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Cell Biology (BY1CB1) > Week 8 (Cell Cycle) > Flashcards

Week 8 (Cell Cycle) Flashcards

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1
Q

What are the stages in a typical cell cycle?

A

G1 (Gap1- Growth)
S (DNA synthesis)
G2 (Gap2- Growth)
M (Mitosis)

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2
Q

Variations in Cell Cycles: Neurons Exit

A

Post-mitotic neurons exit into G0
and usually stop cycling.

Neurons stop cycling*

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3
Q

Variations in Cell Cycles: Stem Cells Are Quiescent

A

LT-HSC: Long Term Hematopoietic Stem Cell, IT: Intermediate, ST: short term
Quiescent —-> ‘quiet’
Not dividing

Stem cells can go into G0 and come out of it (a state of dormancy)

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4
Q

Variations in Cell Cycles: Senescent Cells Stop Dividing and Permanently Exit the Cell Cycle

A

Senescent cells stop dividing

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5
Q

How is proliferation measured directly?

A

To determine if cells in a tissue are proliferating:

 measure the incorporation of 3H-Thymidine
 or bromodeoxyuracil (BrdU) into the DNA.
 after which the tissue can be fixed and prepared for          labeling with fluorescent anti-BrdU antibodies

Application Note: Only cells currently in S phase
will be labeled by BrdU.*

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6
Q

Applying Fluorescence Activated Cell Sorter (FACS) Analysis to the Cell Cycle

A

1) Cells are stained with a dye that becomes
fluorescent when it binds to DNA.

2) The amount of fluorescence is directly proportional
to the amount of DNA.

3) The FACS measures the level of fluorescence of
individual cells.

4) The relative amount of fluorescence (DNA content)
can be plotted on the X-axis against the number
of cells on the Y-axis. A high peak means that lots of cells
are in that cell cycle phase.

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7
Q

Can the cell cycle be altered?

A

Under different conditions, cells cycle
at different rates and spend more or less
time in G1, S, & G2 .

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8
Q

What is a checkpoint?

A

A checkpoint is a mechanism that tells the
cell whether or not it is safe to proceed to
the next stage of the cell cycle.

Checkpoints send negative signals to the cell cycle controller to inhibit the progression of the cell cycle.

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9
Q

Factors affecting Checkpoint Sensitivity

A
  • Cell size
  • Growth Factors
  • DNA damage
  • Incomplete DNA
  • Replication
  • Centrosome Integrity (centrioles)
  • Spindle Alignment
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10
Q

What is The Cell Cycle Controller is Composed of?

A

Protein Kinases and Ubiquitin Ligases

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11
Q

What do checkpoints inhibit?

A

Checkpoints inhibit both protein kinases and ubiquitin ligases.

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12
Q

What is a kinase?

A

An enzyme that attaches phosphate

groups to its substrate

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13
Q

What is an enzyme?

A

a substance (usually protein) that
increases the rate of a chemical reaction
without itself being changed in the process.
At the end of the reaction, the enzyme is
in the same state as before.

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14
Q

What is a substrate

A

the substance that is transformed

by the enzyme.

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15
Q

What is a protein kinase?

A

A protein kinase is an enzyme that substitutes phosphate groups for the hydroxyl groups on amino acids in proteins

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16
Q

Protein kinase: What is a serine/threonine kinase?

A

A serine/threonine kinase substitutes a phosphate group for the hydroxyl group on serine or threonine

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17
Q

Protein kinase: What is a tyrosine kinase?

A

substitutes a phosphate group for the hydroxyl group on tyrosine

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18
Q

Why Phosphorylate?

A

The large phosphate group introduces two negative charges that can change the structure and activity
of proteins.

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19
Q

Is phosphorylation reversible?

A

The phosphorylation is reversible (phosphate added with a kinases and removed with a phosphatase) so the
structural change is reversible–ideal for
regulating function

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20
Q

Is the degradation of proteins essential to the

cell cycle?

A

Yes

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21
Q

What is the degradation of cell cycle proteins is

mediated by?

A

ubiquitin ligases.

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22
Q

What are ubiquitin ligases?

A

Specific target proteins including cyclins are

marked for destruction by the covalent attachment of chains of ubiquitin by enzymes called ubiquitin ligases.

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23
Q

What is ubiquitin?

A

Ubiquitin is a 70 amino acid protein that acts
as a flag to target the ubiquitinylated protein to the
proteasome.

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24
Q

What is the proteasome?

A

The proteasome is a multi-subunit complex
of about 20 subunits that forms a hollow
cylinder. Proteasomes unfold proteins and
digest them into short peptides.

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25
Q

How long is a typical cell cycle?

A

24 hours

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26
Q

Do frog cell cycles have growth phases?

A

No (they are already large)

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27
Q

Can FACS distinguish between G2 and Mphase?

A

No because both have 4n in terms of DNA content per cell

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28
Q

What is a checkpoint for G1?

A
  • Damaged DNA

- Unfavourable extracellular environment

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29
Q

Why do you never go backwards in the cell cycle?

A

Ubiquitin ligases degrade proteins thus not allowing you to go backwards

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30
Q

What is a checkpoint for S phase?

A

Damaged or incompletely replicated DNA

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31
Q

What is a checkpoint for G2?

A

Damaged or incompletely replicated DNA

32
Q

What is a checkpoint for M phase?

A

Chromosome improperly attached to mitotic spindle

33
Q

Explain the action of ubiquitin

A

The enzymes couples its carboxyl end to the amino group of a lysine side chain in a target protein molecule

34
Q

Explain the action of proteasomes

A

-The complex cap (also called the regulatory particle) selectively binds proteins that have been marked by ubiquitin for destruction; it then uses ATP hydrolysis to unfold their polypeptide chains and feed them through a narrow channel into the inner chamber of the cylinder for digestion to short peptides

35
Q

Explain how proteasomes with the help of ubiquitin destroy proteins

A
  • The proteasome cap recognizes proteins marked by a polyubiquitin chain , and subsequently translocates them into the proteasome core, where they are digested.
  • At an early stage, the ubiquitin is cleaved from the substrate protein and is recycled.
  • Translocation into the core of the proteasome is mediated by a ring of ATPases that unfold the substrate protein as it is threaded through the ring and into the proteasome core
  • The proteasome cap includes a ubiquitin receptor, which holds a ubiquitylated protein in place while it begins to be pulled into the proteasome core, and a ubiquitin hydrolase, which cleaves ubiquitin from the doomed protein
36
Q

What are the duplicated chromosomes like at the end of S phase?

A

The DNA molecules in each pair of duplicates Ed chromosomes are intertwined and held tightly together by specialised protein linkages (cohesion’s)

37
Q

What are chromosomes like in prophase?

A

The two DNA molecules are gradually disentangled and condensed into pairs of rigid, compact rods called sister chromatids which remain linked by sister chromatids cohesion’s

38
Q

What happens in prometaphase?

A

Nuclear envelope disassembles

39
Q

What stage of mitosis do sister chromatids attach to opposite poles of the spindle and align at the spindle equator?

A

Metaphase has

40
Q

What is the motor if spindle?

A

A giant bipolar array of microtubules

41
Q

When does the destruction of sister chromatids cohesion occur?

A

At the start of anaphase

The sister chromatids are separated which are pulled to opposite poles of the spindle

42
Q

What phase of mitosis is the spindle disassembled and the segregated chromosomes packaged into separate nuclei?

A

Telophase

43
Q

What occurs during cytokinesis?

A

The cell is cleaves in two, so each daughter cell inherits one of two nuclei

44
Q

What are the 4 sequential phases of the eukaryotic cell cycle?

A

G1, S, G2 and M

45
Q

What are the stages of interphase?

A

G1, S and G2

46
Q

When does cell growth occur?

A

Throughout the cell, except during mitosis

47
Q

Why are the two gap phases more than simple time delays to allow cell growth?

A

They also provide time for cell to monitor the internal and external environment

48
Q

What is G0?

A

A specialised resting state before the cell resumes proliferation

49
Q

What is the restriction point?

A

A commitment point near the end of G1 and is known as start in yeast. After passing this point, cells are committed to DNA replication, even if extracellular signals that stimulate growth and division are removed

50
Q

What is another name for FACS?

A

Flow cytometer
This allows results to be obtained for a proliferating cell population where the DNA content of its individual cells is determined

51
Q

What happens as a result of CDK activity at the G2/M transition?

A

Increase in the phosphorylation of proteins that control chromosome condensation, nuclear envelope breakdown, spindle assembly and other events that occur during early mitosis

52
Q

What are the function of G1/S cyclins?

A

Activate Cdks in late G1 and thereby help trigger progression through start, resulting in a commitment cellcycle entry. Their levels fall in S phase

53
Q

What are the function of S cyclins?

A

They bind Cdks soon after progression through start and help stimulate chromosome duplication. S cyclin levels remain elevated until mitosis (through S and G2) and these cyclins also contribute to the control of some early mitotic events

54
Q

What are the function of M-cyclins?

A

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

55
Q

When do M- cyclin level fall?

A

Mid mitosis

56
Q

What causes partial activation of a CDK?

A

Binding of a cyclin

57
Q

What causes full activation of a CDK?

A

Phosphorylation of the T loop

58
Q

Explain the conflicting effects of Wee1 and CDC25?

A

Wee1 inhibits CDK activity (phosphorylation)

Cdc25 increases CDK activity (dephosphorylation)

59
Q

Describe the mechanism of action of CKI’s

A

P27 binds to both the cyclin and CDK complex distorting the active site of the CDK

60
Q

What is the function of APC?

A

Catalysed ubiquitylation of regulatory proteins involved primarily in exit from mitosis, including securin and S and M cyclins
It’s activator is CDC 20

61
Q

How is each origin fired once and only once?

A

S-CDK inactivates proteins at allow that origin to initiate DNA replication again

62
Q

What happens during prophase?

A
  • the replicated chromosomes, each consisting of two closely associated sister chromatids condense
  • outside the nucleus the mitotic spindle assembles between the two centrosomes , which have replicated and moved apart
63
Q

What happens during prometaphase?

A
  • it starts abruptly with the breakdown of the nuclear envelope
  • chromosomes can now attach to spindle microtubules via their kinetochores and undergo active movement
64
Q

What happens during metaphase?

A
  • chromosomes are aligned at the equator of the spindle midway between the spindle poles
  • kinetochore microtubules attach sister chromatids to opposite poles of the spindle
65
Q

What happens during anaphase?

A
  • the sister chromatids synchronously separate from two daughter chromosomes and each is pulled slowly toward the spindle pole it faces
  • the kinetochore microtubules get shorter and the spindle poles also move apart, both processes contribute to chromosome segregation
66
Q

What happens during telophase?

A
  • the two sets of daughter chromosomes arrive at the poles of the spindle and decondense
  • a new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis
67
Q

What does the division of the cytoplasm begin with?

A

Contraction of the contractile ring

68
Q

What happens during cytokinesis?

A
  • the cytoplasm is divided in two by a contractile ring of actin and myosin filaments which pinches the cell in two to create two daughters, each with one nucleus
69
Q

Explain the structure of the mitotic spindles?

A

-The core of the mitotic spindle is a bipolar array of microtubules, the minus ends of which are focused at the two spindle poles, and the plus ends of which radiate outward from the poles

70
Q

What are the plus ends of microtubules called?

A

Interpolar microtubules
They overlap with the plus ends of microtubules from the other pole, resulting in an antiparallel array in the spindle mid zone

71
Q

Where are the kinetochore microtubules attached?

A

To sister chromatids pairs at large protein structures called kinetochores which are located at the centromere of each sister chromatid

72
Q

When does centrosomes duplication begin?

A

The same time cells enter S phase

73
Q

When does spindle assembly occur?

A

Prophase

74
Q

What is a kinetochore?

A

A giant, multilayered protein structure that is built at the centromeric region of the chromatid

Two sister chromosomes will be attached to the plus ends of kinetochore microtubules during metaphase

75
Q

What is the function of APC?

A

Initiatester chromatin separation by ubiquitnylating several mitotic regulatory proteins and thereby triggering their destruction

76
Q

When does the loss of sister chromatid cohesion begin?

A

Anaphase

77
Q

Why is it only the cells in S phase that will be labelled with Brdu?

A

Only cells that are dividing will incorporating it into their DNA

Cell Biology (BY1CB1) (16 decks)

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