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The Cell Cycle

The Cell Cycle. Review lecture for Kentnisstandprüfung By Joseph Unsay. Overview of the Cell Cycle. The eucaryotic cell cycle is divided into four phases.

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The Cell Cycle

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  1. The Cell Cycle Review lecture for Kentnisstandprüfung By Joseph Unsay

  2. Overview of the Cell Cycle

  3. The eucaryotic cell cycle is divided into four phases.

  4. Cell division cycle genes (Cdc genes) and the various aspects of cell division has been studied in numerous model organisms and cell-free systems.

  5. The Cell Cycle Control System

  6. The cell cycle has check points governed by cyclin-dependent kinases (Cdks). Cyclin concentration oscillates but Cdk concentration remains the same.

  7. Although activity of Cdks are highly related to presence of cyclin, there are other important control mechanisms for Cdk activity: (1) Inhibitory phosphorylation/binding (2) Critical Proteolysis and (3) Transcription control

  8. Although activity of Cdks are highly related to presence of cyclin, there are other important control mechanisms for Cdk activity: (1) Inhibitory phosphorylation/binding, (2) Critical Proteolysis and (3) Transcription control.

  9. Cell cycle regulatory proteins function as switches.

  10. S Phase

  11. S-Cdk initiates formation pre-initiation complex from pre-RC once every cycle.

  12. Formation of pre-RC is mediated by APC/C activation and Cdk inhibition.APC/C activation degrades geminin which then destroys inhibitors of Cdt1.

  13. Cohesins help hold sister chromatids together

  14. Mitosis

  15. M-Cdk drives entry into Mitosis and is controlled by dephosphorylation. Cdc25 is partially activated by S-Cdk. Condensin (similar in structure to cohesins) helps configure chromosomes for separation: condensation and resolution

  16. Mitotic spindles from centrosomes facilitate choromosome separation by attaching to kineticores (and other parts of the cell) in cooperation with kinesins. M-Cdk also plays a role in assembly of spindles as well as destruction of nuclear envelope

  17. Chromosomes promote spindle assembly and attaches to these spindles in Kinetocores (in bi-orientation). Opposing forces create the metaphase plate

  18. Chromosomes promote spindle assembly and attaches to these spindles in Kinetocores (in bi-orientation). Opposing forces create the metaphase plate

  19. Several forces allow for the separation of chromosomes: (1) depolymerization of microtubules, (2) microtubule flux and (3) polar ejection force.

  20. Once conditions are optimal (there’s a checkpoint!), APC/C activates chromosome separation.

  21. Separation occurs in Anaphase A and B. M-Cdk is deactivated and chromosomes are repackaged to nucleus.

  22. Cytokinesis

  23. Cytokinesis accompanies mitosis to separate cells and is done by contractile ring (composed of actin and myosine II). Contractile ring formation is regulated by RhoA protein.

  24. Several models describe how this contractile ring is formed

  25. Plant cytokinesis is somehow different as phragmoplasts guides deposition and construction of cell plate that divides the cell into two.

  26. Other things to consider:1. Organelles are distributed (on the average) evenly between two daughter cells2. Mitosis can occur without cytokinesis3. APC/C is again a major player in resetting the cell cycle back to G1 phase.

  27. Control of Cell Division and Cell Growth

  28. Mitogensstimulate cell divisionSome signals can lead cells to a non-dividing state (G0). Human Cells have also a limit to the number of cell divisions it can undergo (regulated by telomeres)

  29. If there’s DNA damage, ATM and ATR produces a response that ultimately blocks G1/S-Cdk and S-Cdk activity.

  30. Aberrations to mitogenicsignalling can be countered by p53.

  31. Growth factors stimulate cell growth (increase in mass, production of more moacromolecules, etc.). Cell size is controlled by many factors.

  32. Cells also compete with other cells for nutrients and signals (Ex. Culture grown cells will only grow to a certain cell density)

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