By either method, a collection of single-stranded DNA fragments is produced, each fragment one base longer than the next. The length of a fragment depends on where a chemical cleaved the strand (in Maxam-Gilbert sequencing) or where a special terminator base was added (in the chain termination method). The fragments are then separated according to their size by a process called gel electrophoresis, in which the fragments are drawn through a gel material by electric current, with shorter fragments migrating through the gel faster than longer fragments. The DNA sequence is then "read" by noting which reaction produced which fragment.
Maxam-Gilbert Sequencing
All DNA sequencing protocols must incorporate a method for making the DNA fragments generated in the reaction "visible"; they must be capable of being detected. For Maxam-Gilbert sequencing a technique called end labeling is used, in which a radioactive atom is added to the ends of the DNA fragment being sequenced. The first step in this process is to use an enzyme, called a restriction endonuclease, to cut the DNA at a specific sequence. If the restriction endonuclease is Hind III, for example, the sequence AAGCTT will be cut.
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