Life Sciences 1a: Section 3B. The cell division cycle Objectives Understand the challenges to producing genetically identical daughter cells Understand how a simple biochemical oscillator can drive the cell cycle Understand how protein phosphorylation can be used to regulate protein activity Understand why regulated protein destruction is a major mode of biological regulation Understand how cell cycle checkpoints protect the integrity of the genome Introduction Minimal cycle: chromosome replication, chromosome segregation, cell division, and (optionally) cell growth Mitosis: nucleus breaks down, chromosomes segregated using the spindle, a structure made of microtubules The standard cell cycle has four phases (G1, S phase, G2, mitosis) and takes about 24 hours in mammals Most cells are neither growing nor dividing but are resting in G0, which is reached from G1 Challenges:

Nature of the biochemical oscillator that drives the cell cycle (the cell cycle engine) DNA replication: how is it regulated during the cycle The mechanism of chromosome segregation and cell division How are cell growth and proliferation coordinated to regulate cell size How do cells make sure one task is finished before beginning the next

The cell cycle engine Many early embryos are specialized for very rapid cell division. Their moms lay LOTS of eggs. Oocytes get big by growing without dividing. Early embryonic cycles go fast by dividing without growing. An autonomous oscillator drives the cell cycle. In frog embryos:

Oscillator doesn’t require a nucleus Slowing down DNA replication or mitosis doesn’t slow the oscillator All regulation is on the activity and stability of proteins

The rise and fall of cyclin drives the early embryonic cell cycle Cyclin accumulates in interphase and is destroyed at the end of mitosis A minimal model: cyclin induces mitosis and its own degradation Cyclin binds to and activates cyclin dependent kinase 1.

Protein kinases add phosphate from ATP to hydroxyl groups on serine, threonine, or tyrosine Phosphatases remove the phosphate allowing independent regulation of phosphate addition and removal Phosphorylation regulates protein activity, localization, binding, and stability The engine oscillates because of positive and negative feedbacks (reaction kinetics and topology matters) Positive:

Initial Cdk1-cyclin complexes inactive Active Cdk1-cyclin complexes stimulate reactions that turn on inactive Cdk1-cyclin complexes

Negative: With a delay, active Cdk1-cyclin complexes turn on the cyclin destruction machinery

Cyclin is degraded by ubiquitin-mediated proteolysis C terminus of ubiquitin is coupled to lysine side chains of cyclin (and many other proteins) Anaphase promoting complex (APC) is the final coupling enzyme and is activated by Cdk1-cyclin

Ubiquitination is like phosphorylation: irreversible, diverse, regulates protein activity, location, and stability Standard cell cycles have temporally separate G1 (D), S phase (A, E) and mitotic (B) cyclins G1 cyclins overcome a cell cycle roadblock set by inhibitors of cyclin-dependent kinases Cancer mutations remove the roadblock or make enough G1 cyclins to over-ride it. Cell cycle checkpoints Not finishing tasks damages chromosomes: incomplete replication leads to chromosome loss & breakage Checkpoints monitor completion and induce arrest and repair Damaged DNA prevents Cdk1-cyclin B activation, induces DNA damage repair, and can induce cell death Apoptosis: a program of cell death to remove damaged or unwanted cells Reading: Alberts: pp. 611-632

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3C: The cell division cycle

1. Introduction

a. The cell cycle

b. Challenges: order and coordination

2. The cell cycle engine

a. Early embryonic cells cycles are stripped down

b. Cyclins and cyclin-dependent kinases make an oscillator

c. Proteolysis regulates the progress of the cell cycle

d. The cell cycle engine in mammalian cells

3. Cell cycle checkpoints

a. Cell cycle arrests

b. Damage repair

4. Mitosis and the cytoskeletona. Phosphorylation controls cellular architecture

b. The cytoskeleton: roadways & scaffolding

c. Microtubules are dynamically unstable

d. Chromosome capture by exploration with selection

3B: The cell division cycle

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Cell cycle learning objectives

Understand the challenges to producing geneticallyidentical daughter cells

Understand how a simple biochemical oscillator can drivethe cell cycle

Understand how protein phosphorylation can be used toregulate protein activity

Understand why regulated protein destruction is a majormode of biological regulation

Understand how cell cycle checkpoints protect theintegrity of the genome

A minimalist view of the cell cycle

CellGrowth

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Mitosis segregates chromosomes

The standard cell cycle

G0

Cells rest in G0

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Cell Cycle Challenges

The cell cycle engine

Coordinating growth and proliferation

Chromosome segregation & cell division

DNA replication

Finishing tasks

a. Early embryonic cells cycles are stripped down

b. Cyclins and cyclin-dependent kinases make an oscillator

c. Proteolysis regulates the progress of the cell cycle

d. The cell cycle engine in mammalian cells

The cell cycle engine

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The frog egg is specialized for cell division

Fast cycles

LOTS of eggsNatural synchrony

Watching eggs cleave

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Coordinating Cell Growth & Division: reproduction

Typical somatic cells doublein size in each cell cycle

An autonomous oscillator drives the cell cycle

Time in minutes after activation

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Embryonic oscillator conclusions

The oscillator does not require a nucleus or DNA

The oscillator is unaffected by interfering with mitosisor DNA replication

The oscillator depends on post-translationalmodification, and protein degradation

NOTE: First two conclusions are not valid for thestandard cell cycle

The discovery of cyclin

Tim Hunt,discoverer of cyclin

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Cyclin oscillates through the cell cycle

A cyclin-based cell cycle

Cyclin binds to and activatesCyclin-dependent kinase 1

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Protein kinases add phosphates to other proteins

Phosphorylation regulates protein activity

Also regulates: Inter-molecular bindingProtein degradationProtein location

OR

Phosphorylation ACTIVATES

Phosphorylation INACTIVATES