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 understand the relationship between the dose of UV light and the percentage of cells surviving for each E. coli strain.
2. Identify the wild-type strain, which is represented by the green line. Notice that it maintains a high percentage of survival across increasing doses of UV light, indicating it is the least sensitive to UV exposure.
3. Observe the strain with mutations in both uvrA and recA genes, represented by the yellow line. This strain shows a rapid decline in survival percentage with increasing UV doses, indicating it is the most sensitive to UV light.
4. Compare the strains with single mutations: the uvrA strain (red line) and the recA strain (orange line). Both show decreased survival compared to the wild-type, but the uvrA strain is more sensitive than the recA strain.
5. Conclude that the strain with mutations in both uvrA and recA is the most sensitive to UV light, while the wild-type strain is the least sensitive. The uvrA strain is more sensitive than the recA strain, but less sensitive than the strain with both mutations.

Key Concepts

Ultraviolet (UV) Light and DNA Damage

Ultraviolet light is a form of electromagnetic radiation that can cause damage to DNA, leading to mutations and cell death. It primarily induces the formation of pyrimidine dimers, which disrupt normal base pairing during DNA replication. Understanding how UV light affects cellular survival is crucial for analyzing the sensitivity of different E. coli strains.

DNA Repair Mechanisms

E. coli employs several DNA repair mechanisms to fix damage caused by UV light, including the uvrA and recA genes. The uvrA gene is involved in nucleotide excision repair, which removes damaged DNA segments, while recA plays a role in homologous recombination and SOS response, facilitating repair under stress. Mutations in these genes can significantly affect a strain's ability to survive UV exposure.

Strain Sensitivity Analysis

The analysis of survival rates among different E. coli strains after UV exposure allows for the identification of sensitivity levels. In the provided graph, the wild-type strain shows the highest survival rate, while strains with mutations in uvrA, recA, or both exhibit decreased survival. This comparison helps determine which genetic mutations confer increased sensitivity to UV light.