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 TTC TGA 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.

Explanation / Answer

The severe mutation is not observed in both MUTA and MUTB sequences. The point mutation is observed, which is a silent mutation that is showing no effect on enzyme synthesis.

According to the question, the mutation in the DNA sequence is observed in MUTA sequence and MUTB sequence, whereas the WT sequence is the normal.

Study the mutation in MUTA, you will observe that "TGC" is mutated to the "TGA". The mRNA sequence is formed complimentary to the DNA strand as given below:

DNA ................................ mRNA

TGC (normal).................. ACG,    the ACG code for the aminoacid Threonine.

TGA (mutated)................ ACU, the ACU also codes for the aminoacid Threonine.

So, it can be concluded that mutation has no effect on the sequence that is coding for respective protein or enzyme. This type is called silent mutation, which is a type of point mutation.

In the similar way, MUTB shows that

DNA.................................................mRNA

AGG (normal)..................................UCC (code for the serine aminoacid)

AGC (mutated)................................UCG (also code for serine amianoacid)

This type is called the point mutation, where the mutation effect is not observed in the enzyme or protein sequence.

The change in the single nucleotide base does not cause any effect on the synthesis of protein or enzyme.

If an organism is heterozygous for mutation A and when it is crossed with other heterozygous organism with mutation B, then 100% offspring would require leucine in the media.

If the F1 mutant organisms are back-crossed to the MutA parent, 100 % of the results in F2 would need L in the media.

The homozygous true breeding line of Mutant A organisms are crossed with another true breeding mutant line (MutC) that carries a mutation by a different enzyme in the synthesis pathway for L. So, the percent of the F1 generation depends on the MUtC sequence (as MUtC sequence is not shown in question).

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