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Cell Bio Hack > L2-3- cell cycle > Flashcards

L2-3- cell cycle Flashcards

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1
Q

what are the main 5 stages of the eukaryotic cell cycle?

A 
Interphase:
G1 phase
S phase
G2 phase
Mitosis:
M phase
cytokinesis
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2
Q

how long does a complete cell cycle last in actively growing mammalian cells?

A

24 hours

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3
Q

why is yeast a good model for cell cycle control?

A

small genomes, easy to make mutants, rapid growth - cell cycle can be as short as 2 hours

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4
Q

what genes do mutants have mutations in?

A

CDC genes - (cell division cycle)

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5
Q

what are the main ‘checkpoints’ in the cell cycle that control progression onto the next stage of the cycle?

A
  1. commitment to DNA synthesis - in G1 (restriction point in mammals)
  2. commitment to enter Mitosis - in G2
  3. commitment to complete mitosis - in M
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6
Q

how is the cell cycle controlled?

A

by protein phosphorylation and dephosphorylation

proteins that catalyse reactions in processes of the cell cycle regulated by

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7
Q

what are Cdks?

A

cyclin-dependent protein kinases

these are the principle enzymes that control the cell cycle, only active in presence of specific proteins called CYCLINS

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8
Q

What trigger each stage of the cell cycle?

A

specific cyclin-Cdk complexes

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9
Q

how many Cdks do vertebrates have compared to yeast?

A

4 Cdks, several cyclins, whereas yeast only has 1 Cdk

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10
Q

what are the 4 classes of cyclin?

A 
In most cells: 
G1-cyclins (in late G1 phase) - control initiation of processes
IN ALL CELLS: 
G1/S-cyclins - initiate S phase
S-cyclins - initiate DNA synthesis
M-cyclins - initiate mitosis
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11
Q

what is the role of cyclin?

A

cyclin directs Cdk to phosphorylate specific target proteins

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12
Q

what are 2 main specific target proteins of cyclin-Cdk complexes?

A

Lamin - protein of nuclear lamella

Condensin complex proteins - required for chromosome condensation

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13
Q

what controls cyclin concentrations?

A

the concentrations are determined by the balance of:

  1. cyclin synthesis - cyclins accumulate during cell cycle
  2. cyclin degradation - by proteolysis
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14
Q

what are proteasomes?

A

Large complex involved in breakdown of proteins which is ATP dependent with two main parts:
- hollow cylinder in which proteolysis takes place
- cap that binds and unfolds proteins to be destroyed (associated with ATP hydrolysis)
they degrade proteins with small protein UBIQUITIN attached

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15
Q

what is the protein ubiquitin?

A

attaches to target protein (e.g. cyclins) with degradation signal - added via ubiquitin ligases which add the chains of ubiquitin polypeptides to target protein marking them for destruction

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16
Q

which carboxy-terminal of ubiquitin is attached to the amino group of a lysine side chain on the target protein?

A

GLYCINE - forms peptide bond

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17
Q

how do ubiquitin ligases identify target proteins?

A

recognise degradation signals on the target proteins, these signals can be:

  • non-specific - proteins that have abnormal structures
  • specific - proteins with short lives
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18
Q

are ubiquitin ligases specific or not?

A

yes - cells contain multiple ubiquitin ligases, different ones recognise different degradation signals

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19
Q

Cdk activity is regulated by…

A
  1. concentrations of cyclins
  2. phosphorylation and dephosphorylation of Cdk itself
  3. Cdk inhibitor proteins (CKIs) - bind to cyclin-Cdk complexes and inhibit activity
20
Q

which cyclin-Cdk complexes are involved at these main control points of the cell cycle:

  1. G1 to S transition?
  2. entry into mitosis?
  3. transition to anaphase?
A
  1. G1 to S transition - involves G1-Cdk, G1/S-Cdk and S-Cdk
  2. entry into mitosis - involves M-Cdk
  3. transition to anaphase - involves M-Cdk
21
Q

The G1 to S transition

A
  1. start of G1 = no cyclins present
  2. G1-cyclin accumulates in G1 and G1-Cdk activates G1-S-cyclin gene transcription
  3. G1/S-Cdk increases S-Cdk activity by: activating transcription of S-cyclin genes, inhibiting an S-Cdk ubiquitin ligase (more S-Cdk), and inhibits an S-Cdk inhibitor protein (activating S-Cdk)
  4. S-Cdk promotes DNA synthesis
22
Q

what triggers the G1 to S transition?

A

external signals mechanism, in animals: MITOGENS stimulate cell division, GROWTH FACTORS stimulate increase in mass. One protein can have both activities, e.g. Platelet-derived growth factor (PDGF) - stimulates many cells to divide
Erythropoietin - induces proliferation of red blood cell precursors

23
Q

what are Mitogens?

A

External signals that stimulate cell division, they bind to cell surface and indirectly increase cyclin activity
e.g. stimulate transcription of G1-cyclin genes
cause degradation of G1/S-Cdk inhibitor proteins

24
Q

what is the Retinoblastoma protein (RB1)?

A

AN INHIBITOR in normal cells: prevents premature cell division by blocking transition to S phase - it inhibits transcription of genes for G1/S-cyclin and S-cyclin by binding to transcription factor E2F

25
Q

what inactivates RB1 protein by phosphorylating it?

A

G1-Cdk (inhibits RB1 whch is an inhibitor)

26
Q

what is Retinoblastoma in children?

A

inherited eye cancer caused by loss of both copies of RB1 gene, RB1 protein functions in many other cell types.

27
Q

what happens after G1-Cdk inactivates RB1 protein?

A

genes coding for G1/S-cyclin and S-cylcin ARE transcribed so G1/S-Cdk and S-Cdk start to accumulate

28
Q

what happens once G1/S-Cdk and S-Cdk start to accumulate?

A

they inactivate RB1 protein too

EF2 also stimulates own transcription

29
Q

what is the key protein controlling response to DNA damage called?

A

p53 - accumulates when DNA is damaged
normally in very low levels
mutations in this occur in many forms of cancer

30
Q

what is the normal mechanism for p53 and what happens when DNA is damaged?

A

p53 is ubiquitinated and destroyed

DNA damage leads to PHOSPHORYLATION of p53 which prevents addition of ubiquitin

31
Q

What happens when p53 is phosphorylated?

A

activates transcription of gene for p21 - a Cdk inhibitor protein

32
Q

what is p21?

A

a Cdk inhibitor protein, inhibits G1/S-Cdk and S-Cdk allowing time for DNA repair

33
Q

summary of G1 to S transition

A

SYNTHESIS of G1-cyclin, G1/S-cyclin, S-cyclin increases
G1/S-Cdk prevents inhibition of S-Cdk and stabilises it
MITOGENS promote synthesis of G1-cyclin and prevent inhibition of G1/S-Cdk
RB1 PROTEIN prevents early increase in G1/S-cyclin, S-cyclin concentrations and their corresponding Cdk activities
DNA damage causes stabilisation of p53 causing p21 to accumulate - inhibiting G1/S-Cdk and S-Cdk

34
Q

how does entry into mitosis occur?

A

M-cyclin synthesis increases in G2
M-cyclin binds to Cdk = M-Cdk
M-Cdk phosphorylated by wee1 kinase inhibiting Cdk activity, then reactivated by Cdc25 phosphatase by dephosphorylating it

35
Q

why is Cdc25 phosphatase activated in late G2 phase?

A

it is phosphorylated by POLO-LIKE KINASE - activity of polo-like K increases during G2/M phases

36
Q

what is Polo-like kinase?

A

polo-like kinases are common in eukaryotes
activity increases during G2/M phases
phosphorylate Cdc25 phosphatase

37
Q

what are cyclins?

A

group of proteins that control progression through cell cycle by activating cyclin-dependent kinase enzymes

38
Q

what happens during inhibitory phosphorylation by Wee1 kinase?

A

M-Cdk phosphorylated by wee1 kinase inhibiting Cdk activity
late G2 phase: Cdc25 phosphatase is activated by phosphorylation - Cdc25 dephosphorylates M-Cdk activating it
M-Cdk then activates Cdc25 phosphatase and inhibits wee1 kinase = positive feedback

39
Q

what happens during phosphorylation by Cdk-activating kinase

A

full activation of cyclin-Cdk requires phosphorylation by Cdk-activating kinase at the active site

40
Q

what is the purpose of he DNA replication checkpoint?

A

incomplete replication of DNA damage results in Cdc25 phosphatase not dephosphorylating M-Cdk, therefore not progressing to mitosis

41
Q

what happens in anaphase?

A

sister chromatids separate

by separase protease cleaving subunit of cohesin

42
Q

what holds sister chromatids together prior to anaphase?

A

cohesin complex

43
Q

what are the steps in the action of separase?

A
  1. before anaphase: securin inbihits separase
  2. during mitosis: M-Cdk activates APC by promoting binding of Cdc20
  3. ABC ubiquitinates securin - degraded
44
Q

what is APC?

A

Anaphase promoting complex - can be called cyclosome

a ubiquitin ligase

45
Q

what is the exit mechanism from MITOSIS?

A
  1. APC adds ubiquitin to M-cyclin
  2. M-cyclin is destroyed
  3. APC inactivated in absence of M-cyclin

Cell Bio Hack (5 decks)

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