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
3C: The cell division cycle
3B: The cell division cycle
1.
2.
3.
4.
Introduction
a.
The cell cycle
b.
Challenges: order and coordination
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
Cell cycle checkpoints
a.
Cell cycle arrests
b.
Damage repair
Mitosis and the cytoskeleton
a.
Phosphorylation controls cellular architecture
b.
The cytoskeleton: roadways & scaffolding
c.
Microtubules are dynamically unstable
d.
Chromosome capture by exploration with selection
1
Cell cycle learning 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
A minimalist view of the cell cycle
Cell
Growth
2
Mitosis segregates chromosomes
The standard cell cycle
G0
Cells rest in G0
3
Cell Cycle Challenges
The cell cycle engine
DNA replication
Chromosome segregation & cell division
Coordinating growth and proliferation
Finishing tasks
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
4
The frog egg is specialized for cell division
Fast cycles
LOTS of eggs
Natural synchrony
Watching eggs cleave
5
Coordinating Cell Growth & Division: reproduction
Typical somatic cells double
in size in each cell cycle
An autonomous oscillator drives the cell cycle
Time in minutes after activation
6
Embryonic oscillator conclusions
The oscillator does not require a nucleus or DNA
The oscillator is unaffected by interfering with mitosis
or DNA replication
The oscillator depends on post-translational
modification, and protein degradation
NOTE: First two conclusions are not valid for the
standard cell cycle
The discovery of cyclin
Tim Hunt,
discoverer of cyclin
7
Cyclin oscillates through the cell cycle
A cyclin-based cell cycle
Cyclin binds to and activates
Cyclin-dependent kinase 1
8
Protein kinases add phosphates to other proteins
Phosphorylation regulates protein activity
Phosphorylation ACTIVATES
OR
Phosphorylation INACTIVATES
Also regulates:
Inter-molecular binding
Protein degradation
Protein location
9