More Related Content


Eukayotic_cell_cycle Prashant.ppt

  1. EUKARYOTIC CELL CYCLE : -Different Phases and Molecular Events -Control mechanisms: Role of (A) Cyclins and cyclin-dependent kinases (B) Retinoblastoma and E2F proteins -Cytokinesis and cell plate formation PRESENTED BY PRASHANT VC DEPT OF ZOOLOGY GUK
  2. INTRODUCTION  Cell cycle is a complex set of cytoplasmic & nuclear processes performed to accomplish some targets-  Production of a pair of genetically identical daughter cells  Faithful replication of DNA  Segregation of replicated chromosomes in these daughter cells
  3. PHASES OF CELL CYCLE  Cell cycle basically comprises of two phases-  Interphase  Mitosis  Interphase is again subdivided into 1. G1 phase 2. S phase 3. G2 phase  Mitosis is divided into  Prophase, Metaphase,Anaphase,Telophase, Cytokynesis *Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G0 phase
  4. Phases of Interphase  G1 phase is also known as the growth phase. cell continues to grow, synthesis of enzymes mainly required for S phase.  S phase also known as the synthesis phase. DNA replication occurs i.e. the DNA of the cell has effectively doubled though the ploidy remains same.  Rate of RNA transcription & protein synthesis is very low.  G2 phase biosynthesis occurs mainly the production of microtubules for Mitosis.
  5. Fig: Various phases of cell division cycle G1 S G2 M Propha ase Meta- phase Ana- phase Telo- phase Growth & Prepration For mitosis Growth & Normal metabolic roles DNA Replication MITOTIC PHASE SECOND GROWTH PHASE FIRST GROWTH PHASE SYNTHESIS PHASE
  6. Outer ring: I = Interphase M = Mitosis Inner ring : M = Mitosis, G1 = Gap 1, G2 = Gap 2, S = Synthesis , Not in ring: G0 = Gap 0/Resting Start point Nutrients Mating Factors Cell Size G0 2 Daughter cell Fig-Phases of the Cell cycle Outer ring: I = Interphase M = Mitosis Inner ring : M = Mitosis, G1 = Gap 1, G2 = Gap 2, S = Synthesis , Not in ring: G0 = Gap 0/Resting Start point Nutrients Mating Factors Cell Size G0 2 Daughter cell Outer ring: I = Interphase M = Mitosis Inner ring : M = Mitosis, G1 = Gap 1, G2 = Gap 2, S = Synthesis , Not in ring: G0 = Gap 0/Resting Start point Nutrients Mating Factors Cell Size G0 2 Daughter cell Outer ring: I = Interphase M = Mitosis Inner ring : M = Mitosis, G1 = Gap 1, G2 = Gap 2, S = Synthesis , Not in ring: G0 = Gap 0/Resting Start point Nutrients Mating Factors Cell Size G0 2 Daughter cell
  7. FIG :-
  8. Phases of Mitosis  Prophase: centrosome duplicates to form the two poles of the mitotic spindle. Each centrosome nucleates a radial array of microtubules, called an aster. The two asters, which initially lie side by side and close to the nuclear envelope, move apart.  Metaphase: the nuclear envelope breaks down, enabling the spindle microtubules to interact with the chromosomes. The chromosomes are arranged in an equatorial plane or metaphase plate.  Anaphase: the sister chromatids are cleaved & begin to move towards the opposite ends the poles with microtubules gradually shortened, which are elongated at the end of anaphase.
  9.  Telophase: nuclear envelope reappears around each group of daughter chromosomes. The nucleoli which had dissapeared in prophase begin to re-appear. Both sets of chromosomes now surrounded by new nuclei.  Cytokinesis: is basically the division of the cytoplasm. Each daughter cell has a complete copy of the genome of the parent cell. Phases of Mitosis
  10. Resting (G0 phase)  The term "post-mitotic" is sometimes used to refer to both quiescent and senescent cells. Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). This is very common for cells that are fully differentiated. Cellular senescence is a state that occurs in response to DNA damage or degradation that would make a cell's progeny nonviable; it is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis.
  11. Fig: Various phases of cell division cycle
  12. Control mechanisms/Regulation of cell cycle  Regulation of the cell cycle involves processes crucial to the survival of a cell, including the detection and repair of genetic damage as well as the prevention of uncontrolled cell division. The molecular events that control the cell cycle are ordered and directional; that is, each process occurs in a sequential fashion and it is impossible to "reverse" the cycle.
  13.  On receiving intracellular & extra cellular signals like growth factors the cell progresses to the next phase.  Each & every cell has to pass through the START Point or the decision point only after the fulfillment of certain criteria like sufficient size, sufficient nutrients & sufficient mating factors.  After fulfilling all the requirements the cell directly progresses to the S phase of the cell cycle.
  14. Cell cycle checkpoints  Checkpoints are the control mechanisms that ensure the fidelity of cell division in eukaryotic cells.  They are categorized as:  G1 checkpoint  S checkpoint  G2 checkpoint  Metaphase checkpoint
  15.  G1 check point  It allows the repair of the damage to take place before the cell enter S phase where the damaged DNA could be replicate.  S-Phasecheckpoint  Provides monitoring of the DNA to check that damaged DNA is repaired before replication.  It also ensure the repairing of error occurred during replication e.g.. incorporation of incorrect base or incomplete replication of segment of DNA.
  16.  G2 checkpoint  It prevents the migration into mitosis until the DNA replication S-phase completed. If it only senses only DNA damage then cell cycles is stopped & continue only often the damaged DNA repair.  Metaphase checkpoint  It ensure the alignment of chromosomes on the mitotic spindle thus ensuring that a complete set of chromosomes is distributed accurately to the daughter.
  18. Role of cyclins & cdks  Two key classes of regulatory molecules, cyclins and cyclin-dependent kinases (CDKs), determine a cell's progress through the cell cycle
  19. Role of cyclins & cdks  Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activated cyclin-cdks complex (heterodimer).  CDKs are inactive in the absence of a partner cyclin. When activated by a bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins leading entry into the next phase of the cell cycle.  Different cyclin-CDK combinations determine the downstream proteins targeted.  CDKs are constitutively expressed in cells whereas cyclins are synthesised at specific stages of the cell cycle, in response to various molecular signals.
  20. CYCLIN AND CDK [Animal cells have atleast 10 different cyclins (A,B and so forth) and atleast 8 cdks(cdk1 through cdk8), which act in various combinations at specific points in the cell cycle]
  21. Three classes of cyclins in Eukaryotic cells  G1/S cyclins : bind cdks at the end of G1 and commit the cell to DNA replication  S-cyclins : Bind cdks during S phase & are required for the initiation of DNA replication  M-cyclins : promote the events of mitosis
  22. Families of cyclin and cyclin-dependent kinases (combination or partner of cyclins and cdks)
  23. General mechanism of cyclin-CDK interaction  Upon receiving a pro-mitotic extracellular signal, G1 cyclin-CDK complexes become active to prepare the cell for S phase, promoting the expression of transcription factors that in turn promote the expression of S cyclins and of enzymes required for DNA replication. The G1cyclin-CDK complexes also promote the degradation of molecules that function as S phase inhibitors by targeting them for ubiquitination. Once a protein has been ubiquitinated, it is targeted for proteolytic degradation by the proteasome.
  24.  Active S cyclin-CDK complexes phosphorylate proteins that make up the pre-replication complexes assembled during G1 phase on DNAreplication origins. The phosphorylation serves two purposes: to activate each already-assembled pre-replication complex, and to prevent new complexes from forming. This ensures that every portion of the cell's genome will be replicated once and only once. The reason for prevention of gaps in replication is fairly clear, because daughter cells that are missing all or part of crucial genes will die. However, for reasons related to gene copy number effects, possession of extra copies of certain genes is also deleterious to the daughter cells.
  25.  Mitotic cyclin-CDK complexes, which are synthesized but inactivated during S and G2 phases, promote the initiation of mitosis by stimulating downstream proteins involved in chromosome condensation and mitotic spindle assembly. A critical complex activated during this process is a ubiquitin ligase known as the anaphase-promoting complex (APC), which promotes degradation of structural proteins associated with the chromosomal kinetochore. APC also targets the mitotic cyclins for degradation, ensuring that telophase and cytokinesis can proceed.
  26. Regulation of CDK by phosphorylation and proteolysis DBRP- destruction box recognizing protein; U- ubiquitin FIG :-
  27. Retinoblastoma and E2F proteins(Specific action of cyclin-CDK complexes )  Cyclin D is the first cyclin produced in the cell cycle, in response to extracellular signals (e.g. growth factors).  Cyclin D binds to existingCDK4, forming the active cyclin D-CDK4 complex. Cyclin D-CDK4 complex in turn phosphorylates the retinoblastoma susceptibility protein (pRb).  The hyperphosphorylated pRb dissociates from the E2F- pRb complex (which was bound to the E2F responsive genes, effectively "blocking" them from transcription), activating E2F.  Activation of E2F results in transcription of various genes like cyclin E, cyclin A, DNA polymerase, thymidine kinase, etc.
  28.  Cyclin E thus produced binds to CDK2, forming the cyclin E-CDK2 complex, which pushes the cell from G1 to S phase (G1/S transition). (When a DNA damage is detected retinoblastoma protein pRB – arrest the cell cycle in G1)
  29. Regulation of E2F by pRB
  30. Regulation of cell cycle transition from G1 to S phase by pRb Rb Rb P P16 Cell Cycle Blocked Rb inactive Cdk4/cyclin D G1 M G2 S S G1 M G2 Cell Cycle Proceeds Rb active
  31. FIG:- Regulation of passage from G1 to S by phosphorylation of pRB Source:Lehninger principles of Biochemistry
  32. Cyclin D + cdk4 Cyclin D-cdk4 complex E2F+pRb-P Dissociation of pRb (pRb-P=Inactive pRb) Activation of E2F Transcription of genes (cyclin E, cyclin B etc.) Cyclin E + cdk2 Cyclin E-cdk2 complex G1toS transition CyclinB+cdk1 Cyclin B-cdk1 complex G2 to M transition CyclinA+cdk1 Cyclin A-cdk 1 complex S to G2 transition Fig: Specific action of cyclin-cdk interaction Phosphorylation of pRb
  33. CYTOKINESIS AND CELL PLATE FORMATION This is the last stage of the cell cycle & it completes the process of cell division. It is process in which cytoplasm divides to two daughter cells. Organelles are distributed into each of the new cells Initiates during late telophase or even in late anaphase, completes as the next interphase begins.
  34. CYTOKINESISAND CELL PLATE FORMATION  In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow and by the action of the contractile ring.  Actin and myosin II in the contractile ring generate the force for cytokinesis.  In plant cells, a cell plate forms during cytokinesis
  35. Mitotic spindle determines the plane of cytoplasmic cleavage
  36. Actin and myosin II in the contractile ring generate the force for cytokinesis.
  37. Contractile ring Made up of non-muscle myosin II and actin filaments assemble equitorially at cell cortex.  Myosin uses ATP energy which moves along actin filament constricting the cell membrane to form cleavage furrow.  Ingression continues till midbody structure is formed abscission physically cleaves in to two. Contractile ring assembly is dedicated by mitotic spindle.
  38. Contractile Ring of Actin and Myosin
  39.  This ring, shown at the bottom, constricts to divide a proliferating cell into two. It is composed of antiparallel actin filaments, cross-linked by myosin molecules.  The process of filament formation is shown from the top. First, actin monomers assemble into trimers ('nucleation'); trimers act as a seed to which further monomers are added  Arp2/3 complex and the Cdc12 and Cdc3 proteins are needed for nucleation or polymerization. The filaments are then organized into the contractile ring.
  40. The double ring model of cytokinesis positioning
  41. Local activation of RhoA triggers the assembly and contraction of contractile ring : > RhoA,a small GTPase,is activated at the cell cortex at future division site. > activated by RhoGEF found at the cell cortex by binding GTP.
  42. The microtubules of the mitotic spindle determine the plane of animal cell division. Membrane enclosed organelles must be distributed to the daughter cells during cytokinesis.
  43. Plant cell Cytokinesis Due to presence of cell wall plant cell cytokinesis differ from animal cell. Formation of cell plate occurs in the centre and grows outwards to meet the lateral walls. Phragmoplast, array of microtuble that guides and support cell plate formation. Orientation of cell plate is determined by prophase band.
  44. Cytokinesis in Plant cells
  45. Plantcellcytokinesis
  46. REFERENCES  Bruce Albert, Alexender Johnson, Julian lewis, Martin raff, Keith roberts, Peter walter (2002) Molecular biology of the cell fifth edition published by Garland science,Taylor and Francis group.  Gerald karp (1996,1999)Cell and Molecular BiologyConcepts and Experiment Second Edition published by John wiley and Sonc.Inc.  Harvey Lodish,Arnold Berk, Paul Matsurdaria,Chris A.Kaiser, Monty Krieger, Matthew, P.Scott, S.Lawrence,Zipursky, James Darnell (1986,1990,1995,2000,2004) Published byW.W. Freeman and Company.  . Com  http://cytokinesis.wikipedia.encyclopedia