Cell Bio Ch. 18: The Cell Division Cycle

4 phases of the cell cycle

1. G1: preparation, growth, protein synthesis, organelle duplication

2. S: DNA replication

3. G2: ensure replication is proper before mitosis

4. M: mitosis (nuclear division) and cytokinesis

3 major checkpoints

1. G1-S: are we ready to duplicate?

2. G2-M: is all DNA replicated and repaired before we can divide?

3. Anaphase: are all the sister chromatids aligned correctly and attached to mitotic spindle?

what other eukaryotic cells are most used to study the cell cycle

yeast cells

CDKs and cyclins

CDKs: cyclically activated protein kinases that control cell cycle

• not always active/phosphorylating

• cyclins: proteins that bind and activate CDK

  • regulating cyclin concentration regulates CDKs

general cell checkpoint concept: different cyclin-CDK complexes

different complexes of cyclins and kinases trigger different steps in the cell cycle

G1-S and G2-M checkpoints are regulated by what which complexes?

G1-S: active G1/S or S cyclin-CDK complexes

G2-M: active M cyclin-CDK complex

proteolysis (what it is and what effects)

degradation of proteosomes to breakdown cyclin, preventing cell cycle (transcription) from continuing

• silencing CDKs through proteolysis decreases cyclin concentration

how do proteins end up in the proteasome for degradation?

1. active CDK is bound to cyclin

2. tagging with ubiquitin and DAG silences the complex

3. tagging signals protein to be sent to proteasome for proteolysis

4. proteasome degrades cyclin and inactivates CDK to stop cell cycle

APC (anaphase promoting complex)

adds ubiquitin

• promotes the separation of sister chromatids

• wants to ensure cell doesn’t repeat mitosis

• regulates the M-CDK complex (stop cell from entering M)

indirectly promotes anaphase and mitosis, but also prevents cell cycle from re-entering mitosis in the same cycle

how do cyclin-CDK complexes depend on phosphorylation? (Wee1 and Cdc25)

kinases are inhibitory

• phosphorylation inactivates CDKs

• Wee1 phosphorylates CDK

phosphatases are activators

• dephosphorylation activates CDKs

• Cdc25 reverses the action of Wee1

phosphorylation determines whether or not a cyclin-CDK complex is active

If I have a CDK bound to a cyclin, do I know for sure the CDK is active?

NO

Need know if it is phosphorylated or not

three regulations of CDK

1. controlling cyclin concentration

2. (de)phosphorylation of CDK

3. inhibitory proteins

inhibitor proteins on CDK

important for stopping and fixing errors

• bind onto CDK complex like a clamp to inactivate the complex

• p27, p21, p58 are inhibitory proteins

3 ways the cell cycle control system can pause the cycle

1. G1-S: CDK inhibitors (p21,27,58) block entry to S phase

2. G2-M: kinases (Wee1) and phosphatases (Cdc25)

3. Anaphase: ubiquitylation driven by APC

G₀

• rest stop: slow down and assess

• cell can hold here for a while to fix or go through apoptosis

terminal differntiation

cell doesn’t want to divide anymore

• resting favored

• CDKs indefinitely silenced

• cyclin concentration wiped

G₁

cell grows, replicates organelles, makes proteins (cyclins, CDKs), prepares for replication

mitogens

external chemical ligand that promotes cell cycle (proliferation) and triggers G1

promote the production of cyclins that stimulate cell division

mitogens and Rb protein

1. mitogen activates a receptor that activates an intracellular signaling pathway

2. G1-CDK and G1/S-CDK is activated to phosphorylate the active Rb

3. phosphorylation inactivates the Rb and it releases from the transcription regulator, thus activating it

4. transcription can take place, leading to cell proliferation

retinoblastoma

mutant Rb (or none)

constant transcription due to constantly activated transcription regulator causing cancer in the eye

what happens when DNA is damaged

1. DNA damage signals kinases that phosphorylate p53 and activate it

2. p53 binds and activates p21 CDK inhibitor protein

3. p21 inactivates G1-S CDK / S CDK

Damaged DNA halts cell cycle progression to fix

what happens when S initiates?

S-CDK initiates DNA replication and blocks re-replication

DNA is replicated using DNA Polymerase III between two unwinding strands

How is the replication is S phase controlled?

need brakes to ensure DNA replicates only once so sister chromatids are equally separated

1. G1:ORC (origin recognition complex) region with Cdc6 bound

2. as cyclin complexes activate, the Cdc6 dissociates while 2 molecules of helicase bind to the lift of ORC to form pre-replicative complex

3. S: S-CDK phosphorylates ORC and allows helicase to separate strands for DNA Pol III

role of Cdc6

Cdc6 is bound at ORC during G1

dissociates so helicase can bind to initiate S replication

incomplete replication, role of Cdc25

arrests cell in G2

• Cdc25 (dephosphorylates M-CDK and activates it) gets locked by phosphorylation of CDK by Wee1

condensins and cohesins

condensins: compact chromosomes and make them visible

cohesins: keep sister chromatids together until anaphase

cytoskeleton 2 roles in mitosis

1. mitotic spindle (form centrosome MT)

2. contractile ring (actin + myosin)

mitotic spindle

form centrosomes (2 on opposite sides)

• usually we only have 1 so in S they replicate

• initially they are very close, but they move to other sides to make spindle

• aster microtubules come out like a star

cell cycle steps

1. interphase

2. prophase

3. prometaphase

4. metaphase

5. anaphase

6. telophase

7. cytokinesis

interphase

G1, S, G2

• cell grows in size

• DNA and centrosome replicates

prophase

• duplicated chromosomes condense

• mitotic spindle assembles as centrosomes move apart

prometaphase

• nuclear envelope breaks down

• chromosomes attach to spindle via kinetochores

metaphase

• chromosomes align on equator of spindle

• kinetochore MTs attached to kinetochores on each side of sister chromatid

anaphase

• sister chromatids separate and pull toward each pole of mitotic spindle

• two ways they are pulled apart

  • kinetochores shorten and pull

  • centrosomes go further apart

• sisters must be attached to avoid aneuploidy

  • trisomy 21: Down Syndrome

telophase and cytokinesis

• have 2 complete sets of chromosomes

• nuclear envelope reassembles around each

• contractile ring squeezes cell to divide into 2

how do centrosomes duplicate

1. G1: there’s only one centrosome

2. S: centrosomes replicate

3. M: asters form (star-like MTs)

• centrosomes start moving apart to poles

• mitotic spindle forms

• spindles attach to chromosomes

three types of mitotic spindle microtubules

1. kinetochore MTs: attach to kinetochores on chromosomes to pull sister chromatids apart

2. aster MTs: star shape; anchore spindle to cell cortex to position division

3. interpolar MTs: overlap in middle to push poles apart

what action is needed to separate sister chromatids?

APC action required

• APC tags with ubiquitin to be sent to proteosome

• cohesins break down through proteolysis to allow separation

how chromosomes separate in Anaphase A then Anaphase B

A: kinetochore MTs shorten

B: interpolar MTs push poles apart

spindle assembly checkpoint

if attachment is incomplete, anaphase will halt

• unattached chromosomes send a stop signal

• APC inhibited

• sister chromatids stay together

nuclear envelope reassembly in telophase

• nuclear lamins/proteins are dephosphorylated (originally phosphorylated by M-CDKs in prometaphase)

• reassemble around new chromosomes

what determines where the contractile ring will form?

mitotic spindle

what is the contractile ring made of

actin and myosin

they are perpendicular to the plane of cleavage

organelle separation during cytokinesis

nonspecific; organelles can vary by number in cells

• ER

  • is a part of nuclear envelope

  • attaches and rearranges with the microtubule cytoskeleton

  • remains whole then MTs release the ER in daughters during cytokinesis

• Golgi

  • cisternae fragments separate and rearrange to form independent golgi