GaTctA3 OH

DNA template S

The DNA polymerase uses the nucleoside triphosphate form of the deoxynucleotides to build the DNA polymer. The monophosphate form of the deoxynucleotide is incorporated into the growing DNA chain, and a pyrophosphate molecule, a kind of salt, is released. The DNA polymerase can add nucleotides only to the 3'-OH end of the growing DNA chain (see

Base

triphosphate \ P

Base

Base

sugar

OOOCX

DNA polymerase attaches an incoming triphosphate nucleotide to the 3' position on the growing chain. Removal of a water molecule between the two attaches them, and the two terminal phosphates break off.

above diagram). Therefore, DNA polymerization occurs in only one direction. Some DNA polymerases are highly processive, polymerizing many nucleotides to the 3' end of the DNA chain before falling off the DNA template. Other DNA polymerases are distributive in nature, incorporating just one nucleotide and then falling off the DNA template.

Occasionally, the DNA polymerase will incorrectly polymerize a nucleotide onto the growing DNA chain. Removal of this misinserted nucleotide must be performed by a "proofreading" exonuclease, which is a substance that removes nucleotides from the 3' end of the DNA molecule. The combined actions of DNA polymerases and proofreading exonu-cleases improve the accuracy of DNA synthesis and thus minimize introduction of errors into the genome.

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