slide set 16 Flashcards

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1
Q

cell divisions in your body

A

bone marrow stem cells: 1,000,000/min

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2
Q

cell theory

A

all cells are made up of other cells

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3
Q

cell cycle basics

A

Cell must

  1. grow
  2. replicate its genome
  3. centrosomes must be segregated to opposite ends of the still-growing cell
  4. cytoplasm is cleaved in half
  5. 2 daughter cells, identical in size to original cell
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4
Q

S+M phase

A

S Phase: chromosomes are duplicated (entire genome is duplicated)

M Phase: Mitosis and Cytokinesis

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5
Q

4 cell cycle stages

A

Interphase

  • M phase
  • G1 phase
  • S phase
  • G2 phase

G = gap

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6
Q

mitosis is easy to see through a microscope, but what about the other stages

A
  • we can detect S phase
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7
Q

How can we identify S phase?

A
  • detect cells that are replicating their DNA
  • 2 options
      1. Detect S-phase by DNA replication and incorporation of a modified base
        * detect BrdU with an antibody
        * treat cells with BrdU for a certain amount of time
        • if it is dividing, it’ll include BrdU
        • if it isn’t, the cell won’t take in any bases floating around
          * bind antibody to BrdU, cell that fluoresces has been in S phase
      1. Measure relative DNA content per cell by flow cytometry = fluorescent label on DNA is detected
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8
Q

How can we identify S phase (part 2)?

A

harvest cells

isolate cells

stain cells

  1. Measure relative DNA content per cell by flow cytometry = fluorescent label on DNA is detected

fluorescent dye shows us how much DNA is there

height of peak = # of DNA in cells

peaks change bc cells divide at random times

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9
Q

cell cycle controllers

A
  • proteins control when each process occurs
  • These proteins integrate signals and can slow or stop the cell cycle if conditions are unfavorable or if errors have occurred
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10
Q

CDK aka

A

cyclin-dependent kinase

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11
Q

key enzyme family

A

Cdk’s

require a cyclin partner before they are active as a kinase

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12
Q

major Cdk’s and their cyclin partners

A

each active Cdk recognizes and phosphorylates specific target proteins that ultimately drive a cell cycle phase

focus on M-Cdk (mitotic Cdk)

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13
Q

Cdk enzyme activity over course of cell cycle stages

A

Cdk enzyme activity rises abruptly and declines abruptly at cell cycle stages

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14
Q

what happens if we add an inhibitor that blocks cells from getting to S phase

A

we’d have one really large peak in G1

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15
Q

Rise in Cdk activity is based on…

A

synthesis and degradation of cyclins

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16
Q

M phase questions

A

Are all chromosomes attached to the spindle?

(If not, we may get uneven daughter cells)

Metaphase to Anaphase transition

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17
Q

G1

A

restriction point: point of no return, after cell crosses that point cell is committed to rest of cell cycle and dividing

Is environment favorable?

enter cell cycle! proceed to S phase!

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18
Q

G2 questions

A

Is all DNA replicated?

Is environment favorable?

G2/M transition!

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19
Q

Structural basis of Cdk activation and additional regulators

A
  • inactive state: T loop is folded (part of active site of kinase)
    • kinase can’t interact with target protein
    • without cyclin bound, active site is blocked by region of T loop
  • As cyclin is made, cyclin protein can bind to CDK
  • binding of cyclin causes T loop to open up
  • active site of kinase is revealed!
  • CAK adds phosphate to CDK (now it is fully active)

To be active, CDK must

  1. bind to cyclin
  2. be phosphorylated by CAK
20
Q

Cdk’s are regulated by inhibition of enzyme activity

A
  • Wee1 kinase can come in and phosphorylate a different part of the active Cdk-cyclin and inhibit the activity
  • Cdc25 phosphatase can remove this inhibitory phosphate group
21
Q

kinase

A

adds phosphate group using ATP

22
Q

M-Cdk partners

A
23
Q

positive feedback loop

A

produces rapid rise in active Cdk1 to drive mitotic entry

  1. Cdk1 binds to M-cyclin as M-cyclin levels rise
  2. M-Cdk is phosphorylated on active site by CAK and on a pair of inhibitory sites by Wee1 kinase
    1. leads to inactive M-Cdk complex
  3. inactive M-Cdk is activated at end of G2 by Cdc25 (a phosphatase)
  4. Cdc25 is further stimulated by active M-Cdk, results in positive feedback
    1. feedback is enhanced by ability of M-Cdk to inhibit Wee1
24
Q

Active Cdk1/cyclin B

A

Active Cdk1/cyclin B drive drastic changes in mitosis to segregate the replicated genome and produce 2 daughter cells

25
Q

Inhibitors used at G1/S and S phases

A

CKI’s (cyclin-dependent kinase inhibitors)

p27 distorts the kinase active site and inactivates enzyme activity

26
Q

When a process is complete,

A

enzyme activities rapidly fall when a process is complete

cyclins are destroyed by proteolysis

proteolysis: protein is no longer needed

27
Q

Polyubiquitin chains

A

target a protein for degradation by the proteasome

requires an E3 ubiquitin ligase

28
Q

proteolysis example

A

mitotic exit is controlled by proteolysis through APC/C (anaphase promoting complex)

29
Q

CAK aka

A

Cdk-activating kinase (CAK)

30
Q

APC aka

A

anaphase promoting complex

31
Q

transition from G1 to S phase

A

G1 to S phase through degradation of CKI’s

32
Q

environmental cues

A

cell cycle machinery receives environmental info to slow timing as needed

33
Q

proteolysis

A

ubiquitin is added to a protein (in a chain = polyubiquitylation)

indicates degradation by proteasome is necessary

34
Q

G0 phase

A

our cells can enter a non-growing/non-dividing state: G0

in response to cues, cells have to get out of G0 and re-enter the cell cycle

35
Q

How to get G1 going

A

Need active E2F, a transcription factor for expression of S-Cdk, enzymes and proteins for DNA replication

this will drive the cell from G1 to S

36
Q

How is E2F responsive to cues from the environment?

A
  1. Mitogen binds receptor, activates Ras
  2. Ras activates MAP kinase cascade
  3. MAP kinase cascade leads to activation of a transcription factor that leads to expression of Myc and cyclin
  4. Active G1 Cdk phosphorylates Rb, a protein that inhibits E2F

resulting G1/S-Cdk and S-Cdk activities further enhance Rb protein phosphorylation, forming a positive feedback loop

E2F proteins also stimulate transcription of their own genes, another positive feedback loop

37
Q

starting S phase

A
  1. Mitogen binds receptor, which activates Ras
  2. Ras activates MAP kinase cascade
  3. MAP kinase cascade leads to activation of a transcription factor that leads to expression of Myc and cyclin
  4. Active G1 Cdk phosphorylates Rb, a protein that inhibits E2F

positive feedback at end leads to more E2F

38
Q

Rb

A

Retinoblastoma protein

active when not phosphorylated

39
Q

What happens when Rb is mutated (loss-of-function mutation)?

A
  • loss of Rb leads to excessive proliferation of some cells in the developing retina
    • Rb protein is important for restraining cell division
  • complete loss of Rb does not immediately cause increased proliferation, because CKIs also help inhibit progression through G1
40
Q

How can we slow/stop the process so damage can be repaired?

A

*

41
Q

p53

A

p53 prevents cell cycle from proceeding until DNA damage is repaired

p21 binds to active S-Cdk and inactivates it (now S phase cannot continue)

42
Q

When would we need G0 phase?

A

cells at the end of a wound

liver cells only divide once a year (divide, then go into G0)

43
Q

mitogen

A

= stimulates mitosis

44
Q

M-cyclins

A

activate Cdks that stimulate entry into mitosis at the G2/M transition

M-cyclin levels fall in mid-mitosis

45
Q

positive feedback in activation of M-Cdk

A
46
Q

mitogen stimulation of cell-cycle entry positive feedback loops

A

resulting G1/S-Cdk and S-Cdk activities further enhance Rb protein phosphorylation, forming a positive feedback loop

E2F proteins also stimulate transcription of their own genes, another positive feedback loop