What are the mechanisms by which cells correct DNA replication errors?

Cells correct DNA replication errors through mechanisms such as DNA proofreading, mismatch repair, and nucleotide excision repair.

DNA replication is a highly accurate process, but mistakes can occasionally occur, such as the wrong base being inserted during synthesis. These errors can potentially lead to mutations, which can have serious consequences for the cell. To prevent this, cells have developed several mechanisms to correct these errors and maintain the integrity of their genetic information.

The first line of defence is DNA proofreading, which is carried out by the DNA polymerase itself. As the enzyme moves along the DNA strand, it checks each newly added base to ensure it is correctly paired with the base in the template strand. If an incorrect base has been added, the enzyme can remove it and replace it with the correct one. This proofreading process significantly increases the accuracy of DNA replication.

Another important mechanism is mismatch repair. This is a more complex process that involves several different proteins. After DNA replication, these proteins scan the DNA for mismatches - bases that have been incorrectly paired. When a mismatch is found, the proteins cut out a section of the DNA strand containing the mismatch, and a new section is synthesised using the original strand as a template. This process can correct errors that were missed during proofreading.

Finally, there is nucleotide excision repair. This mechanism is used to correct more serious damage that can cause distortions in the DNA helix, such as thymine dimers caused by UV light. In this process, a set of proteins recognise and bind to the damaged area, then cut out a section of the strand on either side of the damage. DNA polymerase then fills in the gap using the undamaged strand as a template, and the strand is sealed by DNA ligase.

These mechanisms work together to ensure the accuracy of DNA replication and protect the cell from potentially harmful mutations. They are a testament to the complexity and precision of cellular processes.