How Pcr Works

In forensic analysis, the polymerase chain reaction makes identical copies of a specific short section of DNA. It targets and multiplies a repeating section of DNA, making it easy to measure. After extracting the small sample of DNA needed, the technician places it in a test tube with the cocktail of chemicals used for the steps below.

1. Split the DNA. By heating the sample, the technician separates or “denatures” the two strands of the DNA molecule. At about 94 degrees centigrade (201 degrees Fahrenheit) the bonds that hold the strands together let go. Each strand forms a template for copying a complementary strand.

2. Fix the primers. Primers, like probes, are lengths of synthetic DNA formulated to bind or “anneal” to specific base-pair patterns. The primers are the complements of sections of DNA that lie just outside the targeted region of repeats. One primer attaches to each strand of DNA, marking the section for copying. The primers adhere best when the temperature of the mixture is around 54 degrees C (129 degrees F).

3. Grow the new segment. The technician has already added a supply of the A, T, C, and G base segments to the solution in free-floating form. These are the building blocks for the new DNA strand. He warms the solution to 72 degrees C (162 degrees F), the temperature at which the polymerase works best. Polymerase attaches to the end of the primer and moves along the template, busily adding bases that are the complements of the ones opposite (A to T, C to G) until a second strand is formed. This process is known as “extension.”

After the polymerase has had time to build the new strand of the targeted section, the technician again heats the solution to near boiling, thereby separating the two strands. With another round of cooling, the primers again attach to the new templates. Then the polymerase goes to work constructing additional copies. After 25 to 30 cycles, the test tube contains millions of copies of the target segment. The technician then analyzes the DNA fragments, separating them to determine their lengths.

Polymerase Chain Reaction

1. The DNA molecule, at top, is split into two strands by being heated.

2. Primers, shorter lengths of DNA, are formulated to attach to the strands at certain points, as at upper right, bracketing areas of targeted repeats.

3. A supply of free-floating base segments will grow in the sections marked off by the primers, creating new strands of the targeted repeats.