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Cells of a triple-mutant yeast strain (leu- his- trp-) were spread either on minimal medium or on minimal medium supplemented with various combinations of histidine, leucine, or tryptophan. The cultures were grown at either 25C or 37C for 3 days. Colony numbers in each plate were counted, and the data are listed in the following table.

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What kind of mutation most probably causes the his- phenotype?What kind of mutation most probably causes the leu- phenotype?

A.Point mutation

B.missense mutation

c.nonsense mutation

d.conditional mutation

e.deletion mutation

My questions : What is the meaning of confluent in this context ? Can anyone give me a hint to solve this question ? I have no clue how to solve this.

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For starters, you can just google "confluent". It means that there are no colonies, but that the cells cover the whole plate.

So when you provide all three amino acids, the cells fill up the whole plate. If you leave His out, but keep the plate at 25 degrees, the plate still fills up with cells, so the His- allele isn't hurting the cells at all at 25 degrees, but it's pretty nasty at 37 degrees. That makes it conditional; the mutation only impairs His metabolism at some temperatures.

There is not a single cell which is surviving a lack of Leu, but there are some which are surviving the lack of Trp, and the lack of His at harmful temperatures. Presumably, those are the small number which are lucky enough to acquire mutations which compensate for the original deleterious alleles, but NO cells were able to overcome the problem with the Leucine metabolism. This suggests that the Leucine mutation might be a total deletion.

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Just to add a useful term to this answer: the rare colonies on the -Trp plates and the -His at 37C are called revertants. Mutation classes (a), (b), (c) and (d) as listed in the question are all capable of reverting. A deletion cannot revert. –  Alan Boyd Jan 10 at 18:37
    
@AlanBoyd Why can all the others revert but not deletion ? –  biogirl Jan 11 at 4:19
    
Most deletions cannot revert because they represent a large loss of information. A deletion of a single base would cause a frameshift, and that could revert as a result of a compensating insertion (or two more single deletions) that restored a functional reading frame. –  Alan Boyd Jan 21 at 19:49

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