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Molecular Cell Biology > (4) Chapter 17: The cell cycle > Flashcards

(4) Chapter 17: The cell cycle Flashcards

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1
Q

What controls the Progression through the cell cycle ?

A

-protein kinases that have been conserved from yeasts to mammals

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2
Q

Four coordinated processes of division

A
  1. cell growth
  2. DNA replication
  3. Distribution of the duplicated chromosomes to daugher cells
  4. cell division
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3
Q

Cycles of the cell phase in eukaryotes

A

M, G1, S, G2

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4
Q

M phase

A

Mitosis(nuclear division that ends with cell division (cytokinesis)

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5
Q

Interphase

A

G1, S, G2 phases

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6
Q

G1 phase

A

interval between mitosis and DNA replication. Cell is metabolically active and growing

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7
Q

S phase

A

DNA replication

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8
Q

G2 phase

A

cell growth continues and proteins are synthesized in preparation for mitosis

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9
Q

Early embryos

A

-may have cell cycles of 30 minutes without growth in G1 or G2 phase

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10
Q

Duration of cell phases in yeasts

A

budding yeasts can progress through all four phases in 90 minutes

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11
Q

During S phase what happens to number of DNA

A

-replication increases the DNA content to 4n

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12
Q

How can DNA content be determined?

A
  • incubation of cells with a fluorescent dye that binds to DNA
  • fluorescence intensity of individual cells is measured in a flow cytometer or fluorescence-activated cell sorter.
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13
Q

How is cell progression through the division cycle regulated?

A

extracellular and internal signals

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14
Q

What cellular processes are regulated by control points?

A

-growth, DNA replication, mitosis

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15
Q

START point

A

controls the progression from g1 to S.

-once cells pass START , they are committed to entering S phase and undergoing one division cycle

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16
Q

How is passage through the START point regulated?

A
  • External signals such
    1. nutrient availability
    2. mating factors
    3. cell size
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17
Q

What sends cell division process into a resting phase?

A

-shortage of nutrients can arrest the cycle at START and cause it to enter resting phase

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18
Q

Restriction point in animal cells

A
  • in late G1 stage
  • passage through this point is regulated by extracellular growth factors.
  • once cell has passed, it is committed to proceed through S phase and rest of cycle.
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19
Q

What happens if the appropriate growth factors are not present in G1?

A

progression stops at the restriction point and cells enter a resting stage called G0

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20
Q

What stage of cell division are skin fibroblasts in?

A

-skin fibroblasts are arrested in G0 until stimulated by platelet-derived growth factor to proliferate and repair wound damage

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21
Q

What is unusual about the fission yeast Schizosaccharomyces pombe?

A

-controlled by transition from G2 to M (the point at which cell size and nutrient availability are monitored)

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22
Q

Vertebrate oocytes

A
  • can remain arrested in G2 for long periods of time (decades for humans)
  • progression to M phase is triggered by hormonal stimulation
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23
Q

Cell cycle checkpoints

A

-function to coordinate the cell cycle phases in a specific order

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24
Q

DNA damage checkpoints

A
  • Ensure that damaged DNA is not replicated and passed on to daughter cells
  • cell cycle is arrested until DNA is repaired or replicated
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25
Q

Spindle Assembly checkpoint

A

-if chromosomes are improperly aligned on the spindle the process is stopped at metaphase during mitosis

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26
Q

Name two types of cell cycle checkpoints

A
  1. DNA damage checkpoint

2. spindle assembly checkpoint

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27
Q

What is MPF?

A
  • Maturation promoting factor
  • general regulator of transition from G2 to M phase
  • present in oocyte and somatic cells
  • a protein kinase
  • composed of Cdk1 & cyclin B
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28
Q

How was MPF discovered?

A
  • Using from oocytes
  • Frog oocytes are arrested in G2 until hormoal stimulation triggers their entry into M phase
  • this was induced by microinjection of the cytoplasm of an already phase changed system into the system that needed to change phase
  • MPF was found to be the promotion factor
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29
Q

cdc

A
  • cell division cycle mutant inducers
  • if an optimal temp isn’t present they create a defect the cell cycle progression
  • (temperature sensitive mutators; if temp is correct, they work)
  • They are required for the passage through START and for entry into mitosis
  • encode protein kinases
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30
Q

How was cdc discovered?

A

through genetic analysis of yeasts

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31
Q

Cdk1

A
  • protein kinase that makes up 1/2 components of MPF
  • cell cycle regulator in all eukaryotes
  • controls passage through START and entry into mitosis in yeasts in associated with Cln’s
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32
Q

Accumulation and degredation of cyclins

A
  • 2 cyclins were identified as accumulating throughout interphase but are rapidly degraded at the end of each motosis
  • suggests a role in inducing mitosis
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33
Q

Cyclin B

A
  • regulatory subunit required for catalytic activity of the CDk1 protein kinase
  • part of MPF complex
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34
Q

How is MPF regulated?

A

-phosphorylation and dephosphorylation of Cdk1

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35
Q

Give process for MPF regulation

A
  1. cyclin B synthesized in G2 phase & forms complex with Cdk1
  2. Cdk1 is phosphorylated & inhibited
  3. inactive Cdk1/cyclin B complexes accumulate in G2 phase
  4. Cdk1 is dephosphorylated & acivated; resulting P’s phosphorylate several proteins that initiate M phase events
  5. (M phase initiated)
  6. cyclin B is degraded by ubiquitin mediated proteolysis
  7. since cyclin B is destructured, Cdk1 is inactivated.
  8. Cell then exits mitosis and undergoes cytokinesis and returns to interphase
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36
Q

Ubiquitylation of cyclin B

A

-mediated by ubiquitin ligase: APC/C

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37
Q

APC/C

A
  • anaphase-promoting complex/cyclosome

- activated as a result of phosphorylation by CDK1/cyclin B

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38
Q

G1 cyclins

A

Cln’s: control passage through START and entry into mitosis in yeasts.

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39
Q

Cdk’s

A

-cyclin-dependent kinases
-Cdk-1 related protein kinases
-present in higher eukaryotes
-a family of proteins that control progression through the phases of the cell cycle
-

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40
Q

Growth factros

A
  • regulates proliferation of animal cells
  • in extracellular material
  • control progression through the restriction point in late G1.
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41
Q

D-type cyclins

A

-provide a link between growth factor signaling and cell cycle progression

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42
Q

Cyclin D1

A
  • synthesis stimulated by growth factors through the Ras/Raf/MEK/ERK pathway
  • always synthesized if growth factors are present
  • rapidly degraded (growth factors removed too)
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43
Q

Which components drive cells through the restriction point through G1 phase?

A

-as along as growth factors are present through G1, Cdk4, 6/cyclin D1 complexes drive cells through restriction point

44
Q

MCM helicase proteins

A

-binds originis of replication together with the ORC proteins in G1 to form a pre-replication compled

45
Q

ORC

A

origin recognition complex

46
Q

What components regulate replication of DNA in the S phase?

A

-Cdk2/cyclin E and the DDK protein kinase

47
Q

How is DNA replication initiated?

A
  • DDK phosphorylated MCM proteins
  • Cdk2 phosphorylates additional proteins that join the complex and activate MCM
  • as MCM is activated it initiates activation of DNA
  • MCM proteins move away from the origin with the replication fork.
  • since Cdk’s are highly active, it prevents MCM proteins from reassociating with origins during S, G2, and M, so pre-replication complexes can only reform during G1
48
Q

What is cell cycle arrest at DNA damage checkpoint mediated by?

A
  • protein kinases ATM and ATR

- activated in response to DNA damage

49
Q

ATM and ATR

A
  • protein kinases activated in response to DNA damage

- activate a signaling pathway that leads to cell cycle arrest, DNA repair, and sometimes programmed cell death

50
Q

ATM

A
  • recognizes double-strand breaks

- phosphorylate and activate checkpoint kinase Chk2

51
Q

ATR

A
  • recognizes single stranded or unreplicated DNA

- phosphorylates and activates checkpoint kinase Chk1

52
Q

Role of checkpoint kinase Chk1 and Chk2

A

-phosphorylate and inhibit Cdc25 prosphatases

53
Q

Cdc25

A

required to activate checkpoint kinases Cdk1 and Cdk2

54
Q

Inhibition of Cdk1 and Cdk2

A

Inhibition of Cdk1 results in arrest in G2.

-Inhibition of Cdk2 results in cell cycle arrest in G1 and S

55
Q

Protein p53

A
  • mediates arrest at G1 checkpoint in mammalian cells
  • phosphorylated by both ATM and Chk2
  • a transcription factor; increased expression leads to induction of Cdk inhibitor p21 (Cip/Kip family)
56
Q

P21 mRNA

A
  • Cdk inhibitor of Cip/Kip family

- inhibits Cdk2/cyclin E complexes, leading to cell cycle arrest in G1

57
Q

Describe what happens during M phase

A
  • major reorganization of virtually all cell components
  • chromosomes condense
  • nuclear envelope breaks down
  • cytoskeleton reorganizes to form the mitotic spindle
  • chromosomes move to opposite poles
  • followed by cytokinesis
58
Q

4 stages of Mitosis in animal cell

A
  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase
59
Q

Prophase

A
  • appearance of condensed chromosomes (2 sister chromatids)
  • proteins bind to centromere to form kinetochore and create 2 poles of mitotic spindle
  • mitotic spindle forms during late prophase
60
Q

Chromatids

A

-chromatids attached at centromere where proteins bind to form the kinetochore

61
Q

Describe the end of prophase in higher eukaryotes

A

-prophase ends when the nuclear envelope breaks down (open mitosis)

62
Q

Describe the end of prophase in yeasts

A
  • the nuclear envelope remains intact (closed mitosis)
  • spindle pole bodies are embeddid in the nuclear envelope; the nucleus divides after migration of daughter chromososmes, then the cell divides as well
63
Q

Propmetaphase

A
  • transition between prophase and metaphase.
  • spindle microtubules attach to kinetochores of condensed chromosomes
  • chromosomes shuffle back and forth due to activity of microtubule motors at the kinetochore and centrosomes
64
Q

Metaphase

A

-chromosomes shuffle back and forth until they align on the metaphase plate

65
Q

Anaphase

A

–links between sister chromatids break, and they seperate and move to opposite poles of the spindle

66
Q

Telophase

A
  • nuclei re-form and chromosomes decondense,

- cytokinesis usually begins during late anaphase and is almost complete by the end of telophase

67
Q

Cdk/cyclin B

A
  • protein kinase (MPF)
  • master regulator of M phase transition
  • activates other mitotic protein kinases and directly phosphorylates structural proteins involved in cellular reorganization
68
Q

How are Cdk1, Aurora, Polo-like kinases activated?

A
  • by positive feedback loop to signal entry into M phase

- Cdk1 activates Aurora kinases, which activate Polo-like kinases, which in turn activate Cdk1

69
Q

Condensation of chromatin

A
  • driven by condensins, SMC proteins
  • contribute to chromosome segregation
  • activated by Cdk1/cyclin B phosphorylation
  • replace most of the cohesions, leaving sister chromatids linked only at centromere
70
Q

Cohesions

A

bind to DNA in S phase and maintain links between sister chromatids
-contribute to chromosome segregation

71
Q

Breakdown of the nuclear envelope

A
  • involves changes in all components
    1. fragmentation of membrane
    2. dissociation of nuclear pore complexes
    3. nuclear lamina depolymerizes due to phosphorylation of lamins by CDK1/cyclin B
    4. allows spindle microtubules to attach to chromosomes at the kinetochore
72
Q

Events of M phase

A
  1. Golgi apparatus breakdown
  2. centrosome maturation/spindle assembly
  3. microtubule turnover rate increases
73
Q

Golgi apparatus breakdown

A

a) mediated by phosphorylation of proteins by cdk1 & other protein kinases
b) fragments into vesicles at mitosis
c) vesicles are absorbed into ER or distributed to daughter cells at cytokinesis

74
Q

What components are involved in the maturation and spindle assembly of centrosomes?

A

-aurora and polo-like kinases

75
Q

What happens during Microtubule turnover?

A
  • rate of microtubule increases and leads to depolymerization and shrinkage of interphase microtubules
  • number of microtubules radiating from centrosome also increases
76
Q

APC/C ubiquitin ligase

A

a) mediates progression to anaphase at the spindle assembly checkpoint
b) phosphorylated by Cdk1/cyclin B

77
Q

Function of APC/C ubiquitin ligase

A

-ubiquilates cyclin B and securin and breaks them down; in turn–> inactivates Cdk1 and separase

78
Q

Separase

A
  1. degrades cohesion
  2. breaks link between sister chromatids
  3. allows them to segregate and move to opposite spindle poles
79
Q

How is APC/C ubiquitin ligase activated & inhibited?

A

a) inhibited as mitotic checkpoint complex (MCC) is assembled ( if unattached kinetochores are present)
b) activated if all chromosomes are aligned on the spindle

80
Q

Cytokinesis of animal cells

A

-starts shortly after anaphase starts
-triggered by inactivation of CDK1
-mediated by a contractile ring made
of (1)actin, (2) myosin 2 filaments

81
Q

How does the contractile ring work?

A

a) forms beneath PM
b) contraction of actin-myosin filaments pulls plasma inward, eventually pinching the cell in half
c) cell cleaved in a plane that passes through the metaphase plate

82
Q

Meiosis

A
  • reduces the chromosome number by half
  • results in haploid daughter cells
  • only happens in germ cells in multicellular plants and animals
  • results in haploid progeny; each with only one member of the pair of homologous chromosomes that were present in the diploid parent cell
83
Q

What happens during meiosis 1

A
  • homologous chromosomes pair with one another and then segregate to different daughter cells
  • results in 4 haploid daughter cells
84
Q

Sister chromatids in meiosis 1

A

remain together so that daughter cells contain a single member of each chromosome pair

85
Q

haploid daughter cells

A

each has only one copy of each chromosome.

86
Q

Stages of Prophase 1

A
  • has five stages, based on chromosome morphology:
    1. leptotene
    2. zygotene
    3. pachytene
    4. diplotene
    5. diakinesis
87
Q

Leptotene stage

A
  • Spo11 induces double-strand breaks in homologous chromosomes
  • leads to single-strand regions that invade a homologous chromosome by complementary base pairing
88
Q

Zygotene stage

A
  • close association of homologous chromosomes begins through synapsis.
  • zipper like synaptonemal complex is formed along the length of the paired chromosome (keeps homologous chromosomes closely associated and aligned)
89
Q

Pachytene stage

A
  • recombination is complete by this stage

- leaves chromosomes linked at sites of crossing over (chiasmata)

90
Q

Chiasmata

A

sites of crossing over

91
Q

Diplotene stage

A
  • synaptonemal complex disappears except at the chiasmata

- each chromosome pair ( a bivalent) consists of four chromatids

92
Q

Diakinesis stage

A

-transition to metaphase, during which the chromosomes become fully condensed

93
Q

Metaphase 1

A
  • Bivalent chromosomes align on the spindle
  • Kinetochore of sister chromatids are oriented in the same direction; kinetochores of homologous chromosomes are pointed toward opposite spindle poles
  • microtubes from the same pole of the spindle attach to sister chromatids
  • microtubules from opposite poles attach to homologous chromosomes
94
Q

Anaphase 1

A

the chiasmata are disrupted and homologous chromosomes separate;sister chromatids remain attached.

95
Q

Meiosis 2

A
  • starts immediatly after cytokinesis, usually before the chromosomes have fully decondensed.
  • resembles mitosis
96
Q

Where can oocytes remain arrested in meiosis?

A

diplotene stage of meiosis 1 for up to 50 years in humans

  • during which chromosomes decondense and are actively transcribed
  • most vertebrate oocytes are arrested again at metaphase 2 until fertilization
97
Q

Oocytes

A

-oocytes grow very large and stockpile materials for early embryonic growth

98
Q

When do most oocytes resume meiosis after being arrested?

A

Oocytes resume meiosis in response to hormonal stimulation and proceed through meiosis 1 prior to fertilization

99
Q

Polar body

A

-results because cell division after meiosis 1 is asymmetric

100
Q

Control of meiosis of oocytes by activity of Cdk1/cyclin B

A
  • by Cdk1/cyclin B complexes
  • hormonal stimulation activates Cdk1/cyclin B and results to progression into metaphase
  • the levels of Ckd1/cyclin B determine progression to the next stages
101
Q

How is inhibition of cyclin B degredation mediated?

A

-mediated by inhibition of APC/C by Emi2/Erp1,

102
Q

Emi2/Erp1

A
  • phosphorylated by Rsk

- inhibits APC/C via interaction with Cdc20

103
Q

Fertilization

A
  • The sperm binds to a receptor on the egg surface and fuses with the egg plasma membrane
  • mixes paternal and maternal chromosomes and induces changes in the egg cytoplasm important for further development
104
Q

Fertilization Process

A
  1. binding of a sperm to its receptor signals an increase in Ca2+ levels in the egg cytoplasm
  2. secretory vesicles release materials that coat the egg and block entry of additional sperm (ensures a nml diploid embryo
  3. increases Ca2+also signals completion of meiosis
  4. asymmetric cytokinesis gives rise to a second small polar body
  5. pronuclei replicate their DNA as they migrate toward each other
  6. as pronuclei meet, the zygote enters M phase of the 1st mitotic division, chromosomes align on one spindle
  7. completion of mitosis gives rise to 2 embryonic cells, each containing a new diploid genome
105
Q

How many haploid nuclei are present after the completion of meiosis in the zygote?

A

-after completion of meiosis, the fertilized egg (zygote) contains 2 haploid nuclei (pronuclei) one derived from each parent.

Molecular Cell Biology (16 decks)

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