Conditional Mutations Can Be Used to Study Essential Genes in Yeast

The procedures used to identify and isolate mutants, referred to as genetic screens, depend on whether the experimental organism is haploid or diploid and, if the latter, whether the mutation is recessive or dominant. Genes that encode proteins essential for life are among the most interesting and important ones to study. Since phenotypic expression of mutations in essential genes leads to death of the individual, ingenious genetic screens are needed to isolate and maintain organisms with a lethal mutation.

In haploid yeast cells, essential genes can be studied through the use of conditional mutations. Among the most common conditional mutations are temperature-sensitive mutations, which can be isolated in bacteria and lower eu-karyotes but not in warm-blooded eukaryotes. For instance, a mutant protein may be fully functional at one temperature (e.g., 23 °C) but completely inactive at another temperature (e.g., 36 °C), whereas the normal protein would be fully functional at both temperatures. A temperature at which the

Yeast in liquid culture

Yeast in liquid culture

Agar

Add mutagen; distribute mto smaller aliquots □□□□! ddd t ir f f I ^ ^ ^

I Plate out 3| I individual Colonies 1 aliquots

Agar

I Plate out 3| I individual Colonies 1 aliquots

Incubate at 23 °C

Replica-plate and incubate

Replica-plate and incubate

Incubate at 23 °C

Temperature-sensitive for growth; growth at 23° no growth at 36°

Wild type

Wild type

cdc28 mutants
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