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Hartwell, L.H. (1967) Macromolecule synthesis in temperature-sensitive mutants of yeast. J. Bacteriol. 93: 1662-1670.

1. What is a conditional mutant? What is a temperature-sensitive conditional mutant?

2. Researchers interested in an essential process, such as cell division, isolate conditional mutants. Explain.

3. What is the name of the parental strain in which the mutations were isolated? Does it carry any mutations in known genes, and if so, what is its genotype?

4. The questions below relate to the approach used to isolate cell cycle mutants.

(a) What is the initial phenotype used to identify the mutant strains?

(b) Are these spontaneous mutants or induced?

(c) How is replica plating used to identify the mutants?

(d) Is this a selection or a screen and why?

5. Describe the methods used to separate the mutants into classes affecting protein synthesis, RNA synthesis, DNA synthesis.

6. What are the phenotypes of cell division mutants carrying alterations in an 'early' step and a 'late' step in cell division?

7. What percentage of the mutants could be clearly categorized as protein synthesis, RNA synthesis, DNA synthesis, cell division, cell wall formation mutants.

8. Of the 400 mutant strains analyzed, 396 were recessive.

(a) Choose one mutant strain and diagram the cross carried out to demonstrate that the mutation in this strain is recessive. Be sure to give the genotype and phenotype of both parents and the diploid.

(b) Are you or are you not surprised by the large percentage of recessive mutations and why?

9. In the next article, Dr Hartwell describes how sporulation of each heterozygous diploid (formed by mating the temperature-sensitive mutants to another strain of opposite mating type which is not temperature sensitive) produced two temperature-sensitive haploid spores and two normal haploid spores. What does this tell you about the alteration in each mutant that is causing the temperature-sensitive phenotype? Compare the genotypes of strains A364A and X1069-2D. Do we know that these strains are identical (that is isogenic) at all other genes, and why might this be important?

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