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BIOL 201 > Topic 8: Cell Cycle > Flashcards

Topic 8: Cell Cycle Flashcards

1
Q

What are the key points about the cell cycle?

A

undergoes cell division only when asked to

makes sure cell division steps proceeds with quality control

makes two daughter cells with exact same chromosomes and chromosome number

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2
Q

What are the four distinct phases of the cell cycle?

A

G1 - interphase
S phase - synthesis phase
G2 - continuum of interphase
M phase - mitosis

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3
Q

What is the G1 phase in the cell cycle?

A

cell growth but no division

awaiting signals to divide

DNA is not yet replicated but potentially ready for division

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4
Q

What is the first checkpoint in the cell cycle?

A

between G1 and S phase

restriction point

are there signals to divide?
is there enough space?
is there enough nutrients?
must answer yes to all three

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5
Q

What is the S phase in the cell cycle?

A

DNA replication
MTOC replication

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6
Q

What is the G2 phase in cell division?

A

cell is growing but not dividing

cell is committed to eventual cell division

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7
Q

What is checkpoint two in the cell cycle?

A

G2-M checkpoint

is DNA replication complete?

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8
Q

What is checkpoint three in the cell cycle?

A

“metaphase-anaphase” checkpoint

within the M phase

are all chromatids attached by MT?

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9
Q

What are centromeres?

A

constitutive heterochromatin region of chromosomes where chromatids attach and the kinetochore assembles

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10
Q

What are kinetochores?

A

protein complex that assembles at centromere to allow MT attachment

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11
Q

What are chromosomes?

A

composed of 2 sister chromatids upon entry into mitosis

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12
Q

What is the structure of the microtubules in mitosis?

A

mitotic spindle contains three classes of microtubules (all have negative ends embedded in MTOC)

upon entry into mitosis, MT network reorganizes

increased catastrophe and reduced rescue

many MT-associated proteins regulate this

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13
Q

What is the function of kinetochore microtubules in mitosis?

A

attach at kinetochore to sister chromatids

function: more chromosomes around cell

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14
Q

What is the function of polar microtubules and overlapping microtubules in mitosis?

A

extend from opposite MTOC and overlap in antiparallel manner

function: elongate cell and stabilize spindle

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15
Q

What is the function of astral microtubules in mitosis?

A

extend from MTOC to cell cortex (periphery)

function: anchor MTOC near poles

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16
Q

Why are microtubules more dynamic in mitosis?

A

MT network is regulated by MAP’s (MT associated proteins)

MAPs are regulated by cell cycle regulators (e.g. CDK/cyclin)

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17
Q

What is kinesin 13?

A

“special” kinesin that does not act as a motor but instead facilitates depolymerization of MT at positive end

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18
Q

What is prophase in mitosis?

A

interphase MTs are replaced by astral MTs

chromosomal condensation

MTOC move to opposite poles

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19
Q

What is prometaphase in mitosis?

A

NEBD from the phosphorylation of lamins to disassemble the nuclear lamina

“search and capture” of MTs: MTs from spindle poles (centrosomes) polymerize and radiate outwards and capture chromosomes at specialized structures called kinetochores

NEBD: nuclear envelope breakdown

nuclear membrane vesiculates

after NEBD, MT rapidly polymerize and get stabilized by kinetochore attachment

prometaphase completes when all chromosomes are attached by MT, then move chromosomes to metaphase plate

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20
Q

How are chromosomes moved toward the metaphase plate?

A

coordinated polymerization and depolymerization

motor proteins: dynein, kinesin 7, kinesin 13

kinesin 13: depolymerizes MT at positive end
dynein: pulls chromosomes toward MTOC, attaches MT to kinetochore
kinesin 7: pushes chromosomes towards metaphase plate

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21
Q

What is metaphase in mitosis?

A

chromosomes are all aligned at the metaphase plate

all chromatids are attached to spindle MTs (checkpoint)

signals originating at cell cortex dictates metaphase plate

22
Q

What is anaphase in mitosis?

A

separation of sister chromatids

degradation of cohesin that is holding chromatids together at the centromere

occurs mainly by depolymerization of MT, some contribution of force from kinesin 13 and dynein

signal promoting entry into anaphase occurs at the APC (anaphase promoting complex)

23
Q

What facilitates the chromosome movement in anaphase?

A

kinetochore MT depolymerizes to generate force

kinesin 13 facilitates depolymerization at kinetochore

polar microtubules: elongate the cell to create more space between separated chromosomes

astral microtubules: anchor MTOC to ensure chromosomes move to poles (MTOC doesn’t move to middle)

depolymerization of astral MT contributes to pulling force

24
Q

What is the telophase in mitosis?

A

nuclear envelope reformation

chromosome de-condensation

restructure MT network

assembly of cytokinetic furrow

each daughter cell gets MTOC and copy of each chromosome

25
Q

What is cytokinesis in mitosis?

A

physical separation of cytoplasm

furrow: a ring of MFs and myosin II that forms to bisect the mitotic spindle

actin filaments form a ring

force is generated by bipolar non-muscle myosin

26
Q

What are the cell signals that control cytokinesis?

A

Rho –> ROCK –> phosphorylation of rMLC –> contraction

myosin contraction is controlled by phosphorylation of MLC

27
Q

What are Rho proteins?

A

small G proteins that can be activated by GEFs (GTP/GDP exchange factors) and inhibited by GAPs (GTPase activating proteins)

Rho is tightly controlled by signals from spindle

28
Q

How did the Heterokaryon experiments show cell cycle regulation?

A

two cells at different points were fused together

when cells in G1 and S phase were fused then DNA synthesis started in G1 nucleus, therefore there are signals that promote S phase

when cells in M phase and G1 phase were fused, G1 cells entered M phase, therefore there are signals that promote M phase

29
Q

How did the discovery of MPF (maturation promoting factor) show cell cycle regulation?

A

used an unfertilized frog oocyte arrested at G2

injected either mitotic cell cytoplasm (which caused cell to enter M phase) or interphase cell cytoplasm (no M phase)

therefore there must be a soluble factor in cytoplasm of mitotic cells that promotes cell division

30
Q

How did the study of S. pombe (yeast) mutants show cell cycle regulation?

A

S. pombe is a unicellular eukaryotic yeast

used to characterize enzymes that modify Cdks by phosphorylation

isolated 2 types of mutants:
elongated cells (deficit of Cdc25 or excess of Wee1): cells were too slow in cell division, therefore Cdc25 normally drives cell division forward
small cells (deficit in Wee1 or excess of Cdc25): cells divided too quickly therefore Wee1 normal function is to slow cell division)

31
Q

What is Wee-1?

A

inhibitory kinase

adds a PO4

32
Q

What is Cdc25?

A

activating phosphatase

removes PO4

33
Q

What is a Fluorescence Activated Cell Sorter (FACS)?

A

a culture of growing cells are exposed to a fluorescence dye for DNA

as cells pass through a small opening the relative fluorescence is counted by the machine

the amount of DNA can be determined thus the phase of the cell cycle can be determined (have they replicated DNA?)

G1 = 1xDNA
G2 = 2xDNA

34
Q

What is the G1 checkpoint?

A

controls entry into S phase

activated G1-cyclin/CDK promotes entry into S phase

targets –> Rb (retinoblastoma)

35
Q

What is the G2 checkpoint?

A

controls entry into mitosis

activated by MPF (Mcyclin/CDK)

target of MPF (lamins, APC, and histone H1 activate mitosis)

36
Q

What is the anaphase checkpoint?

A

controls entry into anaphase

activated by APC (anaphase promoting complex) not a cyclin/CDK

37
Q

How is the cell cycle regulated?

A

MPF was later refined to show that it was a dimer of a cyclin and a cyclin-dependent protein kinase (cyclin/CDK complex)

cell cycle progression governed by key protein kinases in a cyclical manner

these cyclical proteins (called cyclins) regulate passage through the various checkpoints

involves a series of proteins being phosphorylated and dephosphorylated

activation and deactivation are under the control of cyclins

cyclins have cyclical expression but Cdks are always present

38
Q

What is MPF?

A

dimer of cyclin/CDK

association of CDK with cyclin is necessary for activation of MPF, but it is not sufficient

39
Q

What is CDK?

A

cyclin-dependent kinase

levels are consistent in cell cycle

40
Q

What is cyclin?

A

levels vary throughout cell cycle

regulators of CDK activity

levels peak just before entry into mitosis

41
Q

What is the relationship between cyclin and cyclin-dependent kinases?

A

cyclins do not have enzymatic activity but must bind to kinases before they can become active

heterodimer is inactive

dimer only forms when a threshold of cyclin accumulates in the cell

42
Q

How is CDK activity activated?

A

association of CDK with cyclin is necessary for activity but not sufficient

phosphorylation of key resides must be accomplished in order for activation of the CDK-cyclin complex

43
Q

What are the steps in CDK activation?

A

CDK and cyclin come together to form an inactive heterodimer

cyclin activated kinase (CAK) adds one phosphate

Wee1 adds two inhibitory phosphates

Cdc25 removes inhibition phosphorylation by Wee1

44
Q

What is the restriction point?

A

passing through the restriction point commits a cell for eventual division

senses the availability of nutrients, presence of growth factors, and cell size

activation of G1 CDK/cyclin

DNA damage, loss of adhesin and growth factor signaling all slow cell cycle

45
Q

How do Rb (retinoblastoma associated gene) proteins control the restriction point?

A

normally binds and inhibits E2F

E2F is a transcription factor that regulates DNA synthesis

Rb is a target of G1 cyclin/CDK

cyclin/CDK promotes phosphorylation of Rb which inactivates it

E2F is then available to trigger gene expression

46
Q

What controls the G2/M checkpoint?

A

CDK/cyclin complex - MPF

M-CDK

47
Q

What are the functions of active M-CDK during the G2/M checkpoint?

A

phosphorylation of lamin causes NEBD in prometaphase

phosphorylation of nuclear material causes nucleoli dismantling

phosphorylation of condensin cause chromosomal condensation in prophase

phosphorylation of Histone H1 causes chromosomal condensation

phosphorylation of MAPs decreases MT stability and increases MT dynamics

indirect activation of APC

48
Q

What controls the metaphase/anaphase checkpoint?

A

M-CDK activates APC (anaphase promoting complex)

APC acts at the metaphase-anaphase transition

cohesin (complex of Smc1/Smc3/Klesin) which holds sister chromatids together needs to be broken down

49
Q

How is cohesin broken down during anaphase?

A

APC induces polyubiquitination of securin (signal that causes protein degradation by proteasome)

securin normally blocks separase, but now separase can cleave kleisin and allow separation

APC is regulated by Mad/Bub
Mad/Bub is produced by unattached kinetochores, therefore Mad/Bub inhibits APC to prevent passage in anaphase if chromatids are not all bound by MTs

MT attachment inhibits Mad/Bub

50
Q

What if the signals regulating the cell cycle don’t work?

A

if the signals for cell cycle progression are ignored or don’t work, this can result in cancer and uncontrolled cell proliferation

BIOL 201 (13 decks)

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