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Mechanics Of Cell Division Flashcards

1
Q

What is the main purpose of cell division?

A

To allow growth, repair, and reproduction of cells

Cell division is essential for the maintenance of life in multicellular organisms.

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2
Q

What are the phases of the eukaryotic cell cycle?

A

G1, S, G2, M

G1 is the gap phase before DNA synthesis, S is the phase of DNA replication, G2 is the gap phase before mitosis, and M is mitosis.

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3
Q

What triggers cell cycle entry?

A

Mitogens

Mitogens are substances that stimulate cell division.

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4
Q

What are oncogenes?

A

Mutated genes that promote cancer

Oncogenes can lead to uncontrolled cell proliferation.

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5
Q

What role do tumour suppressor genes play in cancer?

A

They inhibit cell division and prevent tumor formation

Loss of function in these genes can contribute to cancer development.

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6
Q

What is the role of M-Cdk1/cyclinB in mitosis?

A

Controls and coordinates mitosis through phosphorylation

M-Cdk1/cyclinB is crucial for the progression of the cell cycle.

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7
Q

What happens during the prophase of mitosis?

A
  • Chromosomes condense as condensins compact the DNA
  • Condensins are activated by M-Cdk phosphorylation
  • Cohesin rings hold sister chromatids together until anaphase
  • Bipolar mitotic spindle starts to form —> MTs growing from the 2 centrosomes meet and start to interact via antiparallel interactions = this overlap zone drives pole separation
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8
Q

How is cytokinesis different in plant and animal cells?

A

Animal cells form a cleavage furrow (a contractile ring of actin & myosin filaments), while plant cells form a cell plate.

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9
Q

What is the function of cohesin during the cell cycle?

A

Holds sister chromatids together until anaphase

Cohesin is crucial for the accurate segregation of chromosomes.

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10
Q

What is the significance of the spindle assembly checkpoint?

A

Ensures proper attachment of chromosomes to the spindle before anaphase

This checkpoint prevents errors in chromosome segregation.

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11
Q

Fill in the blank: The _______ is a specialized protein structure that assembles on the centromere region of the chromosome in prophase.

A

kinetochore

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12
Q

What are the three types of microtubules involved in the mitotic spindle?

A
  • Kinetochore microtubules —> MTs have to find and attach to these, they must be able to undergo coordinated assembly and disassembly
  • Interpolar microtubules —> MTs growing from one pole must meet those from the other pole and form antiparallel interactions
  • Astral microtubules —> are highly dynamic and play a crucial role in anaphase
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13
Q

True or False: The nuclear envelope disassembles during prophase.

A

False

The nuclear envelope disassembles during prometaphase.

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14
Q

What is the role of Eg5 in prophase of mitosis?

A

Cross-links anti-parallel microtubules and pushes centrosomes apart to form the spindle poles

Also required for anaphase & prometaphase

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15
Q

What is a major driver of chromosome movement during mitosis?

A

Microtubule assembly and disassembly

This dynamic process is essential for the accurate segregation of chromosomes.

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16
Q

Describe the changes in membrane dynamics during M-phase.

A
  • Nuclear envelope disassembles
  • Golgi apparatus fragments
  • Membrane traffic stops

These changes facilitate the separation of chromosomes and the formation of daughter cells.

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17
Q

What is the consequence of phosphorylation by M-Cdk?

A

Phosphorylation activates
- Condensin —> chromosomes condense (prophase)
- Microtubule catastrophe proteins —> MTs more dynamic

Phosphorylation inactivates
- MAPs —> MTs more dynamic
- Nuclear lamins —> nuclear envelope disassembles

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18
Q

Fill in the blank: The _______ checkpoint monitors tension across chromosomes to control the exit from metaphase.

A

spindle assembly

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19
Q

What happens to microtubules during anaphase?

A

They undergo coordinated assembly and disassembly to separate sister chromatids

This dynamic behavior is critical for accurate chromosome segregation.

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20
Q

What does phosphorylation control?

A
  • cell growth
  • gene expression
  • cell cycle
  • cell survival
  • metabolism
  • cell division
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21
Q

Role of Cdks

A

Phosphorylate key proteins to control their function in the cell cycle

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22
Q

Structure of Cdks

A
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23
Q

Key things that must occur before mitosis

A
  • Interphase (G2) —> cells increase in size
  • S phase —> DNA of chromosomes is replicated and centrosome is duplicates, organelles must grow
  • Cohesin rings are added when DNA is replicated in S phase and holds sister chromatids together until anaphase
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24
Q

Centrosome duplication in S phase

A
  • Centrosomes are a focus for Microtubule polymerisation
  • These duplicate in S phase = triggered by the Cdk that controls DNA replication
  • Duplicated centrosomes need to separate = 2 spindle poles
  • Centrosome nucleates more Microtubules in mitosis
  • Each daughter cell gets a centrosome after cytokinesis
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25
Set up of the mitotic spindle
- Number of Microtubules nucleated by the centrosomes increases from prophase - Microtubules are more dynamic - They switch from growing to shrinking more often = catastrophe
26
Why do dynamics increase in the mitotic spindle?
- Some MAPs are inactivated when phosphorylated by the mitotic kinase M-Cdk - Proteins that trigger Microtubule catastrophe are activated in mitosis - This results in a greater chance of MTs growing from each centrosome contacting each other or chromosomes
27
Role of Eg5 in the bipolar spindle formation
- Antiparallel interactions are mediated by Eg5 = stabilises the MTs - Eg5 inhibition prevents centrosomes from separating in prophase and prometaphase - forms monopolar spindle
28
Describe prophase
1. Chromosomes condense because condensins compact the DNA (M-Cdk) 2. Duplicated centrosomes start to separate forming 2 spindle poles. Eg5 is needed here where the MTs overlap 3. Centrosomes nucleate more MTs by M-Cdk 4. The mitotic spindle starts to form, starting with the interpolar MTs
29
What happens in prometaphase?
- Nuclear envelope disassembles - Nuclear lamina disassembles in animal cells - Nuclear envelope disassembles in plants - Nuclear envelope and lamins reassemble in telophase
30
Role of nuclear lamins
Are M-Cdk substrates
31
What happens in the Golgi apparatus in prometaphase?
Fragments so that each daughter cell will inherit equal amounts of Golgi apparatus membranes Secretion and endocytosis stop
32
What happens in the transition from prophase to prometaphase?
- Nuclear envelope, nuclear lamina and Golgi apparatus disassemble - Secretion and endocytosis stop - Nuclear envelope disassembly allows MTs to interact with chromosomes - More MTs are nucleated by the centrosome from prophase onwards - MTs are more dynamic - Results in a greater chance of MTs growing from each centrosome contacting chromosomes quickly
33
Role of kinetochores
- Specialised protein structure that assembles onto the centromere region of the chromosome in prophase - Attaches chromosomes to MTs by a dynamic linker that holds onto the MT - Kinetochores can move along MTs in both directions using MT motors - Other proteins act as dynamic linkers between kinetochores and MT plus ends, even when the MT is growing and shrinking - MTs attached to kinetochores grow and shrink in a co-ordinated way
34
Microtubule dynamics at the kinetochores
- Multiple MTs at each kinetochore must grow or shrink together - MT assembly and disassembly is a major driver of chromosome movement - MT motors also play a role
35
Crucial properties of the kinetochore
- Specialised chromosomal structure needed for spindle attachment - Binds multiple MTs at once - MT bundles attached to the kinetochore can switch between growing and shrinking in a regulated way - Move in both directions along MTs by harnessing MT assembly & disassembly and using MT motors - Kinetochores properly attached to MTs from both poles are under tension - Tension is needed before mitosis can proceed
36
Direction of chromosome movement
Constantly changes until anaphase
37
What happens to chromosomes during metaphase?
Chromosomes align in the middle of the spindle and are attached to spindle via kinetochores with 1 kinetochore at each pole ## Footnote Kinetochores can move in either direction by harnessing microtubule dynamics and motors
38
What is the role of the metaphase checkpoint?
It blocks the transition from metaphase to anaphase if certain conditions are not met ## Footnote Conditions include depolymerised microtubules, improperly assembled spindle, or unattached kinetochores
39
What is the outcome if the metaphase checkpoint is functioning correctly?
Each cell waits for all chromosomes to align before transitioning to anaphase ## Footnote Duration of prometaphase can vary significantly among different cells
40
What is the master regulator of mitosis during the metaphase to anaphase transition?
M-Cdk is active during metaphase and turned off during anaphase ## Footnote This regulation is crucial for the separation of sister chromatids
41
What triggers the transition from metaphase to anaphase?
Activation of the anaphase promoting complex (APC/C) ## Footnote APC/C triggers proteolysis of cyclin B and securin, leading to activation of separase
42
What happens during Anaphase A?
Sister chromatids move towards the spindle poles by staying attached to depolymerising kinetochore MTs
43
What occurs during Anaphase B?
- Kinesin Eg5 required to set up the bipolar spindle - Eg5 cross links antiparallel MTs and pushes the centrosomes apart - Interpolar MTs continue growing = get longer by addition if tubulin dimers at MT plus ends - Dynein anchored at the cell cortex pulls on the Astral MTs
44
What are the specific events that occur during telophase?
Reassembly of nuclear envelope, reassembly of Golgi apparatus, restart of secretion and endocytosis ## Footnote The genome is already equally separated before telophase
45
What defines where the cleavage furrow assembles during cytokinesis in animal cells?
The central spindle recruits and activates proteins that signal to the cortex to start contractile ring assembly in anaphase/telophase Actin and myosin filaments are very dynamic and the contractile ring gets smaller over time
46
What is the main difference between cytokinesis in animal and plant cells?
Animal cells use actin and myosin, while plant cells use Golgi-derived vesicles and microtubules ## Footnote Plant cells do not have centrosomes or dynein
47
What is the role of dynein in mitosis?
Dynein anchors at the cell cortex and pulls on astral microtubules ## Footnote This contributes to the separation of spindle poles
48
What happens if a single kinetochore is not attached to the spindle?
The transition to anaphase is blocked, and a stop signal is generated ## Footnote This is part of the spindle assembly checkpoint mechanism
49
Fill in the blank: The anaphase promoting complex triggers proteolysis of __________ and __________.
cyclin B and securin
50
True or False: The metaphase to anaphase transition is loosely controlled.
False ## Footnote It is very tightly controlled by the spindle assembly checkpoint
51
What happens if problems at the metaphase checkpoint cannot be corrected?
The cell undergoes apoptosis ## Footnote This prevents the propagation of cells with incorrect chromosome numbers
52
What is the role of cohesins during mitosis?
Cohesins hold sister chromatids together until they are cleaved at anaphase ## Footnote This cleavage is crucial for allowing sister chromatids to separate
53
What blocks the transition from metaphase to anaphase?
- MTs are depolymerised (e.g. using drug nocodazole) - MTs are stabilised (e.g. using taxol) - Spindle hasn’t assembled properly (e.g. by inhibiting Eg5 with monastrol) - If a single kinetochore is not attached to the spindle
54
What happens when chromosomes are unaligned?
- Kinetochores unattached - STOP signal generated by the spindle assembly checkpoint complex (SAC) at the Kinetochore (KT) - Anaphase is delayed If chromosomes are aligned, KTs remain attached and SAC protein are removed from KTs by cytoplasmic dynein = no stop signal and all clear for anaphase
55
Consequences of entering anaphase too early
Anueploidy - wrong number of chromosomes
56
What happens in the transition from metaphase to anaphase?
- Master regulator of mitosis is active (M-Cdk) - Sister chromatids are held together by cohesins - Chromosomes are aligned so SAC is off - APC/C is active (anaphase promoting complex) - A protease seperase is activated - M-Cdk is turned off - Sister chromatids come apart as cohesins are cleaved
57
How is the metaphase to anaphase transition controlled?
- chromosomes are replicated in S phase to give 2 sister chromatids - after replication, sister chromatids stick together via cohesins - at anaphase, they separate as cohesins are cleaved by separase
58
Role of APC/C
Triggers proteolysis of specific proteins - Covalently attaches ubiquitin - Ubiquitin tagging directs proteins to the proteasome for degradation of cyclin Key substrates are Cyclin B subunit of M-Cdk and Securin (a separase inhibitor)
59
When SAC is active
- stops sister chromatids coming apart - keeps the cell in metaphase by maintaining M-Cdk activity
60
2 phases of anaphase
Anaphase A = sister chromatids move towards the spindle poles Anaphase B = spindle poles move further apart
61
How does animal cell morphology change through mitosis?
- Cells become rounded and less well attached to the surface whilst dividing - Actin & myosin filaments are drastically rearranged - Integrins are phosphorylated and weaken their grip on the extracellular matrix
62
Cell division in plant cells
- plant cells do not have centrosomes or dynein - their spindle poles are broad and organised by minus-end-directed Kinesins
63
Telophase & cytokinesis in plant cells
- Plants and algal cells divide by making a new cell membrane then a new cell wall - Needs Golgi-derived vesicles and MTs (not actin and myosin)

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