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Cell Biology and Biochemistry > 10 Cell Division and Cell Cycle > Flashcards

10 Cell Division and Cell Cycle Flashcards

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1
Q

what are the stages of interphase

A
  • G0
  • G1
  • S phase
  • G2
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2
Q

What are the stages of M-phase

A
  • prophase
  • prometaphase
  • metaphase
  • anaphase
  • telophase
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3
Q

What happens in G1 and G2

A

G1 - is the interval between DNA replication and mitosis where the cell is metabolically growing and active
G2 - the cell continues to grow and proteins are synthesised in preparation for mitosis

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4
Q

What occurs in the S phase

A
  • DNA replication
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5
Q

how are embryonic cells different from regular cells

A
  • they have no growth phase and just divide
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6
Q

What is the significance of the start point in the cell cycle

A
  • This is the first restriction point found in late G1
  • The cell must have the right extracellular growth factors to pass through Start
  • Once passed the restriction point the cell is committed to complete the rest of the cell cycle
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7
Q

What happens if growth factors arn’t present when a cell reaches the restriction point ‘start’

A
  • progression of cell cycle stops and cells enter a resting stage called G0
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8
Q

Which type of cells are arrested at G0 and why

A
  • skin fibroblasts

- are held at G0 until stimulated by platelet derived growth factors to proliferate and repair wounds

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9
Q

What are the roles of the cell cycle checkpoints

A
  • to ensure the cell has completed its phase before moving onto the next
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10
Q

What happens if DNA damage checkpoints detect DNA damage

A
  • the cell cycle is arrested until the DNA is repaired or replicated
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11
Q

Where do the DNA damage checkpoints occur

A
  • at the first restriction point ‘start’ in late G1
  • in mid S phase
  • In late G2
  • In late M-phase
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12
Q

What is a spindle assembly checkpoint

A
  • stops mitosis at metaphase if chromosomes are not properly aligned on the spindle
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13
Q

How does the cell ensure that the DNA is replicated only once in S-phase?

A
  • MCM helicase proteins bind to origins of replication with ORC (Origin recognition complex) proteins and are required for the initiation of DNA replication
  • Once initiation has occurred the MCM proteins are displaced from the origin to replication doesn’t occur again
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14
Q

Describe how DNA replication is initiated in the S-phase

A
  • MCM helicase proteins bind to ORC (origin recognition complex) proteins which are found on DNA
  • MCM sit on either side of the ORC protein on the DNA
  • DNA polymerase comes along looking for a place to start DNA replication with MCM proteins
  • A DNA polymerases initiate DNA replication on each open strand between the ORC and the MCM helicase. the MCM proteins drop off so no more DNA polymerase can bind
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15
Q

What are the 4 levels of regulation within the cell cycle

A

1- cyclin/CDK complex formation
2- Activation of CDK’s by phosphorylation by CAK protein
3- Inhibitory phosphorylation by Wee1 protein
4- Binding of CDK inhibitors (CKI’s) eg, Ink 4a proteins

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16
Q

How can we identify cells in different phases of interphase

A
  • Incubate the cells with a fluorescent dye that binds to the DNA. The fluorescence intensity is measured by a flow cytometer/fluorescence activated cell sorter
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17
Q

What does the graph that teh flow cytometer produce look like

A
  • plots number of cells (Y) against amount of DNA per cell (X)
  • there is a large peak at 2n as most cells are in G1 phase
  • There is a smaller peak at 4n as a lot of cells are in G2 and M-phase with double the amount of DNA (tetraploid)
  • the small dip in between the 2 peaks indicates s-phase where some cells have between 2n and 4n DNA copies
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18
Q

What do protein Kinases do in terms of controlling cell cycle

A
  • they trigger the major cell cycle transition (work by adding phosphate groups)
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19
Q

Give 3 experiments where proteins have been introduced and caused progression in the cell cycle

A

1) frog oocytes that were arrested in G2 until hormonally stimulated to enter M-phase by a kinase
2) Genetic analysis of yeast, investigators found temperature sensitive mutants that were effective in cell cycle progression
3) Protein synthesis in early sea urchin embryos

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20
Q

Describe the frog oocyte experiment:

A
  • researchers found that oocytes could be induced into the M-phase by microinjecting the cytoplasm of oocytes that had been hormonally stimulated
  • The cytoplasmic factor responsible was MPF (maturation promoting factor)
  • So MPF acts as a general regulator of the G2 to M phase transition
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21
Q
  • Describe the genetic analysis of yeast:
A
  • CDC genes are required for passage through start and entry into mitosis as they encode for protein kinase
  • they found temp sensitive CDC mutants in yeast which stopped the cell passing start to begin mitosis if the temp is not right
  • so CDC genes are needed for cell cycle
  • the gene codes for protein kinase now known as CDK1 and is a regulator in all eukaryotic cells
22
Q
  • Describe the protein synthesis in sea urchin experiment:
A
  • Scientists identified 2 cyclin proteins that accumulate through interphase but rapidly degrade at the end of mitosis. Suggesting a role in inducing mitosis
  • They later injecting cyclin A into a frog ooctye and it induced mitosis
23
Q

Describe the structure of MPF

A
  • composed of Cyclin B and CDK 1
24
Q

What happens if cyclin B is not bound to CDK1

A
  • CDK1 cannot phosphorylate molecules/proteins and cell cycle does not progress
25
Q

What happens to MPF in G2

A
  • CDK1 in the MPF complex is phosphorylated and inhibited, leading to accumulation of inactive CDK1/cyclinB complexes in G2
26
Q

Cyclin B is degraded by ubiquitin mediated proteolysis, what does this mean

A
  • This is where cyclin gets covered by lots of little ubiquitin molecules which make it a target for proteolysis (protein breakdown)
27
Q

How does cyclin B degradation lead to the cytokinesis

A
  • degredation of cyclin B renders CDK1 in the MPF complex inactive
  • this causes the cell to exit via mitosis and undergo cytokinesis
28
Q

What us kinetichore?

A

It joins sister chromatids to mitotic spindles in mitosis

29
Q

In between which 2 phases of mitosis does the spindle assembly checkpoint occur

A
  • In between metaphase and anaphase
30
Q

Describe the Ras/Ras/MEK/ERK pathway

A
  • Growth factors bind to an extracellular receptor which begins the chain of phosphorylation
  • Ras phosphorylates Raf, which phosphorylates MEK which phosphorylates ERK
  • ERK promotes protein synthesis of Cylcin D1
31
Q

What is the role of cyclin D1

A
  • forms a complex of CDK4,6/Cyclin D1 which drives the cell passed the restriction point
32
Q

What happens if cyclin D1 stops being synthesised

A
  • It rapidly regrades so the intracellular conc of it would reduce quickly
33
Q

What can defects in Cyclin D1 cause

A
  • loss of growth regulation, so out of control cell growth like cancers
34
Q

What is Rb and its relation to cancer

A
  • Rb is a tumour suppressor gene (prototype) and its inactivation or mutation leads to tumour growth
  • Proteins encoded by this suppressor gene act as breaks slowing down cell cycle
35
Q

Where was Rb first discovered

A
  • Rb was first discovered in a retinoblastoma tumour where it got its name Rb
36
Q

How does Rb prevent cell cycle

A
  • In G0 or early G1, Rb binds to E2F transcription factors forming an Rb-E2F complex which suppress the expression of genes involved in cell cycle progression
  • When Rb is bound to E2F it holds E2F away from the DNA stopping gene expression
37
Q

How does cell cycle continue in the presence of Rb

A
  • Rb gets phosphorylated by CDK4,6/Cyclin D, it dissociates from the E2F
  • This means gene expression + transcription of proteins involved in the cell cycle can continue
38
Q

The activation of which proteins mediates progression through the first restriction point

A
  • The activation of CDK2/Cylcin E complexes
39
Q

How are CDK2/cyclin E complexes deactivated and reactivated

A
  • In G0 and G1 an inactivator protein called P27 is bound to the complex
  • the P27 protein is removed in S phase
40
Q

What causes the synthesis of Cyclin E

A
  • Cyclin E synthesis is stimulated after E2F has been dissociated from Rb due to phosphorylation so transcription of cell cycle genes can continue
41
Q

What inhibits the transcription of P27 inactivator protein

A
  • growth factors
42
Q
  • What does the activation of CDK2/Cyclin E complexes do?
A
  • This leads to the activation of MCM helicase and initiation of DNA replication
43
Q

Summarise the process of cell cycle from the stimulation of growth hormones to the DNA replication

A
  • Gowth factors bind to a receptor on the cell surface initiating the Ras/Raf/MEK/ERK pathway
  • This stimulates the production of Cyclin D1 and progression through the restriction point
  • Cyclin D1/CDK4,6 complexes phosphorylate Rb/E2F complexes so E2F can transcribe Cyclin E
  • Cyclin E/CDK2 complexes activate MCM helicase to start DNA replication
44
Q

Which proteins mediate cell cycle arrest at DNA damage checkpoints

A
  • protein kinases: ATM and ATR
45
Q

Describe how and what happens when ATM or ATR are activated at a DNA damage checkpoint

A
  • ATM or ATR recognise a double or single strand break in DNA
  • They activate a signalling pathway that leads to cell cycle arrest/DNA repair or Cell death
46
Q

How are ATM and ATR different

A

ATM recognises double strand breaks

ATR recognises single strand breaks or unreplicated DNA

47
Q

What proteins do ATR and ATM activate once they have been activated

A
  • ATR activates Chk1 via phosphorylation

- ATM activates Chk2 via phosphorylation

48
Q

How do the protein Chk1 and Chk2 cause cell arrest

A
  • they phosphorylate and inhibit Cdc25 phosphatases which are needed to activate Cdk1 and Cdk2
  • inhibition of Cdk1 results in arrest in G2
  • inhibition of Cdk2 results in arrest in G1 and S phase
49
Q

In mammalian Cells what protein mediates G1 arrest

A
  • P53

- which is phosphorylated by both ATM and Chk2

50
Q

How can increased expression of P53 cause cell cycle arrest

A
  • P53 promotes increased expression of P21

- P21 inhibits Cdk2/Cyclin E complexes leading to cell cycle arrest

Cell Biology and Biochemistry (9 decks)

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