Finding the original mutation | Quick Homework Help

A biologist was testing E. coli for thymidine requirement. She found a thy- point mutant and wanted to study reversion to thy+. She grew 20 cultures of the thy- mutant strain and plated them onto 20 plates that contained media to select for thy+ revertants. She obtained a total of 460 colonies. Six of the plates showed no colonies.
a) If 5 x 108 cells were spread on each plate what was the reversion frequency and the reversion rate?
b) Explain what steps she could have taken to find the thy- mutant in the first place, and why she did each step.
c) She also wanted to study the type of mutation that occurred during formation of the original thy- cells. The mutagens HA (a base modifyer), BU (a base analogue), and acridine (an intercalator) were each aIDed to thy- cells. Only BU reverted the thy- mutants to WT. Briefly describe the types of mutations each mutagen causes. Based on this information what was the original mutation to thy- to begin with? Explain your reasoning.