Deoxyribonucleic acid (DNA) is the hereditary material in humans and almost all other organisms. It is estimated that nearly every cell in a person’s body has the same DNA. Even though most of it is found in the nucleus of the cell, a small amount could also be found in the mitochondria referred to as mitochondrial DNA or mtDNA.
As we all know the hereditary information that determines the characteristics and traits of an individual is stored in the DNA in the form of code made up of four chemical bases, Adenine(A), Guanine(G), Cytosine(C), and Thymine(T).
Human DNA consists of about 3 billion bases and more than 99 percent of those bases are the same in all people. These bases pair up with each other, A with T and C with G to form units called base pairs. Each of them is attached to a sugar molecule and a phosphate molecule.
All three of these, a base, sugar, and phosphate molecules are together called a nucleotide. These nucleotides are arranged in two long strands that form a spiral called a double helix and this is the structure of DNA.
DNA has the unique property of replication. DNA can replicate itself or in simple terms make copies of itself. This feature of our DNA is what makes it extremely important to forensic scientists and in other subfields of biotechnology.
Sources of DNA
As mentioned earlier almost all cells consist of DNA. Thus it is pretty much easy with the advancements in technologies to nowadays obtain DNA from any source. But until a technique like PCR, which can make more numbers of existing DNA samples from a minute trace of a sample is not employed, we require more DNA samples. Thus the term “important sources of DNA” comes into the picture.
1. DNA From Blood
Not only from a crime scene, even for laboratory research employed by biotechnologists blood is the most primary and easily available source of DNA. Blood is rich in Red blood Corpuscles (RBC). However, we do not extract DNA from RBC, because as part of the maturation process, human red blood cells destroy their cell nuclei. They do this to carry as much oxygen as possible and still stay small enough to fit through narrow blood capillaries, thereby maximizing the oxygen delivery.
Human red blood cells simply destroy their nucleus once it is no longer needed as part of the maturation process. A ring of actin within a maturing red blood cell pinches and splits the cell into two parts: one part with the DNA and one part without. Red blood cell enucleation is therefore a special type of cell division.
Macrophages from leucocytes(WBC) then come along and gobble up the parts containing DNA, leaving only the red blood cell parts that don’t have DNA. There is much more in blood than red blood cells. As a result, a blood sample does contain DNA due to the presence of other kinds of cells. Blood contains WBC and platelets as well.
Hence for isolating nuclear DNA leucocytes are used which are primarily nucleated. This is the case of peripheral blood which runs through veins, arteries, etc. Blood plasma could also contain a sufficient amount of nucleic acids, hence these are extracellular nucleic acids or circulating nucleic acids from where we could obtain DNA. Exosomes and microvesicles present in our body could provide this.
2. DNA From Hair
Both scalp and pubic hair are frequently encountered at the crime scene. Human hairs indeed contain DNA and it can be extracted and analyzed for matching.
The hair shaft is keratinized and is cylindrical in structure. The core center of the hair is the medulla, it is surrounded further by the cortex. Finally, the cuticle has a distinct hair pattern and consists of overlapping keratinized cells that provide this pattern. Each hair follicle is located deep in the dermis and holds the root sheath. The hair follicle is composed of two regions mainly, the bulge and bulb. Human hair grows for over a year and sheds according to the follicular cycle.
If the hair sample received consists of its root then nuclear DNA analysis could be performed. But if it does not have roots then one will have to depend upon mitochondrial DNA(mtDNA). But again this is reliable only if the sample has not been contaminated or its cells have not been damaged. If nuclear DNA is obtained regular subjective amplification methods like PCR could be performed. If mtDNA is obtained, a sequence polymorphism analysis could be carried out.
3. DNA From Skin Cells
As we know every cell consists of a nucleus and hence DNA except RBC. As most of the contact a person has is through their bodily skin, it is a major source of DNA. It can be found on worn clothing, surfaces, along with fingerprints or even bloodstains. Just like blood nuclear DNA is obtained from skin contact which can be considered as trace DNA evidence.
4. DNA From Bone & Teeth
When decomposition occurs bones are the last ones to be left behind. Most of them can even turn to fossils without decomposing. In humans, red blood cells are synthesized within the bone marrow. As we know RBCs contain DNA before their maturation while they are inside the bone marrow. Other than these cells other bone cells could also be present and all of them are rich DNA sources.
Even if the body decomposes, bones or the whole skeletal structures are left behind which makes them a very important source of DNA. Most DNA in cortical bones is located in the osteocytes. However during the decomposition process both nuclear and mtDNA are subjected to degradation hence the sooner one recovers the bone the better it would be for DNA analysis.
Other than bones, there has been quite an amount of research going on in the field of dental DNA extraction as well from dental pulp.
Mitochondrial DNA From Cells & DNA From Body Fluids
Mitochondrial DNA cannot prove the whole pedigree linkage; it can only link the mother side of DNA as sperms do not contain mtDNA and it is the ovum that contains the same. Neither are they regenerated during sexual reproduction. Hair cells and all other cells are rich sources of mtDNA, but it is preferred only in the absence of nuclear DNA.
Semen is a common biological or serological evidence encountered in sexual assault cases where DNA evaluation plays a major role. If the received semen stains or swabs collected contain non-damaged sperm cells with its head intact, then DNA extraction and identification is 100% possible. But most of the sexual assault case victims who make out alive are not aware of the importance of this evidence and tend to take a shower or wash themselves post the attack which destroys the sperm cell from the pubic area. Some of them go unreported for years, where semen evidence from clothing or surface can last only up to 8 months.
Other body fluids that contain conditional analysis probability are saliva and sweat, though they are not completely rich in DNA. Only in the absence of all the above-mentioned biological evidence, one would go for saliva or sweat for DNA extraction which is a rare case.