BIOL 212 Lecture Notes - Lecture 31: Spindle Apparatus, Actomyosin Ring, Sister Chromatids

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Lecture 31 Notes
Metaphase
- Chromosomes aligned at equator of spindle, midway between spindle poles
- Kinetochore microtubules on each sister chromatid attach to opposite poles of the spindle
- Equator of spindle is called metaphase plate
- Duplicated chromosome is attached to both spindle poles via 2 kinetochores
o Kinetochores are specialized protein complexes that assemble at the centromere
regions on the chromosomes
Mitotic Spindle
- Contains 3 kinds microtubules
- Aster microtubules (near poles), kinetochore microtubules (attached to chromosomes
kinetochores), interpolar microtubules
Anaphase
- Sister chromatids synchronously separate and are pulled slowly toward the spindle pole
to which they are attached
- Kinetochore microtubules get shorter, spindle poles also move apart, both contributing to
chromosome segregation
- Cohesins destroyed during anaphase due to anaphase-promoting complex (APC)
- Inactive proteolytic enzyme separase activated by active APC cleaved and dissociated
cohesions
- 2 phases, anaphase A and B which occur simultaneously
o Anaphase A: chromosomes are pulled poleward thru a shortening of kinetochore
microtubules forces are generated at kinetochores to move chromosomes toward
their spindle pole
Depolymerization of microtubules is main driving force for chromosome
movement in anaphase A
Is active process that requires special motor protein and ATP hydrolysis
o Anaphase B: poles are pushed and pulled apart
Sliding force is generated b/t interpolar microtubules from opposite poles
to push the poles apart
Pulling force acts to pull poles toward the cell cortex, thereby moving the
two poles apart
Microtubule-associated motors create the pushing/puling forces of
anaphase B
APC
- Ubiquitinates M-phase cyclin (M-cyclin) which leads to M-cyclin degradation thereby
inactivating M-Cdk
- Ubiquitinating activity results in the degradation of securing and activation of separase,
which cleaves cohesion
o Cleavage leads to separation of sister chromatids during anaphase
- Inhibited if chromosomes are not properly attached to mitotic spindle
Telophase
- Two sets of chromosomes arrive at the poles
- New nuclear envelope reassembles around each set, completing formation of two nuclei
and marking the end of mitosis
- Division of cytoplasm begins w/assembly of contractile ring
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Document Summary

Chromosomes aligned at equator of spindle, midway between spindle poles. Kinetochore microtubules on each sister chromatid attach to opposite poles of the spindle. Equator of spindle is called metaphase plate. Duplicated chromosome is attached to both spindle poles via 2 kinetochores: kinetochores are specialized protein complexes that assemble at the centromere regions on the chromosomes. Aster microtubules (near poles), kinetochore microtubules (attached to chromosomes kinetochores), interpolar microtubules. Sister chromatids synchronously separate and are pulled slowly toward the spindle pole to which they are attached. Kinetochore microtubules get shorter, spindle poles also move apart, both contributing to chromosome segregation. Cohesins destroyed during anaphase due to anaphase-promoting complex (apc) Inactive proteolytic enzyme separase activated by active apc cleaved and dissociated cohesions. Ubiquitinates m-phase cyclin (m-cyclin) which leads to m-cyclin degradation thereby inactivating m-cdk. Ubiquitinating activity results in the degradation of securing and activation of separase, which cleaves cohesion: cleavage leads to separation of sister chromatids during anaphase.

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