Cell bio chp. 18 - Cell-division cycle

general cell cycle

• 1: Cell growth chromosome duplication

• 2: Chromosome segregation

• 3: Cell div. for two daughter cells

Some eukaryotic cell cycle times

cell cycle time varies among cells

4 phases of cell cycle

• M phase: mitosis (nuclear div.) and cytokinesis (cytoplasmic div.)

• G1 phase: Cell growth and monitoring of internal and external environment to ensure readiness for S phase

• S phase: DNA replication

• G2 phase: Cell growth and monitoring of internal and external environment to prepare for M phase (checkpoint)

What makes up interphase

G1, S, and G2

cell cycle control system

Ensures key processes are in check to initiate next step in cycle. Is similar in all eukaryotes

locations of cell cycle control system checkpoints and what

• Before S phase: is cell environment ready to duplicate (Biggest one)

• Before M phase: Is DNA replicated and is DNA damage repaired

• in M phase: Are chromosomes properly attached at mitotic spindle to be pulled apart

What happens if chromosomes are not properly aligned and unevenly pulled apart

Could lead to down syndrome (trisomy)

(M checkpoint is important)

What is cell cycle control system dependent of

CDK’s

Cyclin dependent kinases

(Cell cycle is dependent on series of phosphorylation)

Cyclin

regulatory protein that binds to Cdks to control progression of cell cycle

CDK

Enzyme that activates when bound to cyclin. Triggers events in cell division cycle by phosphorylating target proteins

Are all cyclin-cdk complexes the same

No, they are specific to the step in the cell cycle they trigger

• S-Cdk

• M-CDK

How can you regulate CDK activity

regulating cyclin concentration

What regulates cyclin concentration

• transcription: regulate expression of cyclin

• proteolysis: regulate degradation of cyclin (cyclin must be tagged with ubiquitin first)

APC/C

Protein complex that triggers separation of sister chromatids and catalyzes the ubiquitylation of proteins that control cell div. cycle by stopping cell from re entering mitosis.

Anaphase promoting complex

(Not a CDK inhibitor)

cyclin degradation controlling CDK activity process

• Active CDK: cyclin binds to CDK

• Destruction of cyclin: ubiquitin chain sent by APC/C binds to cyclin, directing it towards a proteasome

• Inactive CDK: cyclin is degraded and no longer bound to CDK

What is the activity of Cyclin-CDK complexes dependant of

phosphorylation (inactivating complex) and dephosphorylation (activating complex)

Wee1

inhibitory kinase (adds phosphate to CDK complex)

Cdc25

activating phosphatase (dephosphorylates CDK complex)

M-CDK activation process

• M-CDK is formed but phosphorylated by Wee1 (inhibitory kinase)

• cyclin can bind to inactive M-CDK but it will still be inactive

• Cdc25 (activating phosphatase) will dephosphorylate the complex and activate it

CDK inhibitor proteins

Regulatory proteins that block binding or activity of cyclin-CDK complexes by directly binding to CDK

(APC is not a CDK inhibitor protein because it does not directly act on Cdk)

P27 inhibitory protein example

p27 binds to an active cyclin-CDK complex and inactivates it by preventing it from phosphorylating target proteins essential for progression through G1 to S

Ways that cell cycle control system pauses cycle

• At G1 checkpoint: Cdk inhibitors block entry into S phase (P27 ex.)

• At G2 checkpoint: inhibition of Cdc25 (activator) blocks entry into mitosis

• At M phase: inhibition of APC activation delays exit from mitosis

G1 = inhibitors

G2 = inhibit activator

M = inhibit APC

CDK activity during G1 phase

CDKs are inactivated for majority of the time to ensure that division does not immediately occur before spending time in G1

mitogens

extracellular signals that stimulate cell division

What happens if mitogens are not received by a cell

Cell cycle stays in G1 or will fall back into G0

Mitogen function

Switch on cell signaling pathway that stimulates the synthesis of G1-CDK and G1/S CDKs

(stimulates pathway necessary for DNA synthesis and chromosome duplication)

G1-CDK and G1/S-CDK function

relives negative controls that block progression and start of S phase

Rb protein

cell cycle negative control (inhibitory protein) that inhibits transcription regulators

phosphorylation and dephosphorylation of rb protein

• phosphorylated: inactivated

• dephosphorylated: activated

mitogen inhibiting rb protein process

• dephosphorylated rb protein inhibits transcription regulators

• mitogen binds to cell surface receptor triggering a signaling pathway

• signal pathway forms and activates G1-CDKs and G1/S-GDKs

• CDKs phosphorylate rb protein and inactivate it therefore allowing transcription regulators to transcribe genes needed for S phase

What happens when Rb protein is absent

Transcription factors are not regulated (inhibited) so transcription and translation does not stop and can cause cancer

Cell cycle when DNA is damaged

Cell cycle will halt at G1

p53

Transcription regulator that negatively controls transcription when DNA is damaged by preventing entry into S phase until damage is repaired. Can induce cell death if damage is too severe.

Mutations in p53 leads to what?

Usually cancer as unregulated cell division occurs

p21 gene

encodes for p21 protein, a CDK inhibitory protein, for when DNA is damaged and cell cycle needs to halt at G1

Process of cell cycle arresting in G1

• DNA is damaged acting as a signal

• Protein Kinases phosphorylate p53 activating it

• p53 binds to p21 gene to transcribe p21 mRNA and then translate p21 protein

• p21 protein binds and inactivates G1/S-CDK and S-CDK

• Cell cycle stays in G1

How might cells delay division

entering specialized nondividing states

• G0: temporary arrested state (liver cells)

• Permanent withdraw from cell cycle

What phases do most adult cells spend their time in

G1 or G0

S-CDK function

Initiates DNA replication and bocks re-replication

initiation of DNA replication process

• During G1 Cdc6 binds to ORC to bind a DNA helicase to DNA

• Helicase binds and Cdc6 dissociates forming Pre-RC (Origin loaded)

• S phase starts and S-CDK is activated

• Activated S-CDK can activate the bound DNA helicases and guide DNA polymerase and other proteins that initiate synthesis at the fork (origin fired)

• S-CDK phosphorylates Cdc6 and ORC to inactivate them and prevent re-replication

Where does cell cycle go if error occurs during DNA replication

Halts at G2

How does arrest at G2 occur

Inhibition of Cdc25

When Cdec25 is inhibited it is unable to dephosphorylate M-CDK which will stay inactive and mitosis will not occur

M-CDK positive feedback loop process

• phosphorylated Cdc25 phosphatase is activated

• Cdc25 removes phosphates from inactive M-CDK activating it

• M-CDK phosphorylates more Cdc25 which can then dephosphorylate more M-CDK

cohesin

ring shaped protein complex that holds sister chromatids together. They are broken in late mitosis to allow for seperation.

chromosome condensation

duplicated chromosomes pack into more compact structure so they are not damaged during seperation

condensin

ring shaped protein complex that compacts duplicated chromosomes for segregation by forming loops within loops

cytoskeleton role in mitosis and cytokinesis

• microtubules: mitotic spindle that segregate duplicated chromosomes during mitosis

• actin and myosin filaments: form contractile ring to divide daughter cells

M phase stages

• prophase

• metaphase

• anaphase

• telophase

• cytokinesis

interphase

Cell increases in size, DNA is replicated, and centrosomes duplicate

Prophase (Mitosis begins)

Duplicated chromosomes condense. Outside the nucleus mitotic spindles assemble between two centrosomes that begin moving apart.

Prometaphase

nuclear envelope breakdown allows chromosomes to attach to spindles through their kinetochores

Metaphase

Chromosomes align at center between spindle poles. kinetochore microtubules attached to sister chromatids are at opposite spindle poles

Anaphase

Sister chromatids are pulled apart towards opposite spindle poles. Both kinetochore microtubules and spindle poles contribute to pulling apart by microtubules getting shorter and spindle poles moving further apart

Telophase (Mitosis end)

Two sets of chromosomes arrive at poles. Two new nuclear envelopes assemble around each set creating two new nuclei.

cytokinesis

Cytoplasm divides in two by contractile ring (actin and myosin) that pinches the cell into two daughters.