Each locus that is marked for testing requires its own optimal temperature for amplification. By using repeated chemical reactions in which the strands of DNA bind together and come apart using one compromise temperature, multiplex systems are able to amplify the DNA at more than one loci at the same time. By examining the gene sequence on a specific location on a chromosome and comparing the length with the gene sequence on the same chromosome from a different person, scientists can use the length differences to tell who left the sample. The STR CTT system is a multiplex system that tests for length differences at three loci. Using an electric current, the fragments are moved through the gel.
Longer fragments take longer to move through the gel than the smaller fragments.
As a result, the fragments are separated by length. A stain is then applied to the gel, making the DNA visible in the form of bonds. The DNA fragment s length can then be determined and compared to DNA from other sources that are run in different lanes on the same gel. An allelic ladder, which is a known standard, is also used for comparison purposes. Like the STR CTT system, extraction is followed by amplification.