How DNA Testing Works PCR/STR

Fragments of human DNA are unique to the individual. These fragments are called polymorphic because they vary in shape from person to person. DNA profiling is the process of separating an individual's unique, polymorphic, fragments from the common ones.

With the use of PCR in forensics, law enforcement are revisiting older, unsolved cases. PCR can start with the tiniest DNA sample and rapidly create identical copies until investigators have enough DNA to compare in tests.

"PCR acts as a chemical photocopier. This often works even with degraded DNA," said Dr. Beverly Himick, a forensic scientist at the Washington State Patrol Crime Lab.

PCR breaks apart the double stranded DNA helix and exposes the 2 single strands to enzymes known as "polymerases" and the 4 basic building blocks of DNA, nucleotides known as A, C, T and G.

The polymerases use the nucleotides to rebuild each single strand back into a complete double helix. Where there was just 1 complete strand of DNA, with PCR you get 2, then 4, 8, 16, 32, 64, 128. Polymerase chain reaction is a process catalyzed by an enzyme, polymerase replicating DNA from templates or strands to create a larger pool for extensive testing and characterization.

Now a refined version of PCR, known as "short, tandem repeats" or STR-PCR, is available for cases such as cold felony cases. STR has used skin cells from a doorknob. No two individuals have the same STR patterns.

STR, Short Tandem Repeats are nucleotides along the backbone of a chromosome used as markers.

STR heats a piece of DNA until it separates into the 2 strands of its characteristic double helix, then is incubated with man made DNA matching the genetic code of each strand. This creates 2 new pieces of DNA from the original one. The process is repeated until there are enough copies for capillary electrophoresis, which produces a chart mapping a person's exact genetic makeup. This is the information used to compare suspects to crime.

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