DNA Sequencing

1. Sequencing depends on dideoxynucleotides (compare Figure 9.2) to terminate DNA replication. Each of the four dideoxynucleo-tides (A, C, G, or T) is labeled with a different colored pigment.

2. DNA polymerase uses a section of DNA as a template to synthesize new strands of DNA. Synthesis of each new strand stops when a dideoxynucleotide is added

3. Millions of DNA fragments of different lengths form. All are cop-ies of the template DNA, but most are incomplete.

4. Electrophoresis separates the fragments according to length. Fragments of identical length gather into bands. All of the DNA strands in each band end with the same dideoxynucleotide; thus, each band is the color of that dideoxynucleotide’s tracer pigment.

5. A computer detects and records the color of successive bands on the gel (see Figure 15.8 for an example). The order of colors of the bands represents the sequence of the template DNA.