A technician uses a computer guiding system to align the filter mask in perfect register with the quartz wafer. The filter mask will block light to selected regions of the wafer. More nucleotides can be added to the exposed regions.
Two sequencing techniques were developed independently in the 1970s. The method developed by Fred Sanger used chemically altered "dideoxy" bases to terminate newly synthesized DNA fragments at specific bases (either A, C, T, or G). These fragments are th