Mitosis
Separation of daughter chromosomes, ending with cell division (cytokinesis)
Interphase
The period between mitoses (95% of the time) during which cell growth and DNA replication occur
Cytokinesis
Physical cell division
M phase
Mitosis followed by cytokinesis
G1 Phase
(Gap 1) Corresponds to the interval between mitosis and initiation of DNA replication. Cell is metabolically active and continuously grows, but does not replicate DNA.
S phase
(Synthesis) DNA replication takes place
G2 phase
(Gap 2) Cell growth continues and proteins are synthesized in preparation for mitosis
Flow cytometetry
Experimental method of analyzing the fluorescent intensity of individual cells; can determine where cells are in the cell cycle
START
A regulatory point in yeast, controlling progression from G1 to S; point at which cells decide if there is adquate nutrition to proceed through the rest of the cycle
Restriction Point
A decision point in late G1 in animal cells; regulated primarily by extracellular growth factors; if passed, committed to complete the cycle regardless of further availability of growth factors
G0
Quiescent state where cells are metabolically active but not proliferating; cells enter G0 if they do not pass the restriction point
What are the four discrete phases of the eukaryotic cell cycle?
M, G1, S, G2
What two things regulate progression through specific control points?
Extracellular signals and cell size
Cell cycle checkpoints arrest progression if:
DNA is damaged
Once DNA replication has taken place, progression through a new ____ is prevented until the cell has passed through mitosis.
S phase
Maturation Promoting Factor (MPF)
Cytoplasmic factor found in oocytes which triggers transition from G2 to M; is a general regulator in all cells of the transition from G2 to M
Cdk1
Forms complexes with Cyclin B to form MPF during G2
Cyclin
Accumulate throughout interphase and are then rapidly degraded toward the end of each mitosis
G1 cyclin (Cln)
Associates with Cdk1 to pass yeasts through START; Cdk1 then associates with type B cyclins to enter S
Cdk
Cyclin-dependent kinases; associate with specific cyclins to drive progression through the different stages of the cell cycle
Cdk Inhibitor (CKI)
Ink4 CKIs bind to and inhibit Cdk4 and Cdk6, inhibiting progression through G1. Cip/Kip binds jointly to cyclins and Cdks and can inhibit the activities of the Cdk2/cyclinA or cyclin E, inhibiting G1 and S.
Rb
Key substrate of Cdk4,6/Cyclin D complexes; mutations resulting in the absence of Rb contribute to a variety of human cancers
Tumor Suppressor Gene
A gene whose inactivation leads to tumor development (ex: Rb and Ink4 Cdk inhibitors); slow down cell cycle progression
E2F
Bound by underphosphorylated Rb to repress transcription; when not bound to Rb, activates transcription
ATM
Activated in response to double-strand breaks; phosphorylate and activates Chk2 which inhibits Cdc25 protein phosphatases (required to activate Cdk1 & Cdk2)
ATR
Activated in response to single-stranded or unreplicated DNA; phosphorylate and activates Chk1 which inhibits Cdc25 protein phosphatases (required to activate Cdk1 & Cdk2)
Checkpoint Kinase
Chk1 and Chk2; phosphorylated by ATR and ATM respectively; induce cell cycle arrest by phosphorylating and inhibiting Cdc25 phosphatases
p53
Mediates arrest at the G1 checkpoint; is phosphorylated by ATM and Chk2 which stabilizes it; increases in p53 leads to induction of Cip/Kip
What is the key molecule responsible for regulating the G2 to M transition?
Maturation promoting factor (MPF)
MPF is a dimer of:
The Cdk1 protein and cyclin B
3 regulators of Cdk's
1) Cyclins
2) Activating and inhibitory phosphorylations
3) Binding of Cdk inhibitors (CKI)
Growth factors stimulate animal cell proliferation by inducing synthesis of what type of cyclin?
D
What complex drives cells through the restriction point in G1?
Cdk4,6/cyclin D
What is a key substrate of Cdk4,6/cyclin D complexes which regulates transcription of genes required for cell cycle progression?
Rb
Activation of what complex is responsible for entry into S phase?
Cdk2/cyclin E; inhibited by p27
In what phases of the cell cycle can DNA damage or incompletely replicated DNA arrest progression?
G1, S, G2
Cell cycle arrest is mediated by protein kinases that are activated by DNA damage and inhibit _____, which are required for Cdk activation.
Cdc25 phosphatases
Arrest at the G1 checkpoint is mediated by ____, which induces synthesis of the Cdk inhibitor p21.
p53
Cell growth does not take place in dividing _____ cells
embryonic
In embryonic cells, which to phases of the cell cycle are missing?
G1 and G2
In most cases, cells at different stages in the cell cycle differ only by:
their DNA content
Vertebrate oocytes can remain arrested at the _____ phase for decades until stimulated hormonally.
G2
Cell cycle checkpoints prevent entry into the next phase of the cell cycle until:
The events of the previous stage have been completed
DNA Damage Checkpoints
Ensure that damaged DNA is not replicated and passed to daughter cells
DNA damage checkpoints are present in what stages of the cell cycle?
G1, S, G2
The DNA damage checkpoint in G2 prevents initiation of mitosis if the cell contains:
DNA that has not been replicated or unrepaired lesions
Spindle Assembly Checkpoint
Monitors the alignment of chromosomes on the mitotic spindle, thus ensuring that a complete set of chromosomes is distributed accurately to daughter cells
The spindle assembly checkpoint halts cells at what phase?
Metaphase
MCM helicase proteins serve what function?
Restriction of DNA replication to once per cell cycle; MCM proteins are only able to bind to replication origins during G1 and are removed following initiation of replication
After complexing with Cyclin B, Cdk1 is phosphorylated on threonine-161, which is required for ______.
Cdk1 activity
After complexing with Cyclin B, Cdk1 is phosphorylated on tyrosine-15 and threonine-14, which _____.
Inhibits Cdk1 activity
MPF is activated by:
Dephosphorylation of Thr14 and Tyr15
MPF activity is terminated toward the end of mitosis by:
Proteolytic degradation of cyclin B, followed by dephosphorylation of Cdk1
Progression from G1 to S is regulated principally by:
Cdk2, Cdk4, and Cdk6 in association with cyclins D and E
Progression through the restriction point in G1 is regulated by what Cdks and cyclins?
Cdk4 and Cdk6 with the D type cyclins
What cyclin/cdk complexs are required for G1 to S transition and initiation of DNA synthesis?
Cdk2/cyclin E
What cyclin/cdk complexes function in progression of cells through the S phase?
Cdk2/cyclin A
What cyclin/cdk complexes regulate passage from S to G2?
Cdk1/cyclin A
What cyclin/cdk complexes regulate passage from G2 to M?
Cdk1/cyclin B
In the absence of other Cdks, _____ binds to all of the cyclins and is able to drive progression through all stages of the cell cycle.
Cdk1
The activating phosphorylation of Cdks is catalyzed by what enzyme?
CAK
CAK (Cdk-activating kinase) is a complex of:
Cdk7 and cyclin H
Inhibitory phosphorylation of tyrosine residues near the Cdk amino terminus is catalyzed by what protein kinase?
Wee1
What family of protein phosphatases activate Cdks by dephosphorylation?
Cdc25
Cyclin D1 synthesis is induced in response to:
growth factor stimulation