In your own words, explain why it is necessary to include only one chain-terminating/ synthesis-terminating nucleotide in each well of the electrophoresis instrument.

DNA sequencing methods employ chain termination nucleotides. Furthermore, if two chain-terminating nucleotides are employed in the same well, determining which strand is ended by which dideoxynucleotide will be impossible.

(its my own words, but i did search it to get this answer, hope it helps!)