Which enzyme catalyzes the addition of new nucleotides during DNA synthesis?

DNA polymerase III is the enzyme responsible for catalyzing the addition of new nucleotides during DNA synthesis. This enzyme plays a critical role in the replication of DNA, as it synthesizes the new DNA strands by adding nucleotides complementary to the template strand.

During DNA replication, the process begins with the unwinding of the double helix, which exposes the template strands. DNA polymerase III then adds nucleotides to the growing complementary strand in the 5' to 3' direction. It requires a primer—short RNA strands synthesized by primase—to provide a starting point for synthesis. The nucleotides are added one by one, and DNA polymerase III has proofreading capabilities that ensure that the correct nucleotides are incorporated, reducing the frequency of mutations.

Other enzymes mentioned have different roles: for instance, DNA polymerase I is involved in removing RNA primers and replacing them with DNA; primase synthesizes the RNA primers; and ligase joins Okazaki fragments on the lagging strand by sealing nicks in the sugar-phosphate backbone. Hence, the specificity of DNA polymerase III in adding nucleotides during synthesis makes it the primary enzyme in this critical biological process.