Limitations of Sanger Sequencing

The practical upper limit for Sanger sequencing is a string of about 1,000 nucleotides:

As the sequencing reactions progress, the likelihood of chain termination rises.
As sequence chains lengthen, nucleotides differing by one nucleotide become harder to separate by size.

A laser reads the sequence of several short strings of nucleotides, increasing in length by one nucleotide. The first sequence ends with an A, the second a T, the third a G, highlighted by the laser beam, the forth a T, the fifth a C, and so on. A computer screen displays the letters A-T-G.