Mutation
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You are studying the genetic codes of three phenotypically identical E. coli bacteria: HB99, HB100 and HB101. While studying their DNA sequences, you note that a point mutation has occurred in the same gene in each of the E. coli strains. The gene encodes for an enzyme required to methylate lysine 27 on Histone 3: H3K27.
HB99 sequence: ATGCGTCACGTCGA
HB100 sequence: ATGCGTTACGTCGA
HB101 sequence: ATGCGTGACGTCGA
You find that in HB99 and HB101, H3K27 appears to be methylated normally. Moreover, the methylating enzyme encoded by this gene appears in normal levels and appears to be correctly folded. Therefore, you conclude that a mutation has arisen between these two strains.
However in HB100, the H3K27 is not methylated. Additionally, the methylating enzyme encoded by this gene exists in very low levels and the protein appears to be truncated. Therefore, you conclude that a mutation has arisen in this strain.