Cell Biology Ch 17

In eukaryotic cells, progression through the cell cycle is controlled by?

protein kinases

What is the common cause of the abnormal proliferation of cancer cells?

Defects in cell cycle regulation

The division cycle of most cells consists of four coordinated processes representing 2 main stages. What are they?

Interphase + M phase

What processes happen in interphase?

- Cell growth (G1)

- DNA replication (S phase)

- More growth, and protein synthesis/ preparation for mitosis (G2)

What happens in M phase?

- Mitosis (nuclear division)

- Cell division (cytokinesis)

How does cell growth and DNA replication take place in bacteria?

throughout most of the cell cycle

Duplicated chromosomes are distributed to daughter cells in ________ with the plasma membrane

association

What is duplicated and spreads to opposite ends of the growing cell in bacterial cell cycle?

the ORI

What are the phases of the cell cycle in eukaryotes?

M, G1, S, and G2

Describe the M phase in eukaryotic cell cycle.

Mitosis (nuclear division), usually ending with cell division (cytokinesis)

Describe interphase in eukaryotic cell cycle.

period between mitoses, divided into G1, S, and G2

Describe G1 phase in eukaryotic cell cycle.

interval between mitosis and DNA replication. The cell is metabolically active and growing

Describe S phase in eukaryotic cell cycle.

most DNA replication takes place

Describe G2 phase in eukaryotic cell cycle.

cell growth continues; proteins are synthesized in preparation for mitosis

How long does a human cell proliferation typically take? What is the split?

- 24hrs

- G1=~11h; S=~8h; G2=~4h; M=~1h

Do early embryos have a growth phase?

no

Progression of cells through the division cycle is regulated by both ______ and ________ signals.

extracellular; internal

What regulates ceullular processes?

control points

What is the major control point?

START

What organism uses START?

yeast

What is the function of START?

controls progression from G1 to S in yeast cells

What happens once cells pass START?

they are committed to entering S phase and undergoing one division cycle

Passage through START is highly regulated by what factors?

nutrient availability, cell size, and mating factors

In most animal cells, the restriction point in late G1 is _________ to START in yeast.

analogous

Passage through the restriction point is regulated by extracellular growth factors. What are they?

nutrient availability and cell size

What happens once a cell passes the restriction point?

the cell is committed to proceed through S phase and the rest of the cell cycle

What organism uses restriction points?

animal cells

What happens when appropriate growth factors are not present in G1?

cells enter a resting stage called G0

What is the resting stage (G0) also called?

quiescence

Describe cells following through with quiescence.

are metabolically active, but slow growth and may remain in this stage permanently or may resume proliferation.

How are skin fibroblasts arrested in G0 able to be stimulated by growth factors to divide and repair damage?

Through PDGF (platelet-derived growth factor)

proliferation of most cells is regulated in what phase?

G1

True or false: some proliferation of cells is regulated in G1 as well as G2.

true

What is the purpose of cell cycle checkpoints?

They prevent entry into the next phase until events of the preceding phase have been completed

What is the purpose of DNA damage checkpoints?

ensure that damaged DNA is not replicated and passed on to daughter cells

What is the purpose of the Spindle assembly checkpoint?

stops mitosis at metaphase if chromosomes are not properly aligned on the spindle

How can frog oocytes arrested in G2 be induced to enter M phase? What does this mean?

by microinjection of cytoplasm from a hormonally treated oocyte

- A cytoplasmic factor present in the hormone treated cells was sufficient to trigger the transition

What is the cytoplasmic factor present in the hormone-treated cells that triggered the transition of oocytes from G2 to M phase called?

MPF (maturation promoting factor)

What does two proteins (cyclins) accumulating throughout interphase but are rapidly degraded at the end of each mitosis suggest?

a role in inducing mitosis

eukaryote cell cycles are controlled by a conserved set of?

protein kinases

What is the purpose of protein kinases?

trigger the major cell cycle transitions

What is an example of protein kinases triggering major cell cycle transitions?

Cdk1 was first identified in yeast, has since been shown to be a cell cycle regulator conserved in all eukaryotes

What does Cdk1 mean?

cyclin-dependant kinase 1

MPF (maturation promoting factor) was purified and shown to be composed of?

Cdk1 and cyclin B

Describe Cyclin B.

a regulatory subunit required for catalytic activity of the Cdk1 protein kinase

Cdk1/cyclin B is activated as a result of what?

specific dephosphorylation by Cdc25

What does the specific dephosphorylation of Cdk1/cyclin B by Cdc25 activate?

Cdk1 and triggers the cell to enter mitosis

Activation of Cdk1 results in the degradation of?

cyclin B

Ubiquitination of cyclin B is mediated by a ubiquitin ligase called?

anaphase-promoting complex/cyclosome (APC/C)

In higher eukaryotes, there are multiple cyclins and multiple Cdk1-related protein kinases. What are they known as?

Cdk's for cyclin-dependent kinases

The activity of Cdk's is regulated by multiple mechanisms. What are they?

- Association of Cdk's and cyclin partners

- Binding of Cdk inhibitors (CKIs)

Describe the association of Cdk's and cyclin partners to regulate the activity of Cdk's.

Formation of specific Cdk/cyclin complexes is controlled by cyclin synthesis and degradation

What inhibitory proteins (Cdk inhibitors) are involved in regulating the activity of Cdk's in mammalian cells?

Ink4 and Cip/Kip

InK4 family inhibits what phase of the cell cycle?

G1

Cip/Kip inhibits what phases of the cell cycle?

G1, S, G2

What are D-type cyclins?

the link between growth factor signaling and cell cycle progression

Growth factors stimulate cyclin D synthesis through what pathway?

Ras/Raf/MEK/ERK pathway

Cyclin D is synthesized as long as ______ ______ are present

growth factors

Synthesis of Cyclin D takes place where?

G1

Defects in cyclin D regulation could contribute to what?

the loss of growth regulation characteristic of cancer cells

What is Rb?

a substrate protein of Cdk4, 6/cyclin D complexes, and is frequently mutated in many human tumors

How was Rb first identified?

in retinoblastoma, a rare inherited childhood eye tumor

Describe Rb in terms of gene expression.

the prototype tumor suppressor gene

What is a prototyle tumor suppressor gene?

a gene whose inactivation leads to tumor development

Proteins encoded by tumor suppressor genes (including Rb and Ink4 Cdk inhibitors) act as?

brakes that slow down cell cycle progression

In the Cdk2/cyclin E pathways, in G1, Cdk2/cyclin E complexes are inhibited by?

the Cdk inhibitor p27

In the Cdk2/cyclin E pathway, passage through the restriction point induces the synthesis of _____ __ via activation of _____.

cyclin E; E2F

growth factor signaling in the Cdk2/cyclin E pathway inhibits synthesis of?

p27

In the Cdk2/cyclin E pathway, as Cdk2 becomes activated, it phosphorylates and targets p27 for degradation. What does this result in?

further activation of Cdk2/cyclin E complexes

In the Cdk2/cyclin E pathway, Cdk2/cyclin E also inhibits the

APC/C ubiquitin ligase. What is the result?

prevents cyclin E degradation

What is the function of Cdt1 and Cdc6?

mediate recruitment of MCM hexamers (helicase components) to the ORC (origin recognition complex)

What initiates DNA replication in the S phase?

Cdk2/cyclin E and the DDK protein kinase

What mediates DNA damage checkpoints?

protein kinases ATM and ATR

What do ATM and ATR activate?

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

What does ATR recognize?

single-stranded breaks or unreplicated DNA

What does ATM recognize?

double-strand breaks

ATM and ATR phosphorylate and activate which checkpoint kinases?

Chk1 and Chk2

Chk1 and Chk2 phosphorylate and inhibit what phosphatase?

Cdc25

What is the function of Cdc25?

required to activate Cdk1 and Cdk2

What does Inhibition of Cdk2 result in?

cell cycle arrest in G1 and S

What does inhibition of Cdk1 result in?

arrest in G2

What is arrest mediated by in mammalian cells?

p53

What phosphorylates p53

ATM and Chk2

True or false: p53 is a transcription factor.

true

What does increased levels of p53 lead to?

induction of Cdk inhibitor p21

What does p21 inhibit?

Cdk2/cyclin E or A complexes, leading to cell cycle arrest

What does loss of p53 result in?

prevents cell cycle arrest in response to DNA damage, so the damaged DNA is replicated and passed on to daughter cells

Describe prophase.

condensing of chromosomes, formation of sister chromatids, mitotic spindles form from centrosomes organizing the microtubules

Describe prometaphase.

- transition between prophase and metaphase. Microtubules attach to the condensed chromosomes, which takes time and shuffling of chromosomes

Describe metaphase.

Alignment of sister chromatids at the metaphase plate

Describes anaphase.

triggered by the breakage of the links between sister

chromatids, which separates and move to their respective poles

Describe telophase.

reformation of nuclei, and decondensing of chromosomes, followed by cytokinesis

How does Cdk1/cyclin B act as a master regulator of M phase?

by activating of mitotic protein kinases (aurora kinases; polo-like kinases) and directly phosphorylating some of the structural proteins involved in this cellular reorganization

What are condensins?

Condensation of chromatids driven by protein complexes

Where do condensins occur?

During prophase

What is the function of cohesins?

provide the physical cohesion between sister chromatids to allow for the proper alignment and timing of anaphase

Activation of cohesins and condensins is driven by?

cdk1 and Aurora B kinase (condensins) and Aurora B and Polo-like kinases (cohesins)

What is Cdk1/cyclin B in relation to the nuclear envelope?

breakdown by phosphorylating lamins and other nuclear proteins

In Spindle Assembly Checkpoint, progression to anaphase is mediated by activation of?

APC/C ubiquitin ligase

In Spindle Assembly Checkpoint, what does unattached kinetochores lead to?

the assembly of a protein complex (the mitotic checkpoint complex, MCC) that inhibits APC/C