Proofreading depends on the ability of dna polymerase to reverse and remove. The proofreading mechanism, depends on the availability of dna polymerase to back up and remove mispaired nucleotides from a dna strand. The correct pairs allow the fully stabilizing hydrogen bonds to form. If a newly added nucleotide is mismatched, the dna polymerase reverses using a built-in deoxyribonuclease to remove the newly added incorrect nucleotide. The enzyme resumes working forward, now inserting the correct nucleotide. Any base-pair mismatches that remain after proof-reading faces still another round of correction by dna repair mechanisms. These mismatched repair mechanisms increase the accuracy of dna replication. Because mismatches are too large or too small to form hydrogen bonds, they distort the structure of the dna helix. These distortions provide recognition sites for the enzyme catalyzing mismatch repair. If the repair enzymes encounter a distortion while scanning the dna helix, they remove a portion of the new chain, including the mismatched nucleotides.