Question

A one hour dark incubation period is used between UV exposure and plating. If the incubation period were conducted in the light, what effect would this have on the results? Explain

Answer 1

Solution:-

Bacterial DNA codes for an enzyme called photolyase, which is involved in DNA repair on exposure to UV. These enzymes use the energy from light to irreversibly bind and break nucleotide dimers formed in bacterial DNA due to UV exposure. In the given example, when no dark incubation period is given, the bacteria are exposed to light immediately after the UV treatment, during plating. This is where the photolyase repairs the DNA damage and counteracts the mutagenesis, before the cells can divide, and before the mutation becomes permanent. This results into a low number of sodium-azide resistant mutants after plating.

On the other hand, when a one hour dark incubation period is given, photolyase cannot act on the nucleotide dimers before DNA replication takes place.As a result, a random nucleotide is inserted on the complementary strand, making the mutation permanent. This translates to a high number of sodium-azide resistant mutants, even though the cell number is low.

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