The baker’s yeast Saccharomyces cerevisiae is able to make uracil from simpler c

Question

The baker’s yeast Saccharomyces cerevisiae is able to make uracil from simpler compounds (biosynthesis). A scientist doing research on pyrimidine biosynthesis isolated four mutant strains of yeast that were unable to make their own uracil. These strains are called ura- mutants. To determine if the different strain had mutations in the same gene, or in different genes, she performed complementation tests. She mated all of the strains in all of the possible combinations, and then determined if the hybrids had a ura- phenotype, or if they were normal wild type. The results are given in the table below. A “-” means that when the two strains were mated, their hybrid was still ura-. A “+” means that when the two strain were mated, their hybrid was normal wild type and able to make its own uracil.

A. Which mutant strains have mutations in the same gene, and which strains have mutations in genes that are different from all of the others?

B. How many different genes are represented by these mutant strains?

strain 1 strain 2 strain 3 strain 4 strain 1 - + + + strain 2 - - + strain 3 - + strain 4 -

Explanation / Answer

In the complementation the disability of one gene was compensated by another gene. The positive sign indicates that, when one mutant strain is mixed with another strain so some strains are able to grow because gene transfer make them capable to grow.

The negative sign indicates that when we mix two different strains they are not able to grow because one strain is not able to compensate defficiency of anotger gene.

In this positive signs are five( Five mixing event - 1 and 2, 1 and3, 1&4, 2&4,3&4) and no repetition of group means five complementation group.

So we can say mutant strain one have mutation at particular region and all remaining strin 2,3,4 have mutation on different gene that is why when we mix 2 strain, wild type strains are produced.

In other way, strain 2 and 3 have same mutation that is why by mixing of these 2 & 3 strains wilt type strain are not produced.

When two strain have mutation at different different gene then by mixing of both of these, they produxe normal cells.

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