The polymerase makes 1 error every 10 million base pairs. Before adding a new nucleotide, DNA polymerase checks whether the previous nucleotide is correctly base paired. If not, it is removed by a nuclease activity. Our DNA is continually suffering damage. For example, UV light can cause covalent linkages between pyrimidine bases such as thymine dimers.
Specific enzymes remove specific types of damage.
Yeast- more than 50 repair enzymes. In humans, a predisposition to certain cancers is caused by a mutation in one of the genes responsible for mismatch repair. People with Xeroderma pigmentosum cannot repair thymine dimers and have a high susceptibility to skin lesions and skin cancer. DNA does not direct protein synthesis itself, RNA is used as an intermediate. Usually only one 1 copy of a gene is present in a cell but many identical copies of RNA can be made from the same gene.