Forensic scientists in the DNA section of I-MCFSA analyze DNA (deoxyribonucleic acid) from evidence items. DNA is a chemical found in the nucleus of cells that contains the genetic instructions used in the development and function of living things, including humans. Profiles developed from the DNA collected from evidence items can be used to link victims to suspects, link individuals to crime scenes or objects used in a crime, and to eliminate individuals as contributors of DNA to specific evidence items.
DNA forensic analysis relies on one key characteristic of DNA: it is the same in all cells of an individual. These include muscle cells, brain cells, liver cells, blood cells, sperm cells, skin cells, and others. Every part of the body is made up of these tiny cells and each contains a sample or complement of DNA identical to that of every other cell within a given person. Also, with the exception of identical twins, every individual’s nuclear DNA is different.
Evidence collected from crime scenes often contains cells left behind by individuals present during the crime. Forensic scientists extract or collect the DNA from cells from the evidence and analyze the DNA collected. Forensic scientists focus on certain genetic sequences called short tandem repeats or STR. STR are short repeating DNA sequences found throughout the chromosomes in the non-coding regions of the DNA. The number of repeats at each STR location is highly variable among individuals. By examining the number of repeats in a series of STR regions, forensic scientists are able to identify individual DNA profiles. Currently the forensic scientists at I-MCFSA look at 15 STR regions for each piece of evidence examined.
In order to generate a DNA profile, after DNA has been collected from the evidence, forensic scientists measure how much DNA has been collected and then make millions of copies of the STR regions of the DNA using PCR (polymerase chain reaction). The number of repeats from each STR region is determined using a genetic analyzer and a DNA profile is developed. This DNA profile can then be compared to known DNA profiles from individuals who may have been present at the scene, such as victims or suspects, or if eligible entered into the National DNA database for comparison with other DNA profiles.