01.03.12+-+Cell+Cycle+and+Apoptosis


 * Cell Cycle and Apoptosis**

- Mitotic phase: - Interphase:
 * 1. Describe the phases of the cell cycle and list the events that transpire in each phase.**
 * - prophase (nuclear envelope breakdown)
 * - metaphase (chromosome alignment)
 * - anaphase (attachment of spindles and chromosomes split at centromere)
 * - telophase (new envelope begins to form and cells begin to separate)
 * - cytokinesis (cells split into two daughter cells)
 *  G0 (Quiescence, can last a very long time)
 *  G1 (Cells pass restriction point, immediate early genes activated, expression of secondary genes activated. Substrates of CDK/Cyclin phosphorylated which allows cell to enter S-phase)
 *  S-Phase (DNA replicated)
 * G2 (Cell prepares for division

- Flow cytometry allows identification of the cells in the different cell cycle phases by measuring DNA content. In G1 phase, there are many cells exhibiting 6x10~9 base pairs. Fewer cells exist within the S-phase, and then there is an increase in cell number of cells which are in G2 and M phases, exhibiting 12x10~9 base pairs. - Cancer cells will have fewer cells in G1, more cells in S-phase, and more cells in G2 and M phase.
 * 2. Describe a method used to identify cells in different cell cycle phases**


 * 3. Describe the role that CDKs and cyclins play in cell cycle control and the mechanisms that regulate their activities**

- CDK/Cyclins are involved in cell cycle progression by phosphorylating substrates. There are 7 different CDKs and 5 cyclins.CDK becomes activated with associated with cyclin and phosphorylated. Cyclin levels fluctuate in the cell while CDK remains stable. Cyclins can be rapidly degraded by ubiquitination. Activation of CDKs is terminated when either the cyclins are proteolysed, when CDK is dephosphorylated by phosphatases, or when inhibitors are induced (e.g. p21) G0: CDK4/6-Cyc D- restriction point G1: CDK2-Cyc E- S phase entry S-phase: CDK2-Cyc A and CDK2/1-Cyc A- S phase progression S-phase: CDK2/1-Cyc B – M phase entry


 * 4. Describe the functions of two tumor suppressor genes (p53 and Rb) and the mechanisms by which they affect cell cycle transit**

- Tumor suppressor proteins block cell cycle transit. -p53 becomes activated in damaged cells and causes expression of p21 (which is a cyclin kinase inhibitor). This causes the cyclin/CDK complexes to become inactivated and the cell cannot pass through normal cell cycle progression. -Rb tumor suppressor gene produces pRb. Normally pRb will be hypophosphorylated and bind to transcription factors (E2F/DP). This phosphorylation is regulated by CDK-Cyc D complex. In G1 phase, the CDK/cyclin complex phosphorylates pRb which causes a release of the transcription factors and further cell progression. If pRb is mutated or lost, the cell cycle is unregulated and can causes chromosome abnormalities.


 * 5. Define oncogenes and proto-oncogenes. Name several functions that the proto- oncogenes play in the cell and describe how these functions may be altered as proto-oncogenes progress to oncogenes.**

- Oncogenes are altered cellular genes. Typically have a regulating role in entry or transit through G1 phase. Proto-oncogenes are the normal (or un-altered) version of oncogenes. -Fxns: growth factors, growth factor receptors, intermediate cellular kinases, GTP-binding proteins, transcription factors.


 * 6. How does the MAP kinase cascade lead to the induction of cell cycle transit? Name several intermediate proteins that participate in the MAP kinase cascade**

- Growth factor binds to receptor, causes dimerization and autophosphorylating - Adaptor proteins recognize receptor and activate Ras (G-protein). - Ras then activates Raf (Ser-Thr Kinase) - Raf phosphorylates MKK which then becomes active and phosphorylates MAPK. - MAPK enters the nucleus and targets various transcription factors (Jun, Fos, Myc) and in some human cancers causes disregulation of CDK/Cyclins/E2F (drives S phase progression)


 * 7. Describe the relevance of apoptosis to health and disease**

- Apoptosis is a mechanism for ridding the body of damaged cells. - If apoptosis is suppressed (e.g by survival proteins bcl-2), then can lead to cancer. -Apoptosis is an anti-viral mechanism.

-Intrinsic apoptosis uses intracellular signals which cause mitochondria to release cytochrome C. This then activates procaspase 9 à caspase 9 à procaspases 3,7 (effector caspases which cleave cellular proteins making cells non-viable) -Extrinsic apoptosis is initiated by extracellular ligands that bind to surface receptors which bind to adaptor molecules and activate caspase 8* à caspase 3,7.
 * 8. Define intrinsic and extrinsic apoptosis. Describe the apoptotic pathways and the molecules that initiate and execute apoptosis.**
 * Caspase 8 can also cause mitochondria to release cyctochrome C.