Correction of errors in DNA replication
Although DNA polymerase III only binds complementary nucleotides to the template strand, sometimes there are errors that need to be eliminated. The enzymes nucleases are responsible for removing the mismatched nucleotide. This is why so few errors occur during replication (approximately 1 in 108 bases), although it is not negligible, since in multicellular organisms the number of nucleotides in a chain is very large.
Therefore, there is an error repair mechanism after replication in which several enzymes participate:
- Endonucleases that detect errors and cut the wrong DNA strand. They check that the last deoxyribonucleotide added is the one complementary to that of the template chain, and if not, it replaces it with the correct one.
- Exonucleases that remove improperly placed nucleotides.
- DNA polymerases, which eliminate the erroneous nucleotides by their exonuclease action, and synthesize the part corresponding to the removed piece. Both actions can be carried out by DNA polymerase I.
- DNA ligases that join the generated segments to the rest of the DNA chain.
After correcting for these errors, they drop to one for every 1010 bases added.
In order to correct the errors, it is necessary to distinguish the new chain from the old one. The adenines, after a time methylate, so the adeninas methylated correspond to the old chain and still are unmethylated, the newly synthesized new chain.
Although with a very low frequency, there are errors that escape the control mechanisms. But this does not have to be negative, since it is the origin of the mutations, creating genetic variability and allowing the evolution of the species.