The enzyme that catalyzes the elongation of a DNA strand is DNA polymerase, according to the RCSB Protein Data Bank. This enzyme adds to the growing DNA molecule by adding nucleotide bases. DNA polymerase only makes one mistake every billion bases it adds, making this enzyme extremely accurate.
Once the DNA is unzipped, replication starts on both strands. DNA polymerase binds to a strand of DNA and adds bases only in the 5' to 3' direction. The leading strand of DNA can be replicated continuously; however, the other strand, called the lagging strand, can be replicated only in fragments. In humans, DNA polymerase copies DNA at a rate of 50 bases per second. In addition to adding bases to the DNA strand, DNA polymerase also proofreads its own work, excising bases added mistakenly.