DNA sequencing is the process of determining the order of nucleotides on a segment of DNA. Cycle sequencing is a method used to increase the sensitivity of the DNA sequencing process and permits the use of very small amounts of DNA starting material. This is accomplished by using a temperature cycling process similar to that employed in the polymerase chain reaction.

The Chain Termination Method

The most popular approach to sequencing is the chain termination method, developed in 1977 by Fred Sanger. This technique makes use of a DNA synthesis reaction and a unique form of a base, called a dideoxynucleotide, that lacks the 3′ hydroxyl chemical group involved in forming the link between nucleotides in a DNA chain. A dideoxynucleotide can be added to a growing chain, but, once incorporated, no further nucleotides can be linked to it. Thus, chain growth is terminated.

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