Topic 34: Mitosis (Up to Metaphase)

How is DNA replication ensured to occur only once per cell cycle?

By licensing replication origins in G1 (pre-RCs loaded) and activating them in S phase via S-Cdk which also prevents re-licensing

What is a pre-replicative complex (pre-RC)?

A complex loaded at DNA origins in G1 that licenses DNA for replication including ORC Cdc6 Cdt1 and Mcm helicase

What happens to Mcm helicase during S phase?

It is activated by S-Cdk to initiate DNA replication

What proteins does S-Cdk inactivate to prevent re-replication?

ORC Cdc6 and Cdt1

What makes the G2 to M transition switch-like?

Positive feedback between M-Cdk Cdc25 and Wee1 creates rapid irreversible activation

What does CAK do?

Adds an activating phosphate to Cdk1 priming M-Cdk

What does Wee1 do?

Adds an inhibitory phosphate to Cdk1 keeping M-Cdk inactive in G2

What does Cdc25 do?

Removes inhibitory phosphate from Cdk1 activating M-Cdk

What is the positive feedback loop in M-Cdk activation?

Active M-Cdk activates Cdc25 and inhibits Wee1 amplifying its own activation

What is the role of PP2A before G2/M?

Keeps M-Cdk substrates dephosphorylated and prevents premature mitotic entry

What happens to PP2A after M-Cdk activation?

It is inhibited via the Greatwall kinase pathway allowing mitotic phosphorylation events

What is PP2A's overall role?

A brake that prevents early mitotic entry

What does condensin do?

Condenses chromosomes by forming DNA loops within a single chromatid

What does cohesin do?

Holds sister chromatids together after DNA replication

Why are cohesin and condensin related complexes?

They are structurally related but perform different DNA organization tasks cohesion vs compaction

What does G1/S-Cdk do in spindle formation?

Triggers centrosome duplication

What does M-Cdk do in spindle formation?

Promotes centrosome separation and spindle assembly

What are kinetochore microtubules?

Microtubules that attach to kinetochores and move chromosomes

What are astral microtubules?

Microtubules that radiate to the cortex and help position the spindle

What are non-kinetochore microtubules?

Microtubules that overlap in the spindle midzone and stabilize spindle structure

What does dynein do in mitosis?

Pulls spindle poles toward the cell cortex

What does kinesin-5 do?

Pushes spindle poles apart by sliding antiparallel microtubules

What do chromokinesins kinesin-4 and kinesin-10 do?

Push chromosomes toward the metaphase plate

Can a spindle form without centrosomes?

Yes chromosomes can nucleate spindle formation

What enables spindle formation without centrosomes?

A Ran-GTP gradient around chromosomes promotes microtubule nucleation and stabilization

Where do microtubule plus ends attach in the spindle?

At the kinetochore

Where are microtubule minus ends located?

At the spindle poles

What is depolymerization-coupled pulling DCP?

Chromosome movement driven by microtubule depolymerization at the kinetochore while remaining attached

What is chromosome bi-orientation?

Each sister chromatid attaches to opposite spindle poles

What happens when attachment is incorrect?

It is destabilized by Aurora B kinase

What does Aurora B kinase do?

Phosphorylates Ndc80 to weaken microtubule attachment when tension is low

What does Ndc80 do?

Binds microtubules at the kinetochore

How does tension stabilize attachment?

High tension separates Ndc80 from Aurora B preventing phosphorylation and stabilizing attachment

What is mechanotransduction in the spindle?

Conversion of mechanical tension into biochemical stabilization of kinetochore attachment

What is the spindle assembly checkpoint?

A system that prevents anaphase until all chromosomes are properly attached and under tension

What is the role of Mad2?

Binds unattached kinetochores and inhibits Cdc20

What happens when Cdc20 is inhibited?

APC/C is inactive blocking anaphase onset

What does APC/C do?

Targets securin and M-cyclin for degradation

What happens when securin is degraded?

Separase is activated and cleaves cohesin allowing sister chromatid separation

What happens when M-cyclin is degraded?

M-Cdk is inactivated and the cell exits mitosis

What controls entry into mitosis?

Positive feedback activation of M-Cdk

What controls chromosome alignment?

Aurora B kinase and mechanical tension

What controls anaphase onset?

The spindle assembly checkpoint and APC/C activation