Proof reading is a biological repair process to increase the accuracy of DNA_replication. Infinite accuracy isn’t achieved because this would require infinite time. There is therefore a balance between the speed of DNA replication and the accuracy of the nucleotides added to the strand.
DNA polymerases are the enzymes responsible for DNA synthesis. As well as adding nucleotides onto the strand, they can also function as a repair tool, removing incorrect nucleotides; in this case they are referred to as an exonuclease. If an incorrect nucleotide is added to the expanding strand, DNA synthesis slows down. This is due to weak bonding between the incorrect base on the newly synthesised strand and the base on the template strand. These bases do not follow Watson-Crick base pairings as they are not complementary to each other (If the bases are complementary Adenine (A) will pair with Thymine (T) and Cytosine (C) will pair with Guanine (G)). This weak bond means that the DNA strand can move from the active site of the polymerase to the active site of the exonuclease. The slowdown in DNA synthesis allows time for this to happen. At the active site of the exonuclease, the incorrect nucleotide is cleaved off by hydrolysis of the phosphodiester bond. This process will continue to happen in the 3’-5’ direction as long as a nucleotide is bound to the active site of the exonuclease. When the DNA returns to the active site of the polymerase, this process ceases. The polymerase can then add the correct nucleotide to the DNA strand .
- ↑ Berg J, Tymoczko J, Stryer L. 2012: 869, Biochemistry, 7th Edition, New York: WH Freeman and Company.