The graph that follows shows the survival of four different E. coli strains after exposure to increasing doses of ultraviolet light. The wild-type strain is normal, but the other strains have a mutation in either a gene called uvrA, a gene called recA, or both. (b) What are the relative contributions of these genes to the repair of UV damage?

DNA repair mechanisms are cellular processes that correct damage to the DNA molecule. In the context of UV light exposure, these mechanisms are crucial as UV radiation can cause thymine dimers, leading to mutations. The uvrA and recA genes are involved in different pathways of DNA repair, with uvrA primarily associated with nucleotide excision repair and recA playing a role in homologous recombination and SOS response.

The uvrA gene encodes a protein that is part of the nucleotide excision repair system, which recognizes and removes bulky DNA lesions caused by UV light. Mutations in the uvrA gene can lead to increased sensitivity to UV radiation, as the cell's ability to repair DNA damage is compromised. This is evident in the survival rates of E. coli strains with uvrA mutations when exposed to UV light.

The recA gene is essential for the repair and maintenance of DNA. It plays a critical role in the homologous recombination repair pathway and is involved in the SOS response, which is activated in response to extensive DNA damage. Mutations in recA can impair the cell's ability to repair DNA, leading to lower survival rates under UV stress, as shown in the graph comparing different E. coli strains.