Building Blocks of DNA Replication
The building blocks added on to a growing daughter strand are individual nucleotides. Remember, in DNA the - OH group at the 2' position of the ribose ring is missing. As a result, the substrates for DNA synthesis are called 2' deoxyribonucleotides. Attached to each deoxyribose ring is a base group (C, G, A, or T) and a triphosphate group. The three phosphates are designated alpha, beta, and gamma (alpha being the closest to the ribose ring). These phosphates play key roles in the addition of subsequent nucleotides to the daughter strand.
Addition Occurs Via a Nucleophilic Attack
Deoxyribonucleoside triphosphates, as we just stated, are the building blocks of DNA. Recall, furthermore, that a complete polynucleotide strand of DNA has only one phosphate group and that through this phosphate group each nucleotide is attached to the next. While each nucleotide added to a growing DNA chain lacks an -OH group at its 2' position, it retains its 3' -OH. This hydroxyl group is used to attack the alpha phosphate group of an incoming nucleoside triphosphate. In the attack, the 3' -OH replaces the beta and gamma phosphates that are ejected from the complex as a pyrophosphate molecule. The result is the formation of the phosphodiester bond between the growing daughter strand and the next nucleotide. The 3' -OH of the newly added nucleotide is now exposed on the end of the growing chain and can attack the next nucleotide in the same way.
The Driving Force of the Addition Reaction
Each incoming nucleotide supplies the energy for its addition in the high-energy bond between the beta and gamma phosphates that are ejected upon addition. It is not the release of the pyrophosphate that drives the reaction, but rather the subsequent hydrolysis that takes place. A much larger amount of energy is released when the two phosphates are separated into individual phosphates through the hydrolysis reaction.