# Thymine Dimer --- **Thymine dimers** are a type of damage that can happen to [[DNA]] when two adjacent [[nucleotide|thymine]]] nucleotides [[covalent bond|covalently bond]] together which causes an unreadable kink, or bend in the DNA. Thymine dimers are caused by [[radiation|UV radiation]], which energize them enough that they just pop together. ![[thymine dimer.png]] Thymine dimers are problematic because they change the shape of the DNA, and [[DNA polymerase]] cannot read what type of nucleotides that they are. Polymerases III and I at least will just skip right over it, leaving a blank. ## Fixing a Thymine Dimer There are three different ways for a cell to fix a thymine dimer: *nucleotide excision repair*, *light repair*, and in extreme cases, *SOS repair*. ### Nucleotide Excision Repair **Nucleotide excision repair** is a method of repairing a thymine dimer that involves cutting out the whole section with the dimer and replacing it. ![[nucleotide excision repair.png]] ### Light Repair **Liight repair** is a method of repairing a thymine dimer that can be activated in the presnese of visable light which only [[bacteria]] can do. Bacteria that use this can use the energy from the light to break the covalent bond between the thymines, and let the DNA reshape itself. ### SOS Repair **[[SOS repair system]]** is a cell's *last resort* to fixing extensive damage. SOS repair uses a DNA polymerase that is very prone to errors and cannot proofread itself, so the cell will only resort to this method if there is so much damage that the cell will die. ___