BIO130H1 Lecture Notes - Lecture 26: Spindle Apparatus, Cell Division, Kinetochore

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4 Apr 2019
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BIO130 Lecture 26 03/04/2019
Metaphase
All chromosomes are aligned
o On the metaphase plate
Microtubules from opposite poles:
o Are attached to kinetochores of sister chromatics
Kinetochores are located
o At the centromere
Chromosome alignment on the metaphase plate
1. Kinesin dyneins move the DNA out of the way as mitotic spindle is being formed
2. DNA needs to be attached on an end-way fashion instead of a sideways fashion
3. One microtubule attached to the kinetochore from one and microtubule from the
other end
a. There is first lose binding between microtubule and kinetochore
4. Microtubule pulls a bit on the kinetochore and if there is equal tension from both
sides then the kinetochore will bind tightly
NOTE: 3a) mistakes happen where microtubules from the same centrosome bind to the
DNA
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They pull in the same direction causing the kinetochore to not bind tightly to the
microtubule
This leads to the disassembly of the microtubule (depolymerization)
Chromosomes attachment to microtubules
Exposed plus end of the microtubule allows for polymerization / depolymerization
The kinetochore binds near the plus end of the microtubule but not directly ON
the plus end!
Tubulin flux through microtubules
Centrosome nucleation of microtubules
If red radioactive tubulin dimers are added, the polar microtubules and
kinetochore microtubules become red
This is because there is microtubule treadmill
In the centrosome there are special depolymerases that remove heterodimers
one at the time
o Microtubule treadmilling
There depolymerases are normally not on the centrosomes
They are found only during metaphase
The metaphase-anaphase transition
Also called the spindle Assembly Checkpoint (SAC)
Cell division in a kidney epithelial cell
Notice that anaphase does not start until all the chromosomes are aligned on the
metaphase plate
o This is an important checkpoint
Anaphase
Sister chromatids separate to form the two daughter chromosomes
o Cohesin is cleaved
They are pulled towards the opposite poles
Kinetochore microtubules:
o Shorten
When anaphase is triggered a protease called separase is activated which
cleaves the cohesin complex and allows sister chromatids to separate
Sister chromatic separation
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