DNA synthesis is catalyzed by an enzyme called DNA polymerase. DNA polymerase adds nucleotides to the 3' end of a growing DNA strand by complementary base pairing with a template strand, extending the DNA molecule. This process requires the presence of a primer, which is a short strand of RNA or single-stranded DNA that provides a starting point for the DNA polymerase to add nucleotides. DNA polymerase also has proofreading activity, which allows it to detect and correct errors in the sequence of the newly synthesized DNA strand. DNA polymerase is a crucial component of DNA replication, the process by which cells make a copy of their DNA in preparation for cell division.