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Cell Biology > Cell Cycle > Flashcards

Cell Cycle Flashcards

1
Q

Cell Cycle Stages

A

G1, S, G2, M

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2
Q

G-phase

A
  • ‘gap’ or ‘growth’ phases

- cell regulates conditions to decide whether or not to divide

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3
Q

S phase

A
  • DNA replication
  • many origins of replication
  • sister chromatids are held together by cohesion complexes
  • requires replication of both DNA and chromatin proteins associated with it
  • S CDK activation leads to DNA unwinding protein activation
  • this initiates replication at replication origins and inhibits proteins needed to allow that origin to initiate DNA replication again
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4
Q

Prophase

A
  • chromosome condensation
  • spindle forms, coordinated by centrosomes
  • DNA wound around histones in a bead on a string formation
  • histones wrap around each other and attach to a scaffold protein
  • these loops associate with each other
  • centrosomes are made of 2 centrioles (9x microtubule triplets)
  • microtubules nucleate from centrosomes
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5
Q

Spindle Formation/Structure

A
  • bipolar microtubule array with minus ends of the poles and plus ends radiating out
  • microtubule motor proteins help the spindle function
  • in animal cells, centrosomes provide prefabricated spindle poles to facilitate spindle assembly
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6
Q

Prometaphase

A
  • nuclear envelope breakdown
  • gives more space in cell with which to divide
  • M CDK phosphorylation of nuclear pore complexes in nuclear envelope initiates disassembly of npc / dissociation from envelope
  • this phosphorylation of nuclear lamina = vesicular disassembly of nuclear membranes
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7
Q

Kinetochores

A
  • protein complex bridging between the nucleic acid and microtubules
  • also associate with motor proteins that use ATP to move chromosomes
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8
Q

Chromosome Separation

A
  • chromatids are held together by a cohesin complex
  • APC (anaphase promoting complex) activates separase (protease cleaving cohesins)
  • APC destroys securin (separase inhibitor)
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9
Q

Chromosome Migration

A
  • uses the dynamic instability of microtubules (grow with GTPase and shrink when it is removed)
  • kinetochore microtubules shorten so chromosomes move to poles (ie. force is generated at the kinetochores)
  • overlapping microtubules generate the sliding force from opposite poles to push poles apart
  • astral microtubules pull centrosomes on poles to move them apart
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10
Q

Telophase

A
  • daughter chromosomes arrive at poles/decondense

- reformation of the nuclear envelope

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11
Q

Cytokinesis

A
  • uses myosin and actin to form a contractile ring at equator
  • actin ring contracts to separate two cells
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12
Q

Cytokinesis

A
  • uses myosin and actin to form a contractile ring at equator
  • actin ring contracts to separate two cells
  • cleavage furrow narrows to form the midbody, which persists as a tether between the cells before complete separation
  • mitotic spindle ensures the correct timing and place for cytokinesis & releases signals initiating furrow midbody
  • after cytokinesis the cells enter stable G1 state of low CDK activity (regulatory mechanisms prevent CDK activity)
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13
Q

Hartwell & Nurse

A
  • worked with budding yeast cells and fission yeast and mutated genes important in cell cycle regulation
  • used complementation = method to identify a mutation in the genome / where it is located
  • ## found that CDC2 was a key gene regulator and that it makes protein kinases
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14
Q

Hartwell & Nurse

A
  • worked with budding yeast cells and fission yeast and mutated genes important in cell cycle regulation
  • used complementation = method to identify a mutation in the genome / where it is located
  • found that CDC2 was a key gene regulator and that it makes protein kinases
  • CDC2 activity also correlated with cell cycle stages (cyclic and periodical function)
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15
Q

Tim Hunt

A
  • examined sea urchin oocytes that are all stuck at the same cell phase + are large enough to visualise
  • injected them with radioactive methionine and saw how it was incorporated into the cell cycle activity
  • cyclin proteins appear just before mitosis
  • cyclin is the activating partner of CDC2 - ie. is the on switch for protein kinase activity
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16
Q

Restriction Point

A
  • ‘commitment’ point of the cell
  • retinoblastoma protein serves as the link between the G1 to S phases and is normally phosphorylasted in the G2/S/M transition
  • G1 cyclins bind and activate CDK proteins that hyperphosphorylate it
  • Rb interacting proteins like E2F are found in the G1 phase but not S/G2/M
  • E2F is a transcription factor that is released by a conformational change and that transactivates genes for S phase entry (transcription specific genes)
17
Q

Quiesence

A
  • reversible withdrawal from cell cycle

- referred to as G0

18
Q

S phase entry

A
  • extracellular signal molecule triggers G1 cyclin production
  • signal cascades trigger transcription factor to produce G1 cyclin
19
Q

Directionality

A
  • cyclins give cell cycle directionality
  • S cyclin binds to CDK that phosphorylates the G cyclins that activated them
  • phosphorylated G cyclin becomes unstable and undergoes proteolysis (ubiquitin binding also leads to this)
  • this pattern repeats in a negative feedback cycle
20
Q

Cyclin and CDK functions

A
  • G1 = pass through restriction point
  • S = DNA replication
  • G2 = verify DNA replication
  • M = activates APC & stimulates mitotic entry
21
Q

Summary of Cyclin Activity in the Cycle

A
  1. when cell proliferation conditions are right, internal and external signals activate G1 CDK
  2. gene expression of genes encoding G1/S cyclins
  3. G1/S CDK activation drives the cell past the restriction point
  4. G1/S CDK unleashes S CDK activity to initiate chromosome duplication
  5. M CDK activation triggers progression through the G2/M transition/alignment of chromatids at spindle equator
  6. APC/C and its activator (CDC 2) triggers securin and cyclin destruction causing mitosis completion
22
Q

CDK Activation

A
  • inactive form has the active site partly obscured by a protein loop
  • cyclin binding causes the loop to move away from the active site leading to partial CDK activation
  • CAK (cyclin activation kinase) phosphorylates an amino acid near the active site, ie. a conformational change leads to an increase in CDK activity
  • CKI (cyclin kinase inhibitors) are inhibitor proteins for the cyclin CDK complexes
23
Q

Metaphase Progression

A
  • triggered by protein destruction rather than by cyclin signals
  • ubiquitin ligase enzymes (APC) catalyse the ubiquitylation and destruction of securins
  • securins inhibit separases that cleave chromatid pairs together and the S/M cyclins so their proteins are dephosphorylated
24
Q

Microtubule Instability

A
  • during mitosis you have more microtubules with a shorter half life and are more dynamic and short
  • increased ability to be nucleated by centrosomes
  • result = dense and dynamic array of microtubules suited for sister chromatid capture
25
Q

Chromosomal Environment

A
  • create a local environment promoting spindle formation
  • chromatin bound GEF activates RAN GTP binding
  • RAN GTP releases microtubule stabilising proteins
  • kinetochores attach spindle and sister chromatids
    • end is attached to the ND8 complex which attaches the kinetochore
  • tension regulates incorrect kinetochore attachment, ie. low tension leads to loosened grip of tubules and high tension shuts off inhibitory signals
26
Q

Spindle Forces

A
  1. pulls kinetochore + chromatid along the microtubule to the spindle ( + end depolymerization)
  2. microtubule flux (microtubules are pulled to the poles and dismantled on - ends)
  3. polar ejection force
27
Q

Cell Division Control

A
  1. mitogens = trigger G1/S CDK activity and stimulates cell division
  2. growth factors = stimulates cell growth by promoting macromolecule synthesis
  3. survival factors = promotes cell survival by suppressing apoptosis
28
Q

Mitogens

A
  • stop CDK suppressing activity, ie. ‘releasing brakes’
  • mitogens - Myc transcription protein produced - G1 cyclin - G1 CDK activity
  • G1 CDK complexes activate E2F proteins that promote a wide range of cell proliferation activities
  • without mitogens, E2F dependent gene expression is inhibited by E2F - Rb interaction
  • mitogen stimulation causes G1 CDK to phosphorylated Rb, reducing their E2F binding
  • Positive feedback = complex and irreversible progression
29
Q

DNA Damage

A
  • cell control system can detect and arrest cell cycle entry at S phase or G2/M transition
  • p53 protein stimulates gene transcription of p21 gene that binds to CDKs and inhibits them
  • replicative cell senescence = permanent nondividing state
  • every time a cell divides the telomeres lose some parts and eventually leave DNA exposed leading to cell death

Cell Biology (16 decks)

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