By Jacob Stutsman | Updated March 24, 2022
Deoxyribonucleic acid (DNA) carries the inherited instructions that define every living organism. Its double‑helix structure consists of two intertwined strands, each composed of nucleotides. Adenine pairs with thymine, and guanine pairs with cytosine. Inside cells, these base pairs guide protein synthesis, but they also hold a wealth of information that scientists can decode.
DNA is packaged into chromosomes—tight bundles that vary in number across species. Humans possess 23 pairs; females have two X chromosomes, while males carry one X and a Y. Specific positions on a chromosome are called loci, and different forms at a locus are called alleles. Because each child inherits a unique combination of parental alleles, subtle genetic differences can be used to identify individuals.
Accurate DNA testing requires clean, uncontaminated samples. Cotton swabs are commonly used to harvest cells from a person’s cheek, but virtually any fluid or tissue—blood, saliva, hair, or even a swabbed object—can be analyzed. Humans differ in about one‑tenth of one percent of their DNA, roughly three million base pairs out of a total of three billion.
Polymerase Chain Reaction (PCR) is the cornerstone of modern DNA analysis. A thermal cycler—a programmable instrument that alternates heating and cooling cycles—separates DNA strands and then amplifies them. Even minute or degraded material can be amplified to thousands of copies, enabling downstream testing.
After amplification, DNA probes—short, labeled strands that bind complementary sequences—highlight the target regions. Traditionally, radioactive or fluorescent tags are attached, producing a unique banding pattern for each individual. Using four to six probes provides a robust match while keeping costs manageable.
Short Tandem Repeats, or STRs, are repeating DNA motifs found at 13 loci throughout the genome. After PCR, gel or capillary electrophoresis separates fragments by size under an electric field. Visualization dyes—silver stain, ethidium bromide, or fluorescent dyes—render the patterns visible. The probability that two unrelated people share the same STR profile is about one in a billion, ensuring highly reliable identification.
By combining these techniques—PCR amplification, probe hybridization, and electrophoretic separation—scientists can confidently analyze DNA from diverse sources and match individuals with extraordinary precision.
