# DNA Mistakes --- [[DNA|DNA's]] whole job is maintain its data perfectly, but this doesn't always work out, especially—but not necessarily—during [[DNA replication]] or [[cell division]]. There are also environmental factors that can directly effect the DNA molecule itself can also cause mistakes in the DNA. For example, [[electromagnetic spectrum|UV radiation]] can cause the formation of **thymine dimer**. [[thymine dimers|Thymine dimers]] are a mistake that can occur in DNA that results from damage from UV radiation. Given the sheer number of [[complementary base pairing|base pairs]] in a single eukaryotic, there's just no way to [[DNA replication|replicate DNA]] completely error free, even with precautions like [[DNA proofreading]]. The overall error rate during DNA replication is about one mistake per billion nucleotides[^1] . In humans we have 6 billion base pairs, and only end up having 5 or 6 mistakes per replication. The placement of the error is completely random, so if they're in a really sensitive place like in a [[tumor suppressor gene]], that's a huge problem. Fortunately DNA has a myriad of methods to prevent and correct errors, so most of them do not continue on. The ones that do, and are passed on to the organism's offspring are called [[mutation|mutations]]. [^1]: This level of accuracy is like copying *War and Peace* by hand more than 300 times without making a single spelling or punctuation error! If DNA mistakes become so many that the system has to take a huge risk and forgo standard [[DNA proofreading|proofreading]] precautions, it busts out the [[SOS repair system]]. ## Mismatch Error A **mismatch error** is when [[DNA polymerase#DNA Polymerse III|DNA polymerase III]] is putting in nucleotides, sometimes it grabs the wrong one and puts that in instead. Even under pristine conditions, these kind of errors happen all the time. They can innocuous (if it's [[codon#Redundancy|a redundant codon]], but even a single wrong nucleotide in a vulnerable place can ruin the whole protein. [[DNA polymerase#DNA Polymerase I|DNA polymerase I]] also has the ability to proofread, and it does just that as it follows polymerase III. **Mismatch repair enzymes** are the back-up to the back-up, which come along and check the DNA after DNA polymerase III and I are done. If it catches an error, it takes out the whole section and puts new nucleotides down. It knows which strand is the template strand, because the template strand has been [[DNA#DNA Methylation|methylated]] and the newly made one does not have any methyls on it. ## Insertion/Deletion Error An **insertion/deletion error** is when a single nucleotide is added or forgotten. Unlike mismatch errors, this has a potential of causing a devastating [[point mutation|frameshift mutation]]. %%This line intentionally left blank%%