CELL CYCLE REGULATION

Cell Cycle Review hill.com/sites/ /student_vi ew0/chapter2/animation__mitosis_and _cytokinesis.html

Cell Cycle Checkpoints In order to have normal growth & development cells need to coordinate timing and rate of cell division

Checkpoints Cell cycle has checkpoints ◦ Cell is told to STOP or GO to the next step in the cycle ◦ Controlled by signals  Signals show whether processes were completed right

Signals Animals usually have built-in STOP signals until they are overridden by GO signals

Checkpoints 3 main checkpoints G 1 ◦ Can DNA synthesis begin? G 2 ◦ Was DNA synthesis done correctly? ◦ Can mitosis start? Spindle (during mitosis) ◦ Are the chromosomes attached to the spindles? ◦ Can the chromatids separate correctly?

G 1 Checkpoint Most important decision “restriction point” If GO ◦ Cell goes on to S phase & copies DNA ◦ Internal signals: cell growth (size) and nutrition ◦ External signals: “growth factors” If STOP ◦ Cell exits cell cycle and goes into G 0 phase

G 0 Phase Non-dividing, differentiated state Most human cells are in G 0 ◦ Liver cells – in G 0 but can go back to the cell cycle b/c of external cues ◦ Nerve/muscle cells – very specialized; in G 0 & can never divide

G 1 Checkpoint Most important decision “restriction point” If GO ◦ Cell goes on to S phase & copies DNA ◦ Internal signals: cell growth (size) and nutrition ◦ External signals: “growth factors” If STOP ◦ Cell exits cell cycle and goes into G 0 phase ◦ G 0 phase is a nondividing state

GO Signals What signals cell to “go?” Protein signals that promote cell growth & division ◦ Internal factors  Promoting factors ◦ External factors  Growth factors Primary mechanism of control ◦ Phosphorylation  By kinase enzymes  Either activates or inactivates cell signals

Internal Signals Signals activated by a protein called Cdk: cyclin-dependent kinase (Cdk) ◦ Kinase = enzyme that catalyzes a transfer of a phosphate group from ATP to another molecule  phosphorylation

Internal Signals Cdks need to bind to another protein called cyclin in order to be active as a protein kinase ◦ When cyclin binds to Cdk  create a cyclin-Cdk complex The creation of this complex triggers cell to go from G1 to S

cyclin-Cdk complex

Interaction of Cdk’s & different cyclins triggers the stages of the cell cycle

Internal Signals cyclin-Cdk complex serves as checkpoint ◦ If DNA is damaged in G1, p21 is made ◦ P21 binds to the Cdk’s in G1  this prevents cyclin from activating the Cdks  Cell cycle is paused and DNA is repaired ◦ p21 degrades after DNA repair  now cyclins can bind with Cdk’s

External Signals Growth factors ◦ coordination between cells ◦ protein signals released by body cells that stimulate other cells to divide  density-dependent inhibition  crowded cells stop dividing  each cell binds a bit of growth factor  not enough activator left to trigger division in any one cell  anchorage dependence  to divide cells must be attached to a substrate  “touch sensor” receptors

Growth Factors Experiment

Growth Factor Signals E2F nucleus cytoplasm cell division nuclear membrane growth factor protein kinase cascade nuclear pore chromosome Cdk cell surface receptor P P P P P E2F Rb

G2 Checkpoint Maturation-promoting factor (MPF) = type of cyclin-Cdk complex ◦ peaks of MPF correspond to peaks of cyclin ◦ Promotes mitosis by phosphorylating other protein kinases  triggers fragmentation of nuclear envelope ◦ Cell goes from G 2 into Mitosis

Spindle Checkpoint Spindle (M phase) Checkpoint ◦ Ensures all chromosomes are properly attached to spindles Need an anaphase promoting complex (APC) to move onto anaphase ◦ If kinetochores have not attached to spindles, signal sent and APC is kept inactive ◦ when kinetochores are attached, APC is activated  triggers breakdown of cyclin and of proteins that hold sister chromatids together

Cdk / G 1 cyclin Cdk / G 2 cyclin (MPF) G2G2 S G1G1 C M G 2 / M checkpoint G 1 / S checkpoint APC Active Inactive Active Inactive Active mitosis cytokinesis MPF = Mitosis Promoting Factor APC = Anaphase Promoting Complex Replication completed DNA integrity Chromosomes attached at metaphase plate Spindle checkpoint Growth factors Nutritional state of cell Size of cell

Cancer Transformation = normal cell is converted to a cancer cell

Growth Factors and Cancer Growth factors can create cancers ◦ proto-oncogenes  normally activates cell division  growth factor genes  become oncogenes (cancer-causing) when mutated  if switched “ON” can cause cancer  example: RAS (activates cyclins) ◦ tumor-suppressor genes  normally inhibits cell division  if switched “OFF” can cause cancer  example: p53

Cancer & Cell Growth Cancer is essentially a failure of cell division control ◦ unrestrained, uncontrolled cell growth What control is lost? ◦ lose checkpoint stops ◦ gene p53 plays a key role in G 1 /S restriction point  p53 protein halts cell division if it detects damaged DNA  options:  stimulates repair enzymes to fix DNA  forces cell into G 0 resting stage  keeps cell in G 1 arrest  causes apoptosis of damaged cell  ALL cancers have to shut down p53 activity

DNA damage is caused by heat, radiation, or chemicals. p53 allows cells with repaired DNA to divide. Step 1 DNA damage is caused by heat, radiation, or chemicals. Step 1 Step 2 Damaged cells continue to divide. If other damage accumulates, the cell can turn cancerous. Step 3 p53 triggers the destruction of cells damaged beyond repair. ABNORMAL p53 NORMAL p53 abnormal p53 protein cancer cell Step 3 The p53 protein fails to stop cell division and repair DNA. Cell divides without repair to damaged DNA. Cell division stops, and p53 triggers enzymes to repair damaged region. Step 2 DNA repair enzyme p53 protein p53 protein p53 — master regulator gene

Development of Cancer Cancer develops only after a cell experiences ~6 key mutations (“hits”) ◦ unlimited growth  turn on growth promoter genes ◦ ignore checkpoints  turn off tumor suppressor genes (p53) ◦ escape apoptosis  turn off suicide genes ◦ immortality = unlimited divisions  turn on chromosome maintenance genes ◦ promotes blood vessel growth  turn on blood vessel growth genes ◦ overcome anchor & density dependence  turn off touch-sensor gene

What causes these “hits”? Mutations in cells can be triggered by  UV radiation  chemical exposure  radiation exposure  heat  cigarette smoke  pollution  age  genetics

Tumors Mass of abnormal cells ◦ Benign tumor  abnormal cells remain at original site as a lump  p53 has halted cell divisions  most do not cause serious problems & can be removed by surgery ◦ Malignant tumor  cells leave original site  lose attachment to nearby cells  carried by blood & lymph system to other tissues  start more tumors = metastasis  impair functions of organs throughout body

Traditional treatments for cancers Treatments target rapidly dividing cells ◦ high-energy radiation  kills rapidly dividing cells ◦ chemotherapy  stop DNA replication  stop mitosis & cytokinesis  stop blood vessel growth

New “miracle drugs” Drugs targeting proteins (enzymes) found only in cancer cells ◦ Gleevec  treatment for adult leukemia (CML) & stomach cancer (GIST)  1st successful drug targeting only cancer cells Novartes without Gleevec with Gleevec