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BS31006 > Chromosome Segregation > Flashcards

Chromosome Segregation Flashcards

1
Q

Stages in Mitosis:

A 
Prophase
Prometaphase
Metaphase
Anaphase
Telophase
Cytokinesis
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2
Q

Prophase

A

Chromosomes start condensation.

Centrosomes separate and form asters.

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3
Q

Prometaphase

A

Nuclear envelope is broken down, microtubules interact

with chromosomes.

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4
Q

Metaphase

A

Chromosomes bi-orient on the spindle and align on the metaphase plate.

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5
Q

Anaphase

A

Sister chromatids separate and move towards the spindle poles, spindle poles move apart.

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6
Q

Telophase

A

Chromosomes arrive at poles and decondense, nuclear envelope reassembles.

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7
Q

Cytokinesis

A

Contractile ring generates cleavage furrow and divides the cytoplasm, creating two daughter cells.

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8
Q

What factor promotes mitosis?

A

M-Phase Cyclin/Cdk (Cyclin B and Cdk1)

They promote various mitotic events such as spindle assembly, kinetochore assembly, chromosome condensation etc.

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9
Q

How are M-Phase Cyclin/Cdks regulated?

A
  1. 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).
  2. 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.
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10
Q

How do chromosomes become prepare for mitosis?

A
  1. 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.
  2. 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)
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11
Q

Centrosome

A

Organelles which serve as the main microtubule organizing centres (MTOCs).

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12
Q

Centromere

A

Area of a chromosome that links a pair of sister chromatids. Spindle fibers attach to the centromere via the kinetochore.

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13
Q

Centriole

A

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.

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14
Q

3 Classes of Spindle Microtubules

A

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.

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15
Q

Structure of Microtubules

A

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.

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16
Q

What Regulates the Dynamic Instability Of Microtubules? and how is this utilised in Mitosis?

A
  1. Microtubule-associated proteins (MAPs): Cause stabilisation of the Microtubule = Longer Microtubules.
  2. 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.

17
Q

Kinetochore

A

A complex of proteins associated with the centromere of a chromosome during cell division, to which the microtubules of the spindle attach.

18
Q

Microtubule-Associated Motors

A

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.

19
Q

Microtubules are organized at spindle poles

by BLANK

A

Microtubule-Organizing Centres (MTOCs)

20
Q

MTOC

A

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)

21
Q

Structure of the gamma-tubulin complex:

A

γ-tubulin complexes are arranged in a spiral, providing a template for microtubule nucleation.

22
Q

Centrosome duplication and segregation cycle

A

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.

23
Q

How do microtubules attach to chromosomes?

A

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.

24
Q

Structure of centromeres in eukaryotic cells:

A

Centromeres are chromosome regions supporting kinetochore assembly:
In vertebrate cells, centromeres are embedded within heterochromatin.
In yeast, centromeres are defined by a consensus sequence.

25
Q

Process of kinetochore-microtubule interaction

A
  1. Lateral Attachment: Kinetochore capture by the lateral (i.e. side, not the plus-end) surface of a microtubule
  2. Sliding, End-on Pulling, End-on Attachment: Microtubule-dependent kinetochore transport towards a spindle pole.
  3. Re-orientation/Error Correction: Both sister kinetochores interact with microtubules from the same or opposite spindle poles.
  4. Bi-orientation: Sister kinetochore bi-orientation, equal tension applied to both sides.

Steps 1-3 = Prometaphase
Step 4 = Metaphase

26
Q

Chromosome bi-orientation is [required/unnecessary] for proper chromosome segregation.

A

Chromosome bi-orientation is REQUIRED for proper chromosome segregation

27
Q

Which enzyme has important roles in correcting errors related to chromosome bi-oreintation?

A

Aurora-B Kinase

28
Q

How are chromosomes aligned on a metaphase plate?

A

2 Forces in action:

  • Pulling of kinetochore microtubules.
  • Pushing of interpolar and/or astral microtubules.

These two forces cooperate to align chromosomes
on a metaphase plate (chromosome congression).

29
Q

How do sister chromatids separate from each other

upon the onset of anaphase?

A
  • Securin secures separase in an inactive state.
  • Cdc20 activates APC/C, which causes the degradation of securin
  • This leads to active separase cleaving cohesin
  • Cohesin now no longer holds the sister chromatids together

REMEMBER: Depolymerisation of microtubules is responsible for the separation of chromatids, NOT motor proteins.

30
Q

Chromosome motion in anaphase:

A

Anaphase A: Shortening of kinetochore-microtubules due to microtubule flux and depolymerization at the plus end.
Anaphase B: Centrosomes move away from each other due to sliding between interpolar microtubules and
pulling of poles towards cell cortex.

31
Q

Spindle Assembly Checkpoint

A

Monitors kinetochore-microtubule attachment.
When kinetochores fail to attach microtubules, spindle checkpoint remains unsatisfied and stops anaphase onset.
Prevention of anaphase is crucial because it is a irreversible process.
Spindle assembly checkpoint consists of sensors, mediators and effectors.
e.g. Sensor: Kinetochore components
Mediator: Mad proteins (inhibit Cdc20)
Effector: Anaphase-promoting complex (APC)

32
Q

Overview of Cytokinesis:

A
  1. Cleavage Plane Specification, Loss of M-CDK activity then triggers cytokinesis.
  2. Furrow Assembly and Ingression, Contractile ring is formed by actin and myosin II. Central spindle (formed from interpolar microtubules in-between segregating chromosomes during anaphase ) promotes formation of the contractile ring.
  3. Midbody Formation, due to contraction of the contractile ring.
  4. Cell Separation (Abscission), 2 new daughter cells

Two mechanisms (loss of M-CDK and central spindle) ensure that cytokinesis occurs only after chromosome segregation.

33
Q

What specifies the cleavage/division plane?

A

The central spindle, dynamic and stable astral microtubules.

Ensures that the division plane is perpendicular to the spindle axis and is positioned in the middle of the cell.

34
Q

Contractile ring formation and stimulation:

A

RhoA is a small GTPase. The RhoA cycle is activated by

central spindle-associated factors and stimulates actomyosin ring formation and contraction.

35
Q

Very simple meiosis description, in terms of replication and segregation.

A

DNA replication, then 2 rounds of segregation.
1: Segregation of paired homologs (Anaphase I)
2 Segregation of sister chromatids (Anaphase II)

Note: Pairing of duplicated homologous chromosomes and meiotic recombination occurs at Prophase I.

BS31006 (7 decks)

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