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MBIOS 501 Exam 3 > 12.2 Cell Cycle Control l > Flashcards

12.2 Cell Cycle Control l Flashcards

1
Q

What are the stages in interphase?

A

G1: Gap phase
S phase: DNA synthesis/replication
G2: gap phase

After G2 cell enter mitosis and after mitosis G0: resting state where cells leave the cycle for some period of time

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2
Q

G phases in interphase are involved in making decisions…

A

regarding whether or not to allow a cell to progress through cell division.

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3
Q

What occurs in the S stage of interphase?

A

The S stage is the stage of the cell cycle where DNA replication takes place.

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4
Q

What is G0 in interphase?

A

a resting or dormant stage in which the events of the cell cycle progression have been suspended.

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5
Q

What are gap phases in the cell cycle?

A

Gap phases involve growth and decisions regarding continuation of the cell - a checkpoint to see if the cell should progress through cell division.

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6
Q

Most of what we know about the cell cycle was learned using what?

A

using mutants of yeast; fission yeast and budding yeast.

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7
Q

How does temperature affect cells?

A

cells are functional at low temperatures but become unstable at higher ones.

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8
Q

What do cells display during permissive temperatures?

A

cells can grow in permissive temperature and cells display buds of varying sizes. Some cells can even lack buds, and most have smalls ones.

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9
Q

What happens when a cell strain is placed in a non-permissive temperature?

A

all cells arrest w/large buds. These cells have completed anaphase but fail to complete cytokinesis.

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10
Q

How can we assess how cells progress through the cell cycle?

A
  • mutant yeast and temperature
  • DNA replication
  • BrdU
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11
Q

What is BrdU?

A

a modified nucleic acid that is added into DNA strands during replication. Antibodies can be used to detect cells that have BrdU.

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12
Q

Cells in G1 and G0 are…

A

diploid (2N), where N is a single set of chromosomes.

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13
Q

Cells that are in G2 and M phase are…

A

tetraploid (4N)

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14
Q

Cells that are in S phase are…

A

in between 2N and 4N b/c they are in the process of replication the DNA.

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15
Q

How can analysis of the cell cycle be done using DNA replication?

A

By staining cells w/a general probe for DNA and then passing the cells through an analyzer called flow cytometer.

The result will show the proportion of cells having 2N, 4N or somewhere in the middle amounts of DNA. Changes to the progression of cell population will change the appearance of the graph

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16
Q

What is a flow cytometer? when is it used?

A

Used to analyze cell cycle using cell replication.

The flow cytometer measures the DNA content of large number of cells one at a time and can plot this information out on a graph.

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17
Q

What are the necessary features of the cell cycle control system?

A
  • Clock (timing for each stage and tells a cell when to do things)
  • Initiate stages in correct order
  • Each stage triggered once per cycle
  • Stages triggered in complete, irreversible manner
  • Back-up mechanisms
  • Sensitive to environmental conditions
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18
Q

What happens at each checkpoints in the cell cylce? How does the cell decide if it will move to the next stage?

A

1) G1/S boundary where a cell decides whether or not to begin the process of cell replication and ultimately division.
2) G2/M boundary, where the cell checks to make sure that the DNA has been completely replicated
3) Metaphase-to-anaphase, where cell checks that all of the chromosomes have attached to the mitotic spindle.

A positive answer will allow the cell to proceed to the next stage, while a negative answer will interrupt the process.

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19
Q

What are the main regulators of the cell cycle and what do they do?

A

kinases called cyclin dependent kinases (CDKs), which phosphorylate appropriate targets at every stage of the cell cycle.

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20
Q

What will happen to CDK if it does not have the appropriate cyclin present?

A

the shape of the CDK does not allow it to function. Thus, CDKs are dependent on the presence of specific cyclins.

21
Q

What happens when a cyclin binds to CDK? What happens when cyclin phosphorylates CDK?

A

CDK will undergo a shape change that partially activates it and also reveals a phosphorylation site which is recognized by a cyclin activating kinase.

Phosphorylation of the CDK by the cyclin activating kinase then leads to complete activation of the CDK

22
Q

What are the four classes of cyclin proteins?

A

1) G1
2) G1/S
3) S
4) M

23
Q

What is the expression level/activation state of G1/S cyclin?

A

expression is high in G1 but then falls to a low level at the G1/S boundary

24
Q

What is the expression level/activation state of S cyclin?

A

start out low in mid G1 and reaches a plateau of expression in S phase. It then remains high until mid M phase.

25
Q

What is the expression level/activation state of M phase cyclin?

A

M cyclin starts to be expressed in G2 and reaches a threshold at the G2/M boundary

It then decreases at mid M

26
Q

What is similar between S and M cyclins?

A

Both S and M cyclin expression level decline in the middle of M phase, which signals a cell to finish mitosis and start G1

27
Q

Much control of the cell cycle involves cyclical….

A

cyclical synthesis and degradation of proteins called cyclins.

28
Q

What are the two mechanisms for down regulating the functions of CDKs?

A

1) Wee1 Kinase: can add a phosphate to CDK that inhibits its function.
2) CDK Inhibitor proteins (CKIs), these proteins will bind to the CDK/Cyclin complex and prevent it from acting. (p27 CKI)

29
Q

What is the purpose of down regulation functions of CDKs?

A

it fine tunes the activity of CDKs and allows the changes from one stage of the cell cycle to the next to be more rapid than could be achieved by protein synthesis and degradation alone.

30
Q

How can Wee1 kinase inhibition be reversed?

A

by the activity of a cell division cycle phosphatase called CDC25. Phosphatase is an enzyme that removes phosphate from a protein.

31
Q

What is one signaling protein that becomes activated when it becomes dephosphorylated by a phosphatase?

A

CDKs

wee1 kinase adds phosphate to CDK and inhibits CDK function. Phosphatase CDC25 will dephosphorylated CDK and activate it.`

32
Q

Connections between chromosomes are cleaved when and by what protease?

A

at the entry into anaphase by a protease called separase.

33
Q

How does the cell control the activity of separase?

A

By CDC20

During metaphase, levels of the cell division cycle protein CDC20 build up.

  • CDC20 activates the anaphase promoting complex (APC complex)
  • Active APC complex is capable of proteolysis and degrades inhibitory protein securin, which liberates active separase.
34
Q

What two proteins are degraded during metaphase to anaphase transition phase?

A

securin which releases active separase and
cohesion protein that hold chromosomes together.

M and
S cyclins

When the activation of the
anaphase promoting complex reaches a sufficient level it causes the degradation of M and
S cyclins as well as separase

35
Q

What is CDC20 activated by? what does cdc20 do?

A

cdc20 activates the anaphase promoting complex and cdc is activated by m-Cdk

36
Q

The transition from metaphase to anaphase makes the beginning of…

A

the end of mitosis so changes in cell cycle control are produce that down regulate the proteins that signal cells to conduct mitotic cell division.

37
Q

What roles does anaphase promoting complex play?

A

1) when activated it will degrade securin and release separase
2) when activated it will also target S and M cyclin for degradation.

38
Q

What occurs when S and M cycling are targeted for degradation?

A

this deactivates the mitotic CDks so they no longer phosphorylate the proteins producing the mitotic state.

39
Q

Whaat is poly-ubiquination?

A

a mechanism used by cells to target cell cycle signaling proteins and other proteins for degradation

40
Q

When dos poly-ubiquination occur?

A

When input signal recruits and activates two enzymes called E1 and E2. These enzymes together form an active ubiquitin ligase complex. Ubiquitin is a third protein player in this pathway. The resulting polyubiquitin chain is recognized by another enzyme complex in the cytoplasm called the proteasome which degrades proteins into their component amino acids for re-use

41
Q

What is ubiquitin?

A

a very small and highly conserved protein that can be linked in long chains to a protein by the E1/E2 ligase

42
Q

What is the SFC complex?

A

Degradation of cell cycle control proteins. The SFC complex targets a CdK inhibitor for destruction during the transition from G1 to S phase. The inhibitor is part of the control system that keeps cells in G1

43
Q

Overview of the cell cycle control system from textbook:

A

G1 is maintained by the G1-CdK.

G1-Cdk stimulates production of the G1/S cyclin which then activates cyclin dependent kinases through a checkpoint just before S phase.

If the cell passes through the checkpoint, S phase cyclin is expressed,
which drives the cell into S phase.

M-Cdk performs a similar function by initiating mitosis and activating the anaphase promoting complex.

When the activation of the anaphase promoting complex reaches a sufficient level it causes the degradation of M and
S cyclins as well as separase

44
Q

What conditions can block progession of the cell through the cell cylce?

A
  • DNA damage
  • non-replicated DNA
  • detached chromosomes during mitosis.
45
Q

what protein kinases and phosphotases modify CDKs?

A
  • Cdk-activating kinase (CAK)- phosphorylates activating sites on Cdks
  • Wee1 kinase- phosphorylates inhibitory sites on Cdks
  • Cdc25 phosphatase- removes inhibitory phosphate from Cdks
46
Q

List Protein kinases and phosphatases that modify Cdks

A

Cdk-activating kinase (CAK)- phosphorylates activating sites on Cdks

 Wee1 kinase- phosphorylates inhibitory sites on Cdks

 Cdc25 phosphatase- removes inhibitory phosphate from Cdks

47
Q

List Cdk inhibitor proteins

A

P27- suppresses G1/S-Cdk and S-Cdk activates in G1

 P21- suppresses G1/S-Cdk and S-Cdk activities following DNA damage

 P16- suppresses G1-Cdk activity in G1; frequently inactivated in cancer

48
Q

List Ubiquitin ligases and their activities

A

APC- catalyzes ubiquitylation of proteins involved in exit from mitosis, including securing, S and M-cyclins; regulated by association with activating subunits
(including Cdc20)

 Cdc20—APC subunit in cells; triggers activation of APC at metaphase to anaphase transition; stimulated by M-Cdk activity

 SCF-catalyzes ubiquitylation of proteins involved in G1 control, including some CDI’s; recognizes phosphorylated substrate.

49
Q

What are the methods for analysis of the cell cycle using DNA replication?

A

1) BrdU
2)stain cells with a general probe for DNA
and then pass them through an analyzed, called a flow cytometer

MBIOS 501 Exam 3 (13 decks)

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