Sequencing

Sequencing is the operation of determining the nucleotide sequence of a given molecule. There are There are several approaches to sequencing, but generally, the most successful is based on gel electrophoresis. As mentioned earlier, the DNA polymerase enzyme catalyzes the replication reaction of DNA. DNA polymerase extends the chain by adding nucleotides to its end. Current biotechnology enables synthesis of nucleotides which cause the strand to terminate. For instance, A* denotes an Adenine molecule which does not allow other molecules to extend the strand after itself. By catalyzing DNA replication in an environment containing mixtures of normal Adenine and sythesized Adenine* instead of only Adenine, it is possible to create DNA strands of different lengths. By applying gel electrophoresis to these molecules, it is possible to determine the lengths of all the strings and from it to conclude the location of all Adenines in the tested DNA strand. In a similar fashion it is possible to locate other nucleotides and eventually to fully sequence a whole segment of DNA. Using this method, sequences of 500-800 nucleotides can be mapped.

  

Figure: DNA sequencing

The problem that arises from this sequencing technique is the creation of a long DNA chain from the local sequences. This problem is known as: