Paternity testing and other DNA tests rely heavily on DNA’s structure and function.
DNA Biology: Structure and Replication
Since DNA’s structure was discovered in 1953, numerous techniques have been developed to use this knowledge to learn more about how living things function and solve genetic questions.DNA (Deoxyribonucleic Acid) is the genetic material that contains the “code of life”. The DNA in each cell of a person contains the same DNA code, but each person has a unique DNA code that can be used to identify them and their children. We get our unique DNA code from our biological parents in equal amounts. The building blocks of DNA are known as nucleotides abbreviated A, T, C, and G, and appear in DNA as letter pairs of AT and CG. There are about 3 billion letter pairs in our DNA code.
In humans, most of the DNA is in the form of tightly coiled strands called chromosomes, found inside the cell nucleus. There are 46 chromosomes in a human cell arranged in 22 autosomal (non-sex chromosome) pairs and two sex chromosomes (XY for males and XX for females). If you unwind each chromosome and place them end-to-end, you will have a long, double-stranded DNA helix that is about 3 meters in length—all from one microscopic human cell.
The DNA helix looks like a twisted ladder or spiral staircase. The “steps” are composed of the four bases: adenine (A), thymine (T), guanine (G), and cytosine (C), and are held together by hydrogen bonds that connect specific pairs of these molecules together: A–T and G–C. The arrangement of these molecules, called the DNA sequence, spells out the instructions for our physical characteristics and body functions. These instructions are found in units called genes.
The specific pairing of DNA molecules presents a simple mechanism for their replication. Replication occurs whenever a cell divides in two during growth and development. During replication, the helix unwinds and an enzyme separates the two strands. Another enzyme, DNA polymerase, adds the four molecules A, T, C, and G to each strand according to the strand’s sequence: A is added opposite a T on the strand, and C is added opposite a G. This “complementary base pairing” allows the replication to occur quickly and reliably.
The products of replication are two sets of double-stranded DNA molecules that have exactly the same sequence as the original. During cell division, each cell receives one set of DNA. In this way, all the cells of the body have the same DNA molecules.
DNA is found in the nucleus of a cell, much like the yolk of an egg. DNA is collected from a cheek (buccal) swab sample by breaking open the cells and separating the DNA from the other parts of the cell. DDC uses scientifically accepted methods to perform the DNA extraction step along with robotic instruments for precision and accuracy.
The Polymerase Chain Reaction
Polymerase Chain Reaction or PCR, is a technique that allows the generation of millions of copies of targeted areas (loci) in the DNA. PCR harnesses the cell’s ability to replicate DNA before cell division, but in a test tube. PCR is used commonly in the scientific community to perform a number of different tests and is used by DDC for paternity testing. Once the DNA is extracted, it is combined with chemicals to support the PCR reaction including primers which are small pieces of synthetic DNA labeled with florescent tags that flank the region to analyze as well as the DNA building blocks (A, T, C, G) and an enzyme to support the replication of the DNA (DNA polymerase). For paternity testing, DDC uses a multiplex PCR reaction that tests 21 different locations (loci) in the DNA at once. This is accomplished using an instrument known as a thermocycler that heats and cools the DNA. Each round of heating and cooling doubles the amount of DNA at the target locations, so that after 28 cycles of heating and cooling over a million copies of each starting DNA are present.