1. An auxotrophic E. coli strain requires adenine to grow because of a mutation

Question

1. An auxotrophic E. coli strain requires adenine to grow because of a mutation in a gene for an adenine synthesis enzyme. The following shows part of the wild-type and mutant alleles of the gene, including the start codon. The bottom strand is the template for transcription. ade1 wild-type ade1 mutant allele 5...TTATGGGCAAGATCCCA.. .3' 3.. .AATACCCGTTCTAGGGT...5' 5 '.. .TTATGGGCTAGATCCCA..3 .AATACCCGATCTAGGGT. .5 How does this mutation change the mRNA, and how does it affect the enzyme? 2. A yeast strain was exposed to chemical mutagen. As expected, exposure to mutagen resulted in DNA sequence change in an essential gene you examined. Yet, this mutation did not result in any lethal phenotype. How could you explain this apparent discrepancy?

Explanation / Answer

1. ANS: The third codon which was present in the mRNA was changed from AAG (lysine) to UAG (stop). So because of this reason the protein synthesis did not occurred and the process of translation can also be stopped at the third codon.

2. ANS: The DNA sequence change was taken placed in the amino acid synonymous nucleotide position and these changes can result the protein sequence remained unaltered. Since the protein sequence was not affected, no lethal phenotype manifested. Other possibilities are DNA sequence change was a silent or neutral mutation.

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