11.3 M phase

M-Cdk

• self-reinforcing cyclin-Cdk complex

• explosive activity increase = abrupt transition from G2 to M phase

Prophase

phase in which phosphorylations by M-Cdk often occurs

Phosphorylation

process done by M-Cdk to certain substrates involved in prophase processes

Condensins

• its phosphorylation triggers assembly of its complexes onto DNA

• promote condensation of chromosomes during prophase

Microtubule-associated proteins (MAPs)

• its phosphorylation facilitates the formation of spindle fibers

• for when MTs of centrosomes interact with one another

Lamins

its phosphorylation results in nuclear membrane disassembly

Prometaphase

• phase that starts abruptly with breakdown of the nuclear envelope

• when MTs attach to chromosomes

Kinetochores

• protein complexes that assemble on the centromeres of duplicated chromosome

• recognizes specific DNA sequence at the centromere

Metaphase plate

• Formed when chromosomes line up at spindle equator halfway between the spindle poles

• Event defines beginning of metaphase

Colchicine

• drug that blocks tubulin addition

• tubulin loss continues while metaphase spindle disappears

Sister chromatids

part of DNA that’s suspended under tension as they oscillate back and forth even after they are aligned

Securin

inhibitory protein bound to separase before anaphase

Separase

proteolytic enzyme that destroys cohesin

Anaphase-promoting complex (APC)

• complex involved in ubiquitylation of certain proteins in anaphase

• phosphorylation activated by M-Cdk

Ubiquitylation

process that causes proteins to be targets for proteasomes destruction

6 steps of sister chromatids separation

1. Cohesins hold sister chromatids together

2. Securin bind and inhibits separate from destroying cohesins

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3. Anaphase-promoting complex (APC) is activated by M-Cdk

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4. Ubiquitylation of securin catalyzed by APC

5. Securin destructed by proteasomes

6. Active separase cleave cohesin complexes to let sister chromatids be pulled apart

Anaphase A

• anaphase stage when kinetochore MTs shorten

• attached chromosomes are pulled poleward

Anaphase B

• anaphase stage when spindle poles move apart

• involves kinesin and dynein

Kinesin

• motor protein in anaphase B

• pushing spindle poles apart

Dynein

• motor protein in anaphase B

• pulls the actual poles apart

Spindle assembly checkpoint

• checkpoint that controls onset of anaphase and exit from mitosis

• eg. chromosome missed by the spindle sends “stop” signal to

4 steps of M-Cdk inactivation

1. Phosphorylated active cyclin-Cdx present

2. cyclin-Cdx ubiquitylated by APC

3. M cyclin degraded

4. Inactivated M-Cdk complex exit mitosis

Telophase

• phase when:

  • chromosomes decondense

  • mitotic spindle disassembles

  • Nuclear membrane re-forms

• nuclear pore proteins & nuclear lamins are dephosphorylated