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My Packs: (WIZEPREP) (BIO 2382) > Chapter 8: 8.5 Cell Cycle Checkpoints > Flashcards

Chapter 8: 8.5 Cell Cycle Checkpoints Flashcards

1
Q

Cell Cycle

A

Cycle of events by which a cell duplicates its chromosomes + other cellular components, and divides into 2 daughter cells

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2
Q

What must happen in order for a cell to proceed to the next phases in the cell cycle?

A

It must pass checkpoints
* Confirms that the cell is prepared to enter the next phase

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3
Q

What are the phases of the cell cycle and what happens during them?

A
  • M Phase: Mitosis + Cytokinesis
  • Interphase: Period between M phases, cell grows and replicates its DNA
  • G1 phase: 1st gap phase, Cell is growing
  • S phase: DNA is replicated
  • G2 phase: 2nd gap phase, Cell is growing
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4
Q

What are the checkpoints in the cell cycle?

A
  1. G1 to S
  2. G2 to M
  3. M phase
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5
Q

What does the G1 to S checkpoint check for?

A

Confirms that the environment is suitable for replication

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6
Q

G1 to S Checkpoint:

What is an ideal environment that is suitable for replication?

A
  • Sufficient nutrients
  • Extracellular signals
  • Other
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7
Q

G1 to S Checkpoint:

If the environment is unsuitable for replication, what happens?

A

Cell remains in G1 (also known G0)

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8
Q

Is G0 temporary or permanent?

A

Can be temporary or permanent

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9
Q

What is temporary G0 known as? What happens to cells in this condition?

A

Quiescence
* Cells retain the ability to re-enter the cell cycle

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10
Q

What is permanent G0 known as? When does this occur in cells?

A

Senescence
* Occurs in cells which are terminally differentiated (e.x. nerve cells)

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11
Q

G2 to M Checkpoint

A

Confirms that the DNA is fully replicated and undamaged

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12
Q

M phase Checkpoint

A

Known as the Spindle Checkpoint
* Confirms that duplicated chromosomes are properly attached to the mitotic spindle

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13
Q

The control of the cell cycle depends on how many proteins?

A

Mainly 2 types of proteins

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14
Q

What two types of proteins does the control of the cell cycle mainly depend on?

A
  1. Cyclins
  2. Cyclin-dependent kinases (Cdks)
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15
Q

Cyclins

A

Proteins that bind to and activate Cdks

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16
Q

Describe:

The concentration of cyclins

A

Rise and fall in a cyclical fashion

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17
Q

What causes increases and decreases of concentration of cyclins?

A
  • Increased transcription = Increase in concentration
  • Targeted proteolysis = Decrease in concentration
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18
Q

Cyclin-dependent kinases (Cdks)

A

Enzymes that phosphorylate other proteins involved in the cell cycle

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19
Q

What is the significance of phosphorylation status?

A

Regulates the activity of various proteins

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20
Q

True or False:

There is one cyclin-Cdk pair for the cell cycle

A

False, there is one cyclin-Cdk pair PER PHASE of the cell cycle

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21
Q

G1 cyclins

A

Control the activity of G1/S cyclins

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22
Q

G1/S cyclins

A

Activate Cdks in late G1, triggering progression to S phase

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23
Q

S-cyclins

A

Activate Cdks at the beginning of S phase
* Remain active through S, G2, and early M phases

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24
Q

M-cyclins

A

Activate Cdks that stimulate entry into mitosis at the G2/M checkpoint

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25
Q

In regards to cyclin-Cdk activity, what happens if a checkpoint is not met?

A

The cell cycle can be paused by modulating cyclin-Cdk activity

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26
Q

What is the transition for G1 to S phase blocked by?

A

Cdk inhibitor proteins

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27
Q

What do Cdk inhibitor proteins do?

A

Block assembly of cyclin-CDK complexes

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28
Q

What must the cell do in order to enter S phase?

A

Degrade the Cdk inhibitor proteins

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29
Q

True or False:

Once the G1 to S checkpoint is passed, the cell generally completes the cell cycle

A

True

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30
Q

What are Cdk inhibitor proteins also involved in besides the G1 to S checkpoint?

A

Involved in cells enter G0

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31
Q

Mitogens

A

Extracellular signals that promote progression from G1 to S phase by stimulating synthesis G1 cyclins, G1/S cyclins, and other proteins associated with DNA replication

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32
Q

Retinoblastoma (RB) protein

A

A regulatory protein that binds and inhibits transcriptional regulators required for cell proliferation

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33
Q

What is Rb phosphorylated by?

A

G1-Cdk-cyclin and G1/S-Cdk-cyclin complexes

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34
Q

What does the Rb phosphorylation trigger?

A

Conformational change that leads to dissociation of Rb from transcriptional regulators

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35
Q

What is the end result of phosphorylation of Rb?

A

Promotes cell proliferation

36
Q

DNA damage causes the cell to temporarily halt progression through…

37
Q

What does DNA damage that temporarily halts progression through G1 cause?

A
  • Increase in concentration and activity of protein p53
  • Increase in protein p21
38
Q

p53

A

Transcriptional regulator that activates p21

39
Q

p21

A

Cdk-inhibitor protein, binds G1/S-Cdk and S-Cdk

40
Q

What does temporarily halting in G1 allow for?

A

Cell to repair the damage before proceeding

41
Q

What happens to cell if DNA damage is irreparable?

A

p53 induces apoptosis

42
Q

Apoptosis

A

Programmed cell death

43
Q

Mitogen effects of G1-Cdk and G1/S-Cdk

44
Q

In G1, the cell is prepared for replication by forming the…

A

Pre-replicative complex

45
Q

What is the pre-replicative complex comprised of?

A
  • ORC
  • DNA helicase
46
Q

ORC

A

Origin recognition complex
* A protein complex that binds to origins of replication on DNA throughout the entire cell cycle

47
Q

Cdc6

A

A protein recruited by ORC to help load DNA helicase onto DNA

48
Q

What does DNA helicase do?

A

Unwinds DNA

49
Q

DNA Damage, p53 and p21 processes

50
Q

What happens in S phase with DNA helicase?

A
  • S-Cdk activates DNA helicase
  • Promotes assembly of the proteins required for the replication fork
  • Initiates DNA synthesis
51
Q

How is it ensured the DNA is replicated only once?

A

S-Cdk phosphorylates (and inactivates) Cdc6

52
Q

What is the transition from G2 to M phase blocked by?

A

Inhibition of Cdc25
* A phosphatase needed to activate M-cyclin-Cdk complexes

53
Q

What may result in inhibition of Cdc25?

A

Incomplete or erroneous DNA replication

54
Q

What does M-Cdks contain that determines if it is active or not?

A

Contain an inhibitory phosphate group
* Must be removed before it can become active

55
Q

Wee1

A

Inhibitory kinase that phosphorylates M-Cdk

56
Q

What are M-Cdk levels like during G2? Is it activated?

A

M-Cdk levels increase throughout G2, but remains inactivated until the end of G2

57
Q

Why does M-Cdk become active at the end of G2?

A

Cdc25 removes the inhibitory phosphate group

58
Q

What type of feedback loop is M-Cdk activation?

A

Positive feedback loop
* Activated M-Cdk phosphorylates more Cdc25, which activates more M-Cdk

59
Q

ORC, DNA helicase, and Cdc6 mechanism

60
Q

M-Cdk, Cdc25, Wee1 mechanisms

61
Q

Metaphase plate

A

Halfway point between the two opposite spindle poles
* Chromosomes align bound by microtubules align here

62
Q

How do chromosomes ensure that they are properly aligned at the metaphase plate?

A

Dynamic instability of microtubules (growth and shrinkage) helps adjust

63
Q

Spindle Assembly Checkpoint

A

Unattached chromosomes send a “stop” signal which inhibits activation of the APC

64
Q

APC

A

Anaphase-promoting complex

65
Q

What happens APC is activated?

A
  • APC ubiquitylates M-cyclins and S-cyclins (targets them for degredation)
  • Inactivates the associated Cdks
66
Q

What will prevent the APC from being activated?

A

Chromosomes are not correctly attached

67
Q

What is the result of chromosomes not correctly attached?

A

APC is not activated, thus cyclins will not be degraded

68
Q

When APC is activated, what can the cell do?

A

Progress to anaphase

69
Q

What is cleaved in anaphase? What is the purpose of the cleaved things?

A

Cohesins
* Proteins that bind sister chromatids to each other

70
Q

What does the cleavage of cohesins trigger?

A

Separation of sister chromatids to opposite poles of the cell

71
Q

Separase

A

Enzyme that catalyses the degradation of cohesins

72
Q

APC promotes the degradation of…

73
Q

What does securin do?

A

Normally inhibits the action of separase

74
Q

Describe the process of separation of the sister chromatids to their respective poles

A

A result of two processes affecting the mitotic spindle
* Anaphase A
* Anaphase B

75
Q

What happens in:
* Anaphase A?
* Anaphase B?

A

Anaphase A
* Kinetochore microtubules shorten
* Tubulin subunits lost on both ends

Anaphase B
* Spindle poles move apart

76
Q

Dynein

A

Achored to the cell cortex, pulls apart the spindle poles

77
Q

Kinesins

A

Act on (growing) interpolar microtubules, causing them to slide past each other

78
Q

APC, securin, Anaphase mechanisms

79
Q

What experimental techniques are used in cell biology?

A
  1. Labelled nucleotides
  2. Fluorescence-activated cell sorting (FACS)
  3. Cells with synchronised division
80
Q

How are labelled nucleotides used in cell biology?

A

Radioactive/fluorescent signal can be observed in the nucleus when labelled nucleotides are used during DNA replication in S phase

81
Q

How is FACS used in cell biology?

A
  • Cells are stained with dye that fluoresces when bound to DNA
  • Amount of DNA = Amount of fluorescence emitted
  • Cell sorter used to separate cells with different amounts of fluorescence
82
Q

How are cells sorted in FACS based on amounts of fluorescence?

A

G2 cells have 2x the amount of DNA than G1 cells (double the fluorescence)

S cells have somehwere between G1 and G2 cells as replication wouldn’t be complete

83
Q

What is used in cells with synchronised division?

A

Xenopus (frog) eggs

84
Q

Why are large cells used in cell synchronization?

A

Provide a relatively large amount of cell extract, which can be used to purify proteins involved in the cell cycle

85
Q

What happens in immature (unfertilized) eggs in the cell cycle?

A

Arrest in G2

86
Q

How do we ensure eggs (cells) divide synchronized?

A

Fertilize the eggs at the same time

87
Q

What cytoplasm injection experiments have been done in cell synchronization?

A

Cytoplasm of M phase cell injected into a cell in G2
* G2 cell goes to M phase

Ctyoplasm of S phase cell injected into a cell in G1
* G1 cell goes to S phase

My Packs: (WIZEPREP) (BIO 2382) (27 decks)

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