During DNA replication, complementary base pairing dictates the sequence of nucleotides added to the new strand — adenine pairs with thymine and guanine pairs with cytosine. However, nucleotides can sometimes be incorrectly paired, for instance, adenine with cytosine. Such errors are prevented or repaired by the proofreading properties of DNA polymerases carried out during DNA synthesis. First, DNA polymerase has a high affinity for complementary nucleotides, which improves the accuracy of selecting the correct incoming nucleotide to pair with the template strand. Second, as the nucleotides start pairing, the DNA polymerase undergoes a conformational change that makes the mispaired nucleotides more likely to dissociate but retains the correctly paired nucleotides. Third, if an incorrect nucleotide is somehow added to the growing 3' end, the structural deviation caused by this incorrect pairing causes the DNA polymerase to pause. In a process called exonucleolytic proofreading, the 3' end is then transferred to the exonuclease site of the DNA polymerase. The polymerase subsequently removes the incorrect nucleotide in a 3' to 5' direction, replaces it with the correct nucleotide, and resumes DNA synthesis.