Definition for Dideoxynucleotide

Dideoxy sequencing

Diegesis

Dideoxynucleotides, or ddNTPs, are nucleotides lacking a 3'-hydroxyl (-OH) group on their deoxyribose sugar. Since deoxyribose already lacks a 2'-OH, dideoxyribose lacks hydroxyl groups at both its 2' and 3' carbons. The lack of this hydroxyl group means that, after being added by a DNA polymerase to a growing nucleotide chain, no further nucleotides can be added as no phosphodiester bond can be created based on the fact that deoxyribonucleoside triphosphates (which are the building blocks of DNA) allow DNA chain synthesis to occur through a condensation reaction between the 5' phosphate (following the cleavage of pyrophospate) of the current nucleotide with the 3' hydroxyl group of the previous nucleotide. The dideoxyribonucleotides do not have a 3' hydroxyl group, hence no further chain elongation can occur once this dideoxynucleotide is on the chain. This can lead to the termination of the DNA sequence.