# SOS Repair System --- The **SOS repair system** is a (very error prone) way of repairing highly [[DNA mistakes|damaged DNA]]. Because this method is so error prone, a cell only activates this system in the event of extensive DNA damage, such as excessive [[thymine dimers|thymine dimers]] being present. Its a last-ditch effort to salvage something rather than die out. When it does happen that a cell's DNA is so badly damaged that it might not survive, it will activate the SOS repair system by translating the *RecA* protein. *RecA* cleaves the *LexA* repressor protein, that is normally on and repressing the *UmuD* gene. The *UmuD* gene produces [[DNA polymerase#DAN Polymerase II IV V|DNA polymerase V]] to do the legwork of SOS repair. The SOS system is risky because [[DNA polymerase#DAN Polymerase II IV V|DNA polymerase V]]—unlike it's counterparts DNA polymerase III and I—does not have a [[DNA proofreading|proofreading]] ability. What it *can* do that polymerase III and I cannot do is *guess* at a nucleotide. If pol III or I comes across something it can't read (like, for example at a [[thymine dimer.png|thymine dimer]]), it plays it safe and skips those nucleotides entirely. The danger of this is that it can produce a catastrophic [[point mutation|frameshift mutation]]. Polymerase V on the other hand will randomly insert a nucleotide in it's place, which, while it only has a 1 in 4 chance of being right at least avoids the frameshift mutation. ___