The polymerase chain reaction is used to produce millions of cloned pieces of DNA. However substances are added to randomly stop the creation of DNA at of the four bases. This produces pieces of DNA of almost every legnth as when the material stops being produced is random. The lengths of the DNA strands can be worked out using gel electrophoresis. Markers on each strand show which base it ends with. It is therefore possible to work out the sequence of bases when many of these are assesed together to find the base at any particular point.
The DNA polymerase catalyses the joining of deoxy nucleotides to the correspondng bases. However if by chance a dideoxy nucleotide is joined to a base, then that fragment of DNA will stop further pairing. Fragments of all sizes should be obtained due to the randomness of when a dideoxy nucleotide is added.
When incubation is complete, the fragments are separated (with a resolution of just one nucleotide) by gel electrophoresis, from longest to shortest. As each tagged dideoxy nucleotide fluoresces a different colour under laser light, it is possible to read off which base occurs at each diplacement from the original primer.
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