DNA Testing

How Dna Tests Work

Deoxyribonucleic acid, or DNA, is a long, double-stranded molecule configured like a twisted ladder or “double helix.” The genetic information of all organisms [Page 537]is encoded in the sequence of four organic compounds (bases) that make up the rungs of the DNA ladder. Most DNA is tightly packed into structures called chromosomes in the nuclei of cells. In humans, there are twenty-three pairs of chromosomes; half of each pair is inherited from the individual's mother, half from the father. The total complement of DNA is called the genome.

By some estimates, 99.9 percent of the genetic code is the same in all humans. To identify individuals, DNA tests focus on a few loci (plural of “locus,” a specific location on the human genome) where there is variation among individuals. These loci are called polymorphisms because the genetic code can take different forms in different individuals. Each possible form is called an allele.

Forensic DNA tests have examined two types of polymorphisms. Sequence polymorphisms vary only in the sequence of the genetic code. Length polymorphisms contain repeating sequences of genetic code; the number of repetitions may vary from person to person, making the section longer in some people and shorter in others. Analysts begin the testing process by extracting DNA from cells and purifying it. To do this, they use test tubes, chemical reagents, and other standard procedures of laboratory chemistry.

In sexual assault cases, spermatozoa (containing male DNA) may be mixed with epithelial (skin) cells from the victim. Analysts generally try to separate the male and female components into separate extracts (samples) using a process called differential lysis, which employs weak detergents to liberate DNA from the epithelial cells, followed by stronger detergents to liberate DNA from the tougher spermatozoa. After the DNA is extracted, it can be “typed” using several different methods.