BI503: Cell Division and the Cell Cycle

Key Nobel Prize Winners (2001)

• Prize in Physiology or Medicine: Awarded for discoveries relating to key regulators of the cell cycle.

• Winners: Paul Nurse, Tim Hunt, Lee Hartwell

Classic Experiments on cdc Mutants in Yeast (1970-75)

• Researchers: Hartwell & Nurse

• Objective: Isolation of budding yeast mutants that are temperature-sensitive for cell cycle progression.

  • Conditions:

    • Permissive Temperature: 25°C (Normal growth)

    • Restrictive Temperature: 36°C (Cell cycle arrest).

Cell Cycle Dynamics in cdc Mutants

• Logarithmic Growth: The correlation between cell number and generation time

  • Execution Points:

    • Example Mutants:

      • cdc15: Execution point at the end of the cell cycle.

      • cdc7: Execution point at the beginning of the cell cycle.

    • Definition: The execution point is the phase after which the gene product is no longer necessary for continued division.

Significant Publication on Cell Division Control

• Reference: Molec. gen. Genet. 146, 167-178, March 18, 1976.

• Study Focus: The genetic control of the cell division cycle in the fission yeast Schizosaccharomyces pombe.

• Researchers: Paul Nurse, Pierre Thuriaux, and Kim Nasmyth.

Cellular Responses and Genetic Mutations

• 'cdc' and 'wee' Mutants:

  • 'cdc' Block: Normal cell mass but delayed progression through the cell cycle.

  • 'wee' Mutant: Exhibits accelerated progression through the cell cycle.

• Key Genes:

  • Number of Genes Identified: 2 (wee1 and wee2) responsible for cell cycle regulation.

cdc2 Alleles and Their Effects on Cell Division

• Cell Length at Septation: Detailed data on cell length variation in different strains at different temperatures.

  • Growth Conditions: Studied effects of temperature on heterozygous cdc2+/cdc2- diploids versus haploid strains at 25° and 35°.

• Observations Again:

  • Dominance Relations: Impact of cdc2 mutations on cell division rates.

  • Loss of Function: Related to specific temperature conditions and resultant cell lengths.

Regulatory Role of cdc2 Gene

• Functions of cdc2:

  • Required for cell cycle commitment in G1 phase and for controlling mitosis in G2 phase in fission yeast.

  • Assessment: Shows commitment to division is regulated by cdc2 across different species.

Summary of Cdc2 Activity

• Divisional States:

  • Low cdc2 Activity: Cells remain in interphase (not dividing).

  • High cdc2 Activity: Cells proceed to mitosis, resulting in division.