Cell Cycle II: An Integrated Discussion of G2 and Mitosis

Codensins

Becomes phosphorylated by CDK/cyclin at the onset of M phase --> supercoil DNA in an ATP-dependent manner during M phase

Ki67

Cell cycle marker of mitosis Function - Surfactant --> prevents other chromosome pairs from interacting with each other - Positively charged Leu/Arg repeats on Ki-67 bind to negatively charged phosphate backbone of chromatin --> forms highly charged "brush" structure that ensures chromosome dispersion

Entry/Exit from mitosis

Entry: Increased levels of Cdk1/Cyclin B --> phosphorylates -->>> a) Greatwall [Gwl] --> Gwl kinase phosphorylates 2 regulator proteins --> inhibits PP2A phosphatase b) APC --> APC binds to Cdc20 --> active APC/C cdc20 c) Cdh1 --> inhibition Exit: APC/C cdc20 ubiquinates Cdk1/Cyclin B --> proteasome degradation --> decreasing levels of Cdk1/Cyclin B -->>> a) decreased level of APC/C Cdc20 --> activation of Cdh1 --> APC/C swaps binding partner for Cdh1 --> APC/C Cdh1 --> further ubiquitinates Cdk1/Cyclin B b) inactivation of Greatwall [Gwl] --> Decreased phosphorylation of 2 regulator proteins --> Disinhibition of PP2A phosphatase --> dephosphorylates proteins that were activated by Cdk1/Cyclin B

Nuclear Envelope reassembly

In the absence of active MPF, myosin light chain is dephosphorylated and the actin/myosin contractile ring can initiate cytokinesis BAF - small dimeric protein that binds to DNA and crosslinks chromosomes together --> glues them together and engulfs all the chromosomes --> Allows nuclear envelope to reassemble around all the chromosomes

Centrosome duplication

Interphase 1) Centrosome duplication relies on G1/S-phase CDK and also on a member of the Polo kinase family - Plk4 2) Mitotic CDK/cyclin triggers the disjunction of the duplicated centrosomes

Cdc25 phosphatase

Interphase - phosphorylation on ser216 promotes binding by 14-3-3 protein --> NLS blocked, NES exposed → Cdc is localized to the cytoplasm Mitosis - phosphorylation sites change --> NES blocked, NLS exposed, 14-3-3 displaced → nuclear localization --> activity ↑ by phosphorylation and by Pin1 peptidyl prolyl isomerase --> Cdc2/cyclin B dephosphorylated and active

Anaphase

Phase of mitosis in which the chromosomes separate and move to opposite ends of the cell Two phases: 1) Anaphase A - Kinetochore microtubules depolymerize at the "+" end --> shortening pulls the chromosomes towards the poles 2) Anaphase B - Polar microtubules extend --> the microtubules slide until the overlap zone remains the same --> pushes the chromosomes further apart

G2-M transition

Polo kinase - Phosphorylates Cdc25 --> activating MPF [Mitosis promoting factor] - Cdk1/Cyclin B Activating phosphorylation - CAK --> activating Inactivating Phosphorylation - Wee1 kinase --> inactivating - Cdc25 phosphatase --> activating Positive feedback - MPF phosphorylates Cdc25 --> activation of Cdc25 (also inhibition of Wee1 kinase) --> further activating more MPFs

Spindle Assembly Checkpoint [SAC]

Prevents APC/C activation and anaphase transition Mechanism: - Unattached kinetochores activate the Mitotic Checkpoint Complex [MCC], which keeps APC/C-Cdc20 inactive, ensuring that the metaphase -> anaphase transition doesn't occur prematurely

Chromosome capture by spindle fibers

Prometaphase 3 types of microtubules 1) Kinetochore MT --> "+" ends of two kinetochores are attached to each centromere 2) Polar MT --> overlap 3) Astral MT --> Don't worry about these guys

Nuclear Envelope breakdown [NEBD]

Prometaphase Initiated by Cdk1-Cyclin B The proteins of the nuclear envelope are stored in the ER during nuclear envelope break down Mechanism (Phosphorylation) - Nucleoporins --> causes nuclear pore complexes to dissociate - Lamins (via MPF) --> Disrupt nuclear lamina and leads to the vesiculation of the nuclear envelope - Integral membrane proteins of inner nuclear membrane --> disassembly of the nuclear envelope

Centering of metaphase chromosome

Prometaphase --> Metaphase Mechanism (Trial and error) - Microtubules will continue to try and reach out until stable conformation is achieved 1) Amphitelic attachment (Stable) - Equal and opposite tension --> Aurora B is unable to phosphorylate Ndc80 complex --> microtubules are stable 2) Monotelic attachment (Unstable) - Aurora B is phosphorylates Ndc80 complex --> microtubules are unstable

Cohesins

Protein complex 1) 2 Coiled coil proteins [Smc] 2) ATPase Subunits come together during S phase --> Hold sister chromatids together In yeast --> remain on chromosome until the onset of anaphase In mammals --> cohesins on chromosome arms are cleared at the onset of mitosis while centromeric cohesins are protected from phosphorylations (via protein shugoshion) and remain until anaphase

Anaphase transition

Requires activation of the APC/C [Anaphase promoting complex/cyclosome] Activation of APC/C(cdc20) requires two things: 1) MPF phosphorylation of APC 2) Binding to Cdc20 Function: - Activated APC/C ubiquidinates securin --> degradation of securin --> disinhibition of separase --> activated separase cleaves cohesins --> Anaphase progression Inhibition - Activated APC/C is inhibited by Mad2 --> spindle assembly checkpoint [SAC]