazpolice.gov/about-mesa-pd/forensic-services/biology-dna-serology
https://www.sciencelearn.org.nz/resources/1980-dna-profiling
https://dps.mn.gov/divisions/bca/bca-divisions/forensic-science/Pages/dna-procedures.aspx
http://courseresources.mit.usf.edu/~test/lecture/files/pdf/cas_sample.pdf
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