Researchers identified 3 haploid yeast strains that were unable to grow unless u
ID: 144835 • Letter: R
Question
Researchers identified 3 haploid yeast strains that were unable to grow unless uracil was added to the media 4. a. why is uracil essential for life? b. Why is it significant that these yeast strains are haploid? Each of the 3 strains was transformed with a plasmid that contained the URA3 gene. The transformed yeast were now able to survive in media lacking uracil suggesting they each had a mutation in their own URA3 gene. The researchers sequenced the URA3 gene in each of these strains The first yeast strain had a mutation in the fourth codon, UAC was replaced with UAA Explain why this mutant was unable to grow in media lacking uracil The second yeast strain had a mutation in the 15th codon, there was a "C" missing from the sequence. Explain why this mutant was unable to grow in media lacking uracil. The third yeast strain had no mutation in the URA3 coding sequence. What possible reason could there be for the fact that this mutant is unable to grow in media lacking uracil? c. d. e.Explanation / Answer
a. Uracil is a nitrogenous base
It forms nucleosides and consequently nucleotides which are essential in the production of genetic material (RNA), which is one of the basic requirements for life to exist.
b. Since, the yeast stains are haploid, they have a single copy of each gene per cell.
hence, the genetic defects due to any mutation has direct effects which may have been corrected if the stains were diploid. Here, the non-mutated copy of the gene could help in coding properly.
The mutations in haploid cells are more directly and frequently expressed than those in diploid cells.
c. UAA is a stop codon.
Its presence leads to the termination of the process of translation and hence, the synthesis of the required protein is stopped.
No useful protein can be obtained.
Hence, the mutant was unable to grow.
d. This lead to a frameshift mutation which not only affected the particular codon but also changed the reading frame from that point onwards.
The next codons were all translated wrongly and a defective protein was synthesized.
Hence, the mutant was unable to grow.
e. In the absence of uracil, no genetic material can be formed and hence growth of the strain cannot be acheived.
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