DNA fingerprinting is a technique to create an image of someone’s DNA. Aside from identical twins, each person has a unique pattern of short DNA regions that are repeated. These stretches of repeated DNA are of different lengths in different people. Cutting out these pieces of DNA and separating them based on their lengths gives an image that represents a person’s unique DNA signature. DNA can be extracted from any cell that contains DNA. Common types of tissues from which DNA is extracted include blood, saliva, hair, sperm, skin and cheek cells.
Blood and Saliva
Blood contains many types of cells. The most abundant are red blood cells, but those do not contain DNA. However, blood contains many immune cells that patrol the body looking for foreign invaders. These cells have DNA that can be extracted. Cells called neutrophils, eosinophils, basophils and monocytes circulate in the body through the bloodstream. T cells and B cells, or lymphocytes, are also in blood. Saliva is another bodily fluid that contains DNA. The DNA isn’t free-floating in saliva, but can be extracted from the white blood cells that are in saliva.
Hair is one of the most popular sources of DNA in movies and TV shows that involve crime scenes. However, there isn’t as much DNA in a hair as most people think. A hair has a shaft that extends out of the skin and a base that is stuck inside the skin. The cells in the shaft are already dead, and have degraded their DNA. The cells in the base are the ones that have lots of DNA. However, a hair at a crime scene may not contain the base of the hair, which is called the hair follicle. Nonetheless, sometimes there are cells in the shaft that have not completely degraded their DNA, so some of it can be extracted from a hair that is disconnected from its follicle.
Sperm is a key tissue from which to extract DNA when trying to solve cases of sexual assault. Men release sperm during sexual intercourse, so evidence of a man's DNA that is found in or on a sexual assault victim can be indicting in a court of law. A typical human cell has 46 chromosomes that carry DNA. Sperm cells have only half that number, since the job of a sperm is to fuse with a woman's egg, which has the other 23 chromosomes to make a cell that has 46. The average man releases 180 million sperm each time he ejaculates.
Skin and Cheek Cells
Human skin is made of several layers of cells. A person sheds 400,000 skin cells a day, but that’s dead skin on the top layer. The skin underneath the shedding layer is what contains the DNA. A DNA fingerprint technology called “Touch DNA” needs just 5 to 20 skin cells from this bottom layer to make a fingerprint. The cells in the bottom layer come off the skin when something rubs against it. These objects can be clothes, weapons, or even food. In addition to skin cells, the cells that line the inside of your cheek are easily removed with a cotton swab.