Under the effect of many enzymes including the polymerase, the double chain is split in two and desoxynucleotidetriphosphates provide the bases and the chain portions necessary for making two chains identical to the dissociated chains:
All of this happens during mitosis to ensure the identity of daughter cells after division.
The polymerase uses as tools deoxynucleosidetriphosphates, of global structure (similar to ATP) ..
.. where P are phosphate groups and B is → a purine or pyrimidine base for example:
A is one of the two chains of the initial DNA B is the coupled chain being formed (1) is the provisional end of this last chain (2) a deoxynucleotidetriphosphate couples to A (3) B elongates by bonding by esterification to the first phosphate of the deoxynucleotidetriphosphate while repelling the other two phosphates
And here it is!
Unfortunately, things are not that "simple"! Depending on whether it is the replication of a (5 '- 3') or (3 '- 5') chain. , the synthesis is different! See for example → this page from Wikipedia