The special nucleotides used in sequencing are known as
dideoxynucleotide triphosphates(ddNTPs).
The four nucleotides - ATGC are present in the form of
deoxynucleotide triphosphates(dNTPs) in prokaryotic DNA.
During the process of DNA synthesis the 3` hydroxyl group of
one dNTP chemically binds to the 5` phosphate group of the other
dNTP through the formation of phosphodiester bond thus resulting in
the elongation of the DNA strand.
When ddNTP is added during sequencing it serves as a chain
elongation inhibitor because they lack the essential 3` hydroxyl
group which is required for the formation of phosphodiester bond,
thereby halting the chain elongation.