A 1-mL sample of the bacterium E. coli is exposed to ultraviolet light. The sample is used to inoculate a 500-mL flask of complete medium that allows growth of all bacterial cells. The 500-mL culture is grown on the benchtop, and two equal-size samples are removed and plated on identical complete-medium growth plates. Plate 1 is immediately wrapped in a dark cloth, but plate 2 is not covered. Both plates are left at room temperature for 36 hours and then examined. Plate 2 is seen to contain many more growing colonies than plate 1. Thinking about DNA repair processes, how do you explain this observation?

Key Concepts

Ultraviolet (UV) Light and DNA Damage

Ultraviolet light is a form of electromagnetic radiation that can cause damage to DNA by inducing the formation of pyrimidine dimers, primarily thymine dimers. These dimers disrupt normal base pairing during DNA replication, leading to mutations if not repaired. Understanding how UV light affects DNA is crucial for explaining the differences in bacterial growth observed in the experiment.

DNA Repair Mechanisms

Bacteria possess several DNA repair mechanisms to correct damage caused by UV light, including nucleotide excision repair (NER) and photoreactivation. NER removes damaged sections of DNA and synthesizes new DNA to replace them, while photoreactivation directly reverses thymine dimers using light energy. The effectiveness of these repair processes is essential to understanding why one plate shows more colonies than the other.

Effect of Light on Bacterial Growth

In the context of the experiment, light plays a critical role in the photoreactivation repair process. Plate 1, which is covered and kept in the dark, prevents the bacteria from utilizing photoreactivation to repair UV-induced damage, leading to fewer viable colonies. In contrast, Plate 2, exposed to light, allows for effective repair of DNA damage, resulting in a higher number of growing colonies.