Genetics of cell cycle control

Question 1

Budding yeast: main points

Answer 1

G1 = growth + bud emergence
G2 = nuclear migration, spindle formation
e.g. S. cerevisiae

Question 2

What is the main control point of the yeast cell cycle?

Answer 2

at the G1/S transition

Question 3

How were the genes required for cell cycle progression identified? (4 + name)

Answer 3

Hartwell

• used selective temperature to identify the genes
• Ts mutants grow at 25degrees but not higher temps
• process of complementation to identify mutation
• addition of gene rescues cells at non-permissive temps

Question 4

Ts mutants

Answer 4

~ temperature sensitive mutants

~ affect protein stability and function

Question 5

What did gene complementation identify?

Answer 5

~ compensatory mechanisms which identified G1 cyclins

~ performed on the mutants who stopped in G1 phase

Question 6

SPF

Answer 6

S-phase promoting factor

Question 7

Fission yeast: main points

Answer 7

G1 = cell growth/elongation

e.g. S. pombe

Question 8

How wan a G2/M arrest identified?

Answer 8

~ in cdc mutants of S. pombe
~ gene complementation used to identify cell cycle regulators
~ done using a S. cerevisiae gene library

Question 9

Permissive temp

Answer 9

~ 25oC

~ the temperature at which a temperature sensitive mutant gene product takes on a normal, functional phenotype

Question 10

G2/M transition regulated by…

Answer 10

~ low levels of Cgc25 and high levels of Wee1 = elongated cells = increased G2
~ + vice versa

Question 11

Cdc13

Answer 11

cyclin B

Question 12

Cdc25

Answer 12

~ activates MPF (cyclin B + Cdc2)

~ protein phosphatase

Question 13

Wee1

Answer 13

~ inhibits MPF (cyclin B + Cdc2)

~ protein kinase

Question 14

Mitotic CDKs (a.k.a. MPF) + their regulation

Answer 14

~ (mitotic cyclin + CDK)
~ inactive in G2 due to inhibitory phosphorylations ( + Y15 molecule) mediated by Wee1
~ Cdc25 phosphatase removes phosphorylation = rapid transition to mitosis

Question 15

Mitosis is triggered by:

Answer 15

~ rapid feedback loops

1. Cdc25 activates CDK
2. CDK inactivates Wee1
3. CDK enhances Cdc25 activity

Question 16

Why are yeast excellent genetic tools for identification of cell cycle regulators?

Answer 16

cell cycle regulators are conserved from yeast to man (most cases)