DNA ligase will seal the strand C to U transitions are spontaneous, but can be accelerated by nitrites and carcinogens. I and ligase will fill and seal. All of these repair systems are performed on a strand when there is a template available. RecA is a critical protein involved in this kind of repair. When this response is activated by 'stress' it will permit DNA pol III to read through the lesion.
Many errors are inevitable.
RecA also plays a role in this response. Mutations lead to cancer. Problems with repair enzymes can lead to a major problem. Xeroderma pigmentosa is one example. People with this disorder have a defect in nucleotide excision repair and cannot, therefore, correct pyrimidine dimers caused by uv light.