Proofreading

DNA polymerase has a high affinity for complementary nucleotides, which improves accuracy when selecting the correct incoming nucleotide to pair with the template strand. As nucleotides begin pairing, the polymerase undergoes a conformational change that makes mispaired nucleotides more likely to dissociate while retaining correctly paired ones. This selectivity mechanism ensures base pairing and its significance in DNA replication by favoring adenine-thymine and guanine-cytosine pairings over incorrect combinations.

Exonucleolytic proofreading is a DNA polymerase mechanism that removes incorrect nucleotides from the growing DNA strand. When a mispaired nucleotide causes structural deviation, the polymerase pauses and transfers the 3' end to its exonuclease site. The polymerase then removes the incorrect nucleotide in a 3' to 5' direction, replaces it with the correct one, and resumes DNA synthesis, maintaining replication fidelity.

When mutations alter the exonuclease domain of DNA polymerase, the enzyme loses its ability to remove incorrect nucleotides during proofreading. This failure allows errors to accumulate rapidly throughout the genome, generating numerous mutations in newly synthesized DNA. Such widespread genomic mutations have been linked to various types of cancer, as they can affect genes controlling cell growth and division.

When an incorrect nucleotide is added to the growing 3' end, the structural deviation caused by improper pairing signals the DNA polymerase to pause. This pause triggers the exonucleolytic proofreading mechanism, where the polymerase reverses direction and uses its exonuclease domain to excise the wrong base before continuing synthesis with the correct nucleotide.

Eukaryotic cells contain multiple DNA polymerases with specialized roles. DNA polymerase delta and epsilon possess proofreading ability when replicating nuclear DNA. These polymerases read each base after it is added to the new strand and use their exonucleolytic domains to cut off and replace incorrect bases, ensuring high fidelity during replication in eukaryotes.

Error-prone PCR deliberately generates mutations in DNA sequences for research purposes. This technique uses polymerases with compromised proofreading ability, allowing incorrect nucleotides to be incorporated intentionally. In contrast, high-fidelity polymerases are used when accurate replication is critical, such as when producing perfect DNA copies for cloning or sequencing applications.

DNA polymerase employs three proofreading mechanisms: first, high affinity for complementary nucleotides improves selection accuracy; second, conformational changes during pairing favor correct base retention while promoting mispaired nucleotide dissociation; and third, structural deviations from incorrect pairings cause polymerase pausing, triggering exonucleolytic removal and replacement of the wrong nucleotide.