L20 Flashcards
what are chromatids
the individual replicated chromosomes held together by proteins that allow cohesion
What needs to happen during mitosis?
Chromosomes condense
Chromosomes attach to spindle microtubules
- Allow chromosomes to be moved around
Chromosomes align on the spindle
Sister chromatids separated
- Allow chromosomes to be moved into
correct daughter cells
Chromosomes decondense and cell divides into two
- To produce two diploid G1 cells
what are the stages of mitosis
prophase
prometaphase
metaphase
anaphase
telophase
cytokinesis
what drives Entry into mitosis
M-Cdk (Cyclin B-Cdk1)
how is M-Cdk activated
CAK (CDK-activating kinase) phosphorylates a specific threonine residue on M-Cdk
Wee1 adds inhibitory phosphatase keeping M-Cdk inactive
Cdc25 removes inhibitory phosphates added by Wee1. this step activates M-Cdk
active M-Cdk promotes activation of more Cdc25 creating a positive feedback loop
active M-Cdk inhibits Wee1 creating another positive feedback loop
what does M-Cdk do
- directly phosphorylates key substrate proteins
- regulates downstream mitotic kinases (eg Aurora and Polo kinases) which then phosphorylate additional substrates
what is the kinetochore
a large macromolecular complex that assembles on the centromere
(the microtubule binding site on a chromosome)
what factors are required to recruit kinetochore proteins in early mitosis
M-Cdk (Cyclin B-Cdk1) and Aurora B kinases
how are sister chromatids held together in prophase
cohesin proteins
in what stage are cohesins loaded and why
G1
to form loops
what releases cohesin first
mitotic kinases e.g. polo
leaving some in the center to form X shape
how is the rest of cohesin released
via Separase, which cleaves cohesin
how do Chromosomes condense in prophase
Condensins I and II co-operate to condense chromosomes in mitosis
what happens to cohesins and condesins in prophase
Cohesin removed, Condensins recruited
Interphase chromosome structure lost, mitotic structure gained
summarize what happens in prophase
Interphase chromosome structure is lost
Chromosomes condense
Kinetochore assembly begins
Microtubule dynamics change so that the spindle starts to form
what happens in prometaphase
Nuclear envelope breaks down
- allows access of microtubules to chromosomes
Spindle assembles
Microtubules attach to chromosomes
Microtubule adapter proteins and motor proteins become active
- allows chromosomes to be moved on the spindle
what is the mitotic spindle
A microtubule-based machine required to align and segregate chromosomes
consists of:
- interpolar microtubules
- astral microtubules
- kinetochore microtubules
- spindle poles
what are the features of microtubules
made of tubulin dimers
Nucleated at the minus end
Can grow and shrink at the plus end
(dynamic instability)
what proteins are present on microtubules
Microtubule adapter proteins (MAPs)
motors
what is the function of Microtubule adapter proteins (MAPs) give examples
Allow cell components to bind microtubules
Modulate the stability of microtubules
eg Ndc80/Nuf2 at kinetochores
what is the function of motors give examples
Allow cell components to move along microtubules
eg Kinesin-5 (walks to plus ends)
Dynein (walks to minus ends)
give examples of microtubule motors
Kinesin-5: forms dimers and cross-links microtubules (cargo is another kinesin-5 molecule)
CENP-E: binds to kinetochores (cargo is a kinetochore)
what does kinesin-5 do in prophase
cross-links anti-parallel microtubules and pushes them apart
what does CENP-E do in prometaphase
moves kinetochores towards the cell equator
what mechanisms allow for Bi-polarity of the spindle and bi-orientation of chromosomes
Making correct attachments (error correction)
Preventing cell cycle progression until suitable attachments are made (spindle checkpoint)
what chromosomes are stably attached to microtubules
bi oriented chromosomes
what detects bi-orientation
Aurora B kinase localizes to centromeres
By a mechanism that it is not fully understood, it detects tension
how does Aurora B remove microtubules from kinetochores
Aurora B phosphorylates Ndc80
what happens in metaphase
Chromosomes are all bi-oriented
As a consequence, they align on the “metaphase plate”
what do spindle checkpoints do
If chromosomes are incorrectly attached to the spindle, error correction produces unattached kinetochores
The spindle checkpoint detects unattached kinetochores
Unattached kinetochores produce the Mitotic Checkpoint Complex (MCC)
The MCC inhibits the APC/C and so prevents M-cyclin (Cyclin B) degradation
This keeps cells in mitosis
Once all kinetochores are occupied with microtubules, the MCC is no longer produced
M-cyclin (Cyclin B) is degraded, and the cells “biochemically” exit mitosis
Numerous mitotic processes are terminated
what proteins are in the MCC
Cdc20
Mad2
BubR1
Bub3
how does securin inhibition affect the cell cycle
MCC inhibits APC/C
securin is degraded by APC/C
securin inhibits seperase, which is a cohesin cleaver in the centromere during metaphase
When APC/C is inhibited, securin remains intact, keeping separase inactive.
This prevents cohesin cleavage and the premature separation of sister chromatids
what happens in Anaphase A
chromosomes move towards the spindle poles
what drives anaphase A
Driven largely by microtubule depolymerization at the plus ends of kinetochore microtubules (in human cells)
what happens in Anaphase B
spindle poles move apart
what drives anaphase B
Driven largely by microtubule motors eg kinesin-5
relative importance and timing of anaphase A and B varies between cell types and species
true
what happens in telophase
Need to re-establish interphase nuclear and chromosome structure
Nuclear envelope reforms, and nuclear pores inserted
Chromosomes decondense
Condensins dissociate
Cohesins re-associate and enable the
formation of chromosome looping structures
needed for correct gene expression
what happens in cytokinesis
A contractile ring of actin and myosin drives cleavage furrow formation and pinching of the dividing cell into two
what signals are involved in cytokinesis
From:
- central spindle
- spindle poles
- chromosomes
what is the contractile ring made of
actin and myosin
