DNA: What Is It?

Comparing a DNA profile with a sample retrieved from a crime scene can establish a suspect's innocence or guilt with a very high degree of certainty.

DNA (deoxyribonucleic acid) is found in all cells containing a nucleus, including those belonging to plants and animals. A DNA molecule consists of two strands that wrap around each other, forming the familiar double-helix shape. The strands are made of sugar and phosphate; the rungs are called bases.

Every base is made from adenine (A) and thymine (T) or cytosine (C) and guanine (G) bonded weakly by hydrogen. These bonded units are called base pairs. The specific order of these base pairs determines everything that makes up an organism, from the function of a heart cell to the enzymes used to digest food.

There are about 3.3 billion base pairs in the human genome, making up physically distinct units called chromosomes. The nuclei of most human cells contain one set of 23 chromosomes from each parent. DNA is nearly identical among all individuals of a species. In humans, only .1% - approximately three million base pairs encoding more than 100,000 genes - differ between any two people. With the exception of identical twins (formed when a fertilized egg splits), no two people share the same DNA composition.

Variability is the basis for DNA profiling. Molecular biologists use enzymes to cut certain sections of those variable regions of DNA into short sections called restriction fragments. The process, called RFLP analysis, focuses on the length and shape of the restriction fragments, which differ widely between individuals.

Essentially, a small sample DNA is given a computerized numeric value that resembles a bar code.

Another form of variability is called VNTR (variable number of tandem repeats). DNA is identified by segments called loci. A locus known as D1S80 is found on human chromosome number one and comprises repeating units of 16 segments. The number of repeats varies between individuals and is known to range from 15 to more than 40. Combining all of the variations, a profile develops that is unique to the individual.

There are a variety of ways to get a sample of DNA, which is found in blood, semen, skin cells, tissue, organs, muscle, brain cells, bone, teeth, hair, saliva, mucus, perspiration, fingernails, and urine. Liquid evidence - blood, semen, wet tissues, saliva - is collected on a sterile swab and allowed to dry in a paper envelope to prevent contamination. Dry evidence - hair, bones, nails - are gathered and packaged individually. Dried body fluids should be submitted to the lab on the surface where they dried, if possible, or a small sample may be scraped carefully into a sterile paper envelope.

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