Chromosome Segregation Flashcards
(35 cards)
Stages in Mitosis:
Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis
Prophase
Chromosomes start condensation.
Centrosomes separate and form asters.
Prometaphase
Nuclear envelope is broken down, microtubules interact
with chromosomes.
Metaphase
Chromosomes bi-orient on the spindle and align on the metaphase plate.
Anaphase
Sister chromatids separate and move towards the spindle poles, spindle poles move apart.
Telophase
Chromosomes arrive at poles and decondense, nuclear envelope reassembles.
Cytokinesis
Contractile ring generates cleavage furrow and divides the cytoplasm, creating two daughter cells.
What factor promotes mitosis?
M-Phase Cyclin/Cdk (Cyclin B and Cdk1)
They promote various mitotic events such as spindle assembly, kinetochore assembly, chromosome condensation etc.
How are M-Phase Cyclin/Cdks regulated?
- Regulated by phosphorylation of Cdk by CAK (Cdk-activating kinase) and Wee1 (Cdk-inhibitory kinase) and and dephosphorylation by Cdc25 (phosphatase).
CAK and Wee1 dual phosphorylate Cdk. Cdc25 is activated by phosphorylation by an active M-phase Cyclin/Cdk complex. Active Cdc25 removes the inhibitory phosphate from the inactive dual phosphorylated M-phase Cyclin/Cdk (This causes a positive feedback loop). - Regulated via proteolysis of M-Phase Cyclins. Cdc20 activates an inactive APC/C (Anaphase-promoting complex, an E3 ubiquitin ligase), causing ubiquitination and degradation of M-Cyclins by the proteasome.
How do chromosomes become prepare for mitosis?
- Sister chromatid cohesion is established by the cohesin complex during S phase. Cohesin is a ring like structure made of 4 proteins, with a hinge domain. Sister chromatid cohesion is important for chromosome bi-orientation and ‘tells’ which chromatids are sisters to segregate in mitosis.
- Chromosomes are then condensed in prophase. Condensin, which has a similar structure to cohesin, removes cohesins from chromosomes (except at the centromeres), making the chromatids become visible.
Condensin I: Active in Prometaphase, Lateral Compaction (squished along width, tall and thin)
Condensin II: Active in Prophase, Axial Shortening (Squished from top to bottom, short and chubby)
Centrosome
Organelles which serve as the main microtubule organizing centres (MTOCs).
Centromere
Area of a chromosome that links a pair of sister chromatids. Spindle fibers attach to the centromere via the kinetochore.
Centriole
A bound pair of centrioles, surrounded by a shapeless mass of dense material, called the pericentriolar material (PCM), makes up the centrosome.
Each centriole has nine-fold symmetry: triplets of short
microtubules are arranged around the cartwheel structure.
3 Classes of Spindle Microtubules
Astral Microtubules: Radiate in all directions from centrosomes and properly orient the spindle.
Kinetochore Microtubules: Attach to kinetochores and regulate the chromosome motion.
Interpolar Microtubules: Inter-digitate at the spindle equator and generate forces for separation of the poles.
Structure of Microtubules
Composed of alpha and beta tubulin dimers.
Form protofilaments, with a plus-end and a minus-end.
These protofilaments form long and hollow cylinders.
Both alpha and beta tubulin have GTP, however only the beta tubulin GTP is hydrolisable, because the alpha tubulin GTP is used to make the alpha-beta association constantly strong.
Microtubules are stabilized by a ‘GTP cap’ on the plus -end, a region of a polymerizing microtubule where GTP hydrolysis has not yet occurred
GTP hydrolysis on beta tubulin promotes microtubule depolymerisation.
Growing and shrinking state of microtubules can rapidly interchange (dynamic instability).
Shrinking -> Growing Microtubule = Rescue
Growing -> Shrinking Microtubule = Catastrophe
Protofilaments “peel” off during catastrophe.
What Regulates the Dynamic Instability Of Microtubules? and how is this utilised in Mitosis?
- Microtubule-associated proteins (MAPs): Cause stabilisation of the Microtubule = Longer Microtubules.
- Catastrophe Factor (e.g. kinesin-8, kinesin-13). Cause destabilisation of the Microtubule = Shorter Microtubules.
Polymerisation and depolymerisation of kinetochore microtubules regulate chromosome motion during metaphase and anaphase.
Kinetochore
A complex of proteins associated with the centromere of a chromosome during cell division, to which the microtubules of the spindle attach.
Microtubule-Associated Motors
Transport cargo along microtubules, and regulate sliding between microtubules.
2 types of motor proteins: Kinesin and Dynein
Both have ATPase activity.
Dynein shows minus end-directed motion
while the majority of kinesins show the plus end-directed motion (Kinesin-13 promotes catastrophe, kinesin-14 shows minus-end directed motion)
Motor proteins help to organise the mitotic spindle
and to adjust chromosome position.
Microtubules are organized at spindle poles
by BLANK
Microtubule-Organizing Centres (MTOCs)
MTOC
Called centrosome in animal cells.
Organises microtubules at spindle poles.
Two centrioles organize the centrosome matrix, forming a centrosome.
The gamma-tubulin complex gives nucleation sites of microtubules.
Microtubules extend from MTOCs with the distal plus end. (i.e. minus-end at MTOC, plus-end further away)
Structure of the gamma-tubulin complex:
γ-tubulin complexes are arranged in a spiral, providing a template for microtubule nucleation.
Centrosome duplication and segregation cycle
In S/G2 phase: Centrosomes are duplicated, stimulated by S-CDK. A mother centriole promotes elongation of a daughter centriole.
In early prophase: Aster formation and centrosome separation occur.
In late prophase-prometaphase: The mitotic spindle is formed between two centrosomes.
How do microtubules attach to chromosomes?
Kinetochores are the main site of microtubule attachment. Microtubules attach to kinetochores by their plus-end. Each chromatid has only one kinetochore (otherwise separation would occur incorrectly)
In humans, 20-30 microtubules attach to a single kinetochore during metaphase.
Structure of centromeres in eukaryotic cells:
Centromeres are chromosome regions supporting kinetochore assembly:
In vertebrate cells, centromeres are embedded within heterochromatin.
In yeast, centromeres are defined by a consensus sequence.
