Lecture 1 - Genetics of the Cell Cycle

Question 1

Why use budding yeast as a model system?

Answer 1

eukaryotic
Single celled
Microscopic
Can be grown on an Agar plate

Question 2

What are temperature sensitive mutants?

Answer 2

Have a wild type phenotype at permissive conditions
‘Conditional’ mutants
Can be ‘rescued’

Question 3

How do temperature sensitive mutants work?

Answer 3

at the restrictive temperature you can see the mutant phenotype
Mutations in genes required to progress to the next phase of the cell cycle cause it to arrest at the end of that phase
All cell division cycle (cdc) mutants arrest at a specific point in the cell cycle

Question 4

What happens to a non cell cycle division mutant at the restrictive temperature?

Answer 4

arrests immediately irrespective of the phase of the cell cycle

Question 5

How do you screen for temperature sensitive mutants?

Answer 5

Wild type cells are unaffected as you transfer them between the different temperatures
CDC mutants are affected because they have a mutation in one gene (but are otherwise identical to wild type). The gene is inactive so cells arrest at one point in the cell cycle.
Mutant in non cell cycle function cells can also be identified as they arrest immediately when transferred.

Question 6

What did hartwell’s genetics reveal?

Answer 6

1) each step and checkpoint in the cell cycle
2) all the genes required for cell cycle progression
3) the point at which these genes are required in the cell cycle
4) the order in which these genes act

Question 7

What is plasmid rescue?

Answer 7

1) grow cdc2- mutants at permissive temperature
2) transform mutant cells with plasmid library of human genes
3) shift transformed cells to restrictive temperature

Only cells transformed with the wild type copy of the mutated gene will grow

Question 8

What are wee mutants?

Answer 8

Smaller cell size divide very quickly

Question 9

Describe how cdc2 regulates the cell cycle

Answer 9

Dominant & recessive mutations in the same gene have opposite phenotypes
Both phenotypes are consistent with the cdc2 gene having a key role in regulating progression through the cell cycle
cdc25 and wee1 mutants have similar phenotypes to cdc2 indicating that they are likely to act at the same point in the cell cycle
cdc25- and wee1- have opposite mutant phenotypes
This indicates that wee1 and cdc25 have antagonistic (or opposite) functions

cdc25 & wee1 act before cdc2 and have opposite functions
Conclusion - These three genes regulate entry into mitosis
The Cdc25 and Wee1 proteins regulate Cdc2 protein activity by a sequential series of phosphorylation events