Below are the DNA sequences for a section of the coding regions of an enzyme in

Question

Below are the DNA sequences for a section of the coding regions of an enzyme in the pathway for the production of leucine. Note this section begins at the start of a codon for the amino acid A. Explain what type of mutation has occurred in both DNA sequences (A and B) (changes are in bold type and underlined). Suggest a hypothesis for which will be the more severe mutation and explain your reasoning. If an organism heterozygous for mutation A is crossed with the organism heterozygous for mutation B, then what ratio of the offspring would require leucine in the media? If the F1 mutant organisms are back-crossed to the MutA parent what percentage of the resulting F2 would need L in the media? Assume a homozygous true breeding line of Mutant A organisms are crossed with another true breeding mutant line (MutC) that carries a mutation at a different enzyme in the synthesis pathway for L. What percent of the F1 generation would require L in the media? If the F1 are backcrossed to MutA organisms what percentage will require L in the media?

WT sequence: GCA GCC CCA ACA AGC AGG TGG GGG GTA TTC TGC GAT GUA

MUTA sequence: GCA GCC CCA ACA AGC AGG TGG GGG GTA TTCTGA GAT GUA

MUTB sequence: GCA GCC CCA ACA AGC AGC TGG GGG GTA TTC TGC GAT GUA

When answering this question, please be every detailed. I am not just looking for a quick answer but a strong explanation that I can use to help learn the topic. I have gotten two very different answers. I would like a third opinion with more detail if possible.

Explanation / Answer

The mutant A has stop codon in the mutation. Hence we could get truncated non functional protein for the mutant gene A.

In mutant B the codon for the amino acid arginine is changed to serine. The consequence of mutation depends on the region in which it has occurred. If it has occurred in the active site of the enzyme it would change the affinity for the substrate.

If the organism heterozygous for mutant A is crossed with heterozygous mutant B then 50% of F1 will require lysine in the media. Being heterozygous there is a possibility that 50% will posses normal genes.

If F1 mutant is crossed with mutant A then Ll Ll then the genotype LL or Ll will not require leucine and ll will require leucine. Here 25% of the F2 will require leucine in the media.

If homozygous mutant A ll is crossed with another mutant c in a different enzyme hence it is considered as non mutant for the Leucine LL. None of the F1 will require leucine supplementation since minimum one normal copy of gene will be present in the offspring.

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