Cell Cycle

Front 1

G₁ Phase

Back 1

First component of interphase

Major growth phase

First major checkpoint in the cell cycle occurs at the G1/S border

- cell must determine if it has the necessary energy and nutrients to begin DNA synthesis and enter cell cycle

- cell must determine if the environment is right to divide

- cell must synthesize proteins and buildup redox potential

Front 2

S Phase

Back 2

Second component of interphase

DNA replication occurs in this phase in preparation for division later on in the cell cycle

Front 3

G₂ Phase

Back 3

Third and final component of interphase

Second growth period and second checkpoint of cell cycle

- cell must check to make sure there were no issues with DNA replication before committing to cell division

- cell grows to create enough organelles and cytoplasm for division into two daughter cells

Front 4

M Phase

Back 4

Cell division occurs during this stage

Once a cell commits to M phase it must complete the process; there is no going back

Third checkpoint occurs before the transition from metaphase to anaphase

- cell checks to make sure all chromosomes are attached to their spindles

- if so, anaphase and cytokinesis are initiated

Front 5

G₀ Phase

Back 5

Resting phase partially removed from regular cell cycle

Cells maintain normal cellular function w/o replication

Often occurs in senescent cells and terminally differentiated cells (e.g. cardiomyocytes, neurons)

Front 6

Cell Cycle Control System

Back 6

A series of biochemical switches induce and inhibit the stages of the cell cycle

Major component = cyclin-dependent kinases

Biochemical processes involved

- protein phosphorylation

- protein-protein interactions

- protein degradation

- telomeres

Front 7

Cyclin-Dependent Kinases (Cdks)

Back 7

Phophorylate proteins to drive the progession of the cell cycle

Only active when bound to a cyclin

- causes binding loop in Cdk to become more accessible

Fully activated when binding loop is phosphorylated by a Cdk-activating kinase

Specificity for activity is cyclin dependent

Front 8

G₁-CDK Cyclin-Cdk Complex

Back 8

Cyclin D and Cdks 4 & 6

Front 9

G₁/S-CDK Cylcin-Cdk Complex

Back 9

Cyclin E and Cdk 2

Front 10

S-Cdk Cylcin-Cdk Complex

Back 10

Cyclin A and Cdks 2 & 1

Front 11

M-Cdk Cyclin-Cdk Complex

Back 11

Cyclin B and Cdk 1

Front 12

Time Dependent Control of Cell Cycle Using Cyclins

Back 12

each progressive Cdk-cyclin pair acting in the cell will influence expression levels of other Cdk-cyclin pairs by up-regulation or down-regulation

Front 13

Inhibition of CDK activity

[Diphosphorylation]

Back 13

Kinases can also inhibit Cyclin-Cdk complexes by phosphorylating them on a different site

- Wee1 phophorylates the complex to inhibit it

- Cdc25 phosphatase dephosphorylates it

Front 14

Inhibition of CDK activity

[CKIs]

Back 14

Cyclin dependent kinase inhibitory proteins (CKIs) for a heterotrimeric complex with the active Cyclin-Cdk complex

Example: p27 binds and obscures the active site on the active Cdk2/4-cyclin complex

Front 15

Inhibition of CDK activity

[INK4]

Back 15

INK4 proteins A and B can bind to Cdk 4 and 6 and sequester them away from Cyclin D thus preventing their activation

Front 16

Cell Cycle Regulation via Proteolysis

Back 16

Controlled by multi-protein ubiquitin ligases

Can stop or induce a phase of the cell cycle depending on what type of protein it degrades

Front 17

Cell Cycle Regulation via Proteolysis

[APC/C]

Back 17

Stops phase of cell cycle by degrading cyclins

Specifically targets Cyclin B for degradation at the end of M phase of cell cycle

Inactive APC/C binds with Cdc20 to become active

Adds poly-ubiquitin tail to Cyclin B targeting it for degradation by proteosomes

Front 18

Cell Cycle Regulation via Proteolysis

[SCF]

Back 18

Induces phase of cell cycle by degrading CKIs

CKIs are first phosphorylated by a kinase (ex: PLK 3)

Then can then bind to the F-box protein in the activated SCF complex and be poly-ubiquitinated and targeted for proteosomal degredation

Front 19

Replicative Cell Senescence

Back 19

Cells can only divide a limited number of times before being undergoing permanent cell cycle arrest

-typically 50-70 times

This occurs because our telomeres are shortened after each successive cell division

- when they reach a critical length the cell is forced to stop dividing

Front 20

Control of S Phase by S-Cdks

Back 20

Pre-Replication complex, containg Cdc6 and Cdt1, bind to DNA and Helicase; prevent helicase from proceeding down DNA

Active S-Cdk phophorylates Cdc6, priming it for polyubiquitination and degradation

- this also causes it to dissociate from the DNA and Cdt1

Cdt1 can then be sequestered by geminin

The pre-initiation complex can now bind to the DNA where it is phosphorylated by S-Cdk

This causes the pre-IC to recruit polymerase and initiates replication

Front 21

Control of M Phase by M-Cdks

Back 21

Cdk1 and Cyclin B bind, but the complex in inactive

It is simultaneously diphosphorylated by both CAK and WEE1 which causes the complex to remain inactive

When the cell is ready to enter mitosis, Cdc25 phosphatase removes the inhibitory phosphate that Wee1 added thus activating the M-Cdk complex

The M-Cdk complex then up-regulates its own activity by

1. Phosphorylating and inactivating Wee1

2. Phosphorylating more Cdc25

This is why once you commit to M phase there is NO GOING BACK!

Front 22

APC/C M-Cdk Dependent Initiation of Anaphase

Back 22

M-Cdk complex serves to further activate APC/C

M-Cdk also activates ERK which phosphorylates Securin

Activated APC/C polyubiquitinates the phosphorylated Securin which is inhibiting the action of Seperase

Once the Securin is degraded, Seperase goes on to seperate chromosomes during anaphase

Front 23

Control of Cytokinesis by GTP-dependent Formation of Contractile Ring

Back 23

Actin/Myosin rings form around cell during cytokinesis

RhoA activation causes activation of these filaments in the ring which leads to contraction and formation of the cleavage furrow

Regulated by Cdk1 and cyclin B because this occurs in M phase

Front 24

Induction of G1 Cell Cycle by Growth Factors

Back 24

GF binds to RTK which autophosphorylates itself and recruites PI3K

PI3K phosphorylates PI(4,5)P₂ into PI(3,4,5)P₃

PIP₃ activates AKT(Protein Kinase B) which phosphorylates MTOR

MTOR activates the TOR complex which initiates translation and protein synthesis by activating S6K and inhibiting 4E-BP

Front 25

Mitogen Activated Cell Signaling and S Phase Entry

Back 25

Mitogen binds to receptor which leads to phosphorylation and activation of Ras

Ras phosphorylates MEK which phosphorylates ERK

ERK phosphorylates the transcription factor Myc

Myc facilitates transcription of G1 cyclin (D)

Cyclin D leads to activation of Cdks 4 and 6

Cdks 4 and 6 phosphorylate Rb causing it to dissociate from E2F

E2F leads to transcription of G₁/S cyclin (cyclin E) and S cyclin (cyclin A)

Front 26

Excessive mitogen-activated signaling

can induce apoptosis or arrest

Back 26

Myc facilitates trascription and translation of protein Arf

Arf sequesters Mdm which usually targets p53 for degradation

Non sequestered, active p53 can

1. Arrest the cell cycle in favorable environment

2. Initiate Apoptosis in unfavorable environment

Front 27

DNA Damage Halts the Cell Cycle

Back 27

Cell damage activates ATM/ATR kinases

ATM/ATR phosphorylate Chk1/Chk2 which phosporylate p53

Phosphorylated p53 induces expression of p21, a CKI

p21 binds to G₁/S and S-CdkC

Front 28

Cdks and Cancer

Back 28

Normal Cell:

- Highly active CKIs

- Normal levels of Cdk-cyclin pairs

Cancer Cell:

- CKI expression remains relatively the same

- Highly overactive Cdk-Cyclin expression