Читать книгу Cell Biology - Stephen R. Bolsover - Страница 99

The genome of E. coli consists of about 4.6 × 10⁶ base pairs of DNA. DNA polymerase III makes a mistake about every 1 in 10⁵ bases and joins an incorrect deoxyribonucleotide to the growing chain. If unchecked, these mistakes would lead to a catastrophic mutation rate. Fortunately, DNA polymerase III has a built‐in proofreading mechanism that corrects its own errors. If an incorrect base is inserted into the newly synthesized daughter strand, the enzyme recognizes the change in shape of the double‐stranded molecule, which arises through incorrect base pairing, and DNA synthesis stops (Figure 4.2b). DNA polymerase III then uses its 3′ to 5′ exonuclease activity to remove the incorrect deoxyribonucleotide and replace it with the correct one. DNA synthesis then continues. DNA polymerase III hence functions as a self‐correcting enzyme.