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. (a) Which strains are most sensitive to UV light? Which strains are least sensitive?

Verified step by step guidance

1. Examine the graph to identify the different strains of E. coli: wild type, uvrA, recA, and uvrA and recA.
2. Observe the survival rate of each strain as the dose of UV light increases.
3. Note that the wild-type strain (green line) maintains the highest percentage of cells surviving across all doses of UV light, indicating it is the least sensitive to UV light.
4. Compare the survival rates of the mutant strains: uvrA (red line), recA (orange line), and uvrA and recA (yellow line).
5. Identify that the uvrA and recA double mutant strain (yellow line) has the lowest survival rate, making it the most sensitive to UV light.

Key Concepts

E. coli Strains and Mutations

The question involves different strains of E. coli, specifically a wild-type strain and mutant strains with alterations in the uvrA and recA genes. Understanding these strains is crucial, as the wild-type typically exhibits normal resistance to UV light, while mutations can affect DNA repair mechanisms, influencing survival rates under UV exposure.

UV Light and DNA Damage

Ultraviolet (UV) light is a form of radiation that can cause significant damage to DNA, leading to mutations and cell death. The ability of an organism to survive UV exposure often depends on its DNA repair capabilities, which are influenced by specific genes like uvrA and recA. This concept is essential for interpreting the survival data presented in the graph.

DNA Repair Mechanisms

The uvrA and recA genes are involved in different DNA repair pathways. The uvrA gene is part of the nucleotide excision repair system, while recA plays a role in homologous recombination and SOS response. Understanding these mechanisms helps explain why certain strains are more sensitive to UV light, as deficiencies in these pathways can lead to increased cell death when exposed to UV radiation.