Lecture 1 - Eukaryotic Cell Division Flashcards
What is the cell cycle?
Ordered sequence of events that ensures duplication of the entire genome and equal partitioning of the two copies into two genetically identical daughter cells
What separates the major events of S phase and M phase?
Gap phases
Four sequential phases of cell divison
M phase, G1 phase, S phase, G2 phase
During which phase are chromosomes replicated?
S (synthesis) phase (Each chromosome now consist of two sister chromatids)
During which phase do cells carry out nuclear and cytoplasmic division?
M phase
Why do cells enter a quiescent phase (G0)?
When cell division is not required or desirable
Which cells do not spend their time in G0?
Stem cells
What is the evidence that eukaryotic cell division occurs in distinct phases? (Genetic)
Genes that encode for specific cell cycle regulatory proteins can be mutated in the lab, resulting in cells that stop dividind at specific points in the cell cyle
What is the evidence that eukaryotic cell division occurs in distinct phases? (Biochemical 1)
Different cell cycle regulatory proteins are needed to promote the different events that make up the cell cycle. Ex. Mutation in some yeast cdc genes blocks cell division but not growth.
What is the evidence that eukaryotic cell division occurs in distinct phases? (Biochemical 2)
Certain drugs will inhibit the cell cycle at particular stages.
What is the evidence that eukaryotic cell division occurs in distinct phases? (Biochemical 3)
Specific biochemical activities drive different phases of the cell cycle and can dominate over the activities of other cell cycle phases.
What happens during G1 phase?
- Period of cell growth (Single copy of each chromosome) Cellular components are duplicated not chromosomes
- Metabolic changes prepare the cell for cell division
- At the Restriction point (R) the cell is committed to a new cycle and enters into S phase
What happens during G0 phase?
Cells remain metabolically active but no longer proliferate unless called on to do so by appropriate extracellular signals
What extracellular signals cause a cell to exit G0 phase?
When needed to replace cells that have been lost because of injury or cell death
What happens during G2 phase?
- Cell growth
- Metabolic changes assemble the cytoplasmic materials necessary for M phase
Any DNA errors are corrected by specific DNA repair mechanisms
Length of cell cycle in human body cells
24 hrs
Length of cell cycle in budding yeast
90 mins
Length of cell cycle of cells in the early embryo
30 mins.
- Cell growth does not take place and there is no G1 or G2 phase and DNA replication occurs very quickly.
- Early embryonic cells rapidly divide forming smaller and smaller daughter cells
Length of S phase in human body cells
10-12 hrs (about 1/2 the entire length of a cycle)
Length of M phase in human body cells
Around 1 hr
What is the cell cycle controlled by?
Three internal checkpoints that evaluate the condition of DNA content of the cell and the external and internal environment
Name the three check points
- G1/S phase checkpoint
- G2/M phase checkpoint
- Metaphase/Anaphase checkpoint
What happens at the G1/S phase checkpoint? [Restriction point (R)]
It is determined whether all condition are favorable for cell division to proceed.
- Damage to DNA?
- Presence of Growth factors/mitogens
- Correct size?
- Adequate energy reserves?
What happens if requirements aren’t met at G1 checkpoint?
- The cell can halt the cycle and attempt to remedy the problematic condition
- The cell can advance into G0 phase and await further signals when conditions improve
- apoptosis can occur if DNA is damaged beyond repair
What happens at G2/M phase checkpoint?
- Ensures that all of the chromosomes have been accurately replicated without mistake or damage
- Cell size and energy/protein reserves are assessed
Where does the Metaphase/Anaphase checkpoint occur?
Near the end of the metaphase stage of mitosis
What happens at the Metaphase/Anaphase checkpoint? [Spindle Assembly Checkpoint (SAC)]
Check chromosomes are correctly aligned and attach to the spindle microtubules
What controls progression through checkpoints?
Activity of protein complexes called Cyclin-CDKs
Regulatory subunit of Cyclin-CDKs
Cyclins
Catalytic subunit of Cyclin-CDks
Protein kinases (CDKs)
What does a Cyclin binding to a CDK cause?
When inactive, the active site of CDKs is sterically occluded by a loop referred to as the T loop. Cyclin binding to CDKs pulls the T loop away from the active site and exposes the bound ATP, allowing it access to target proteins
Levels of what oscillate during the cell cycle?
Cyclins
In late G1, levels of which Cyclin rise?
G1/S-cyclins (levels fall in S phase) and S-cyclins (S-cyclins start to rise in late G1 and continue to do so in S phase)
What do G1/S-cyclins do?
- They bind to and activate specific G1/S-phase CDKs
- Help progression through the Restriction point resulting in commitment to S phase entry
- Levels fall in S phase as they are no longer needed
What do S-cyclins do?
- They bind to specific S-phase CDKs soon after progression through the Restriction Point
- Help stimulate DNA replication
- Help to control some early steps in mitosis so their levels remain high until they have completed these tasks
How is the cell cycle regulated by transcription and translation?
Growth factors/mitogens can induce expression of G1-cyclins, which initiate cell cycle entry.
Changes in cyclin gene transcription help control cyclin levels in most cells
How is the cell cycle regulated by Ubiquitin ligases (e.g. APC/C)?
The APC/C degrades S-phase and M-phase cyclins during the cell cycle.
Progression through the metaphase to anaphase transition is triggered not by protein phosphorylation but by protein destruction, leading to the final stages of cell division.
How is the cell cycle regulated by Ubiquitin ligases (e.g. SCF)?
The major role of SCF ubiquitin ligase is to ubiquitylate certain CKI proteins in late G1, thereby helping to control the activation of S-Cdk and DNA replication.
It is constant during the cell cycle.
Process of Protein Ubiquitinylation
- Involves the addition of small ubiquitin (Ub) molecules to target proteins
- Ub molecules are added by E3 Ubiquitin Ligases and can be removed by Deubiquitinating enzymes (DUBs)
- Addition of Ub molecules promotes degradation of target protein by the proteasome
What do M-cyclins do?
They activate Cdks that stimulate entry into mitosis at the G2/M transition. M-cyclin levels fall in mid-mitosis
What is required for full activation of a Cyclin-Cdk complex?
Phosphorylation of Cdk’s
How does inhibitory phosphorylation work?
Phosphorylation at a pair of amino acids in the roof of the kinase active site inhibits the activity of cyclin-Cdk comples
What does Wee1 kinase do?
Phosphorylates the pair of amino acids in the roof of the kinase active site of the cyclin-Cdk Complex inhibiting Cdk activity
What does Cdc25 phosphatase do?
It dephosphorylates the kinase active sites in the kinase active site increasing Cdk activity.
How is the cell cycle regulated by Cdk inhibitors (CKIs)?
Binding of these inactivates cyclin-Cdk complexes.
CKI binding stimulates a large rearrangement in the structure of the Cdk active site, rendering it inactive.
p27 is an example of this
What are CKIs primarily used for?
To help govern the activities of G1/S and S-Cdks early in the cell cyle
