DNA
Knowledge of DNA and laboratory procedures will help Officers of the Court prepare for and participate in criminal cases involving DNA evidence. Without DNA evidence, the cold case depicted in the case study would likely not have been reopened.
It is important to understand how proper collection and storage techniques can affect DNA evidence and how results of DNA testing can impact the outcome of a criminal case.
DNA Defined
All organisms are made up of cells. A human has about 100 trillion cells; all of them (except for red blood cells) contain genetic material known as deoxyribonucleic acid (DNA). DNA is sometimes called a "genetic blueprint" because it contains all of the instructions that determine an individual's genetic characteristics.
Within an individual, the DNA profile obtained from all tissue types (blood, saliva, tissue, hair, semen, etc.) will be the same.
DNA is used to differentiate individuals, but nearly all human DNA is the same; less than 0.1% is highly variable. This variability is referred to as polymorphism. These highly variable regions of the DNA (those with a high degree of polymorphism) are tested in forensic DNA analysis.
In the case study, critical biological material was present at the scene of the crime (the rape and murder), even though much of it was not visible to the naked eye. This material, which was carefully collected and stored, became key evidence for the reopening of the case and the filing of formal charges against a former person of interest.
Nuclear vs. Mitochondrial DNA
There are two types of DNA used in forensic testing: nuclear DNA and mitochondrial DNA (mtDNA).
Nuclear DNA is found in cells that have a nucleus. These cells are referred to as nucleated. In nucleated cells, DNA is packaged into 23 pairs of chromosomes, or 46 total. Half are inherited from each parent. The only exceptions are sperm and egg cells, or sex cells, which contain 23 pairs of chromosomes.
Mitochondrial DNA is found in cell cytoplasm and is passed only from a mother to her children. A man inherits mtDNA from his mother but does not pass it to his children. In this respect, it has an advantage and a disadvantage.
An advantage is that the mtDNA of only a single maternal relative, even distantly related, can be compared to the mtDNA from skeletal remains believed to be from a missing person. Comparisons can also be conducted between maternal relatives and crime scene samples in a "no body" homicide.
A disadvantage of mtDNA is that it only excludes individuals from being sources of a sample; it does not identify them. Maternal relatives share the same mtDNA type; therefore, the source of a biological sample cannot be conclusively identified with mtDNA.
Nuclear STR Testing
The most common method used for nuclear DNA testing is called short tandem repeat (STR) technology. There are segments of DNA that have sequences that repeat in short, adjacent, or tandem segments. STR testing has a very high degree of discrimination and can be used on partially degraded, small-quantity DNA evidence.
Mitochondrial DNA Testing
Mitochondrial DNA can be useful for samples that either do not contain a sufficient quantity of nuclear DNA or are not of sufficient quality for nuclear DNA testing. Shed body, head and pubic hairs with no cellular material (hair follicles) and aged skeletal remains are the samples that are most commonly analyzed for mtDNA.
DNA Degradation
When DNA degrades, the DNA strands start to break apart, and the breaks may occur at regions evaluated in forensic DNA testing. For this reason, degradation of DNA may affect the ability to obtain a DNA profile, thus limiting its evidentiary value.
Degradation of evidence is caused or hastened by:
- Prolonged exposure to UV light.
- Heat, humidity and moisture.
- Bacteria and fungi often found in soil and foliage.
- Acids or chemical cleaning solutions such as bleach.
Poor evidence handling and storage techniques may have contributed to the degradation of the DNA evidence collected from the vaginal swabs in this case study. For example, evidence may not have been properly dried or may have been subjected to extreme heat and humidity for a prolonged period of time.
Biological evidence should be properly dried, packaged and stored to minimize degradation. Note that cool environments are preferred, although research has shown room temperature is suitable for storing dried stains, as long as humidity is controlled.
