The function of DNA polymerase is to replicate, proofread and repair DNA. Several DNA polymerases exist, but DNA polymerase I, or Pol I, and DNA polymerase III, or Pol III, are the main ones involved in DNA replication.
DNA polymerase adds nucleotide bases only when an RNA primer, a short piece of RNA, is already present. Nucleotide bases for DNA consist of adenine, thymine, guanine and cytosine. DNA polymerase then adds nucleotide bases to the RNA primer at a rate of 1,000 bases per second. DNA polymerase is an extremely accurate enzyme, inserting only one erroneous base for every 10,000 bases added.
Before the next base is added, DNA polymerase checks to see if the last base added was the correct one. DNA replication is not 100 percent accurate; therefore, incorrect bases can sometimes be added. DNA polymerase removes the incorrect base and adds the correct one. DNA replication is extremely accurate because of DNA polymerase's proofreading function, resulting in one mistake for every 1 billion bases added.