Didesoksynukleotidia

Didesoksynukleotidia, also known as dideoxynucleotides or ddNTPs, are synthetic analogs of deoxyribonucleotides, the building blocks of DNA. The key difference lies in the absence of a hydroxyl group at the 3' carbon position of the sugar molecule. This structural modification is crucial for their function in DNA sequencing.

In the process of DNA replication, DNA polymerase adds incoming deoxyribonucleotides to the growing DNA strand by forming a phosphodiester bond between the 3' hydroxyl group of the existing strand and the 5' phosphate group of the incoming nucleotide. Since dideoxynucleotides lack this 3' hydroxyl group, they cannot form this bond, and the elongation of the DNA chain terminates when a dideoxynucleotide is incorporated.

This chain-terminating property is exploited in the Sanger sequencing method, also known as the dideoxy chain-termination method. In this technique, a DNA sample is amplified in the presence of a mixture of all four regular deoxyribonucleotides (dNTPs) and a small amount of one of the four dideoxynucleotides (ddATP, ddTTP, ddCTP, or ddGTP), each labeled with a different fluorescent dye. Each ddNTP terminates the synthesis of a DNA fragment at a specific position corresponding to its base. By separating these terminated fragments by size, and detecting the fluorescent label at the end of each fragment, the nucleotide sequence of the original DNA can be determined. Didesoksynukleotidia are therefore essential reagents in molecular biology for DNA analysis and sequencing.