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1、3. Cytokinesis,(1) In animal cells,The MT of mitotic spindle determine the plane of animal cell division; Mitosis can occur without cytokinesis.,Contractile ring: Actin and myosinII in the contractile ring generate the force for cytokinesis,(2) In plant cells: The phragmoplast guides cytokinesis in
2、higher plants;,The assembly of the cell plate begins in late anaphase and is guided by phragmoplast,4. Meiosis,The comparison of meiosis and mitosis,Two major contributions to the reassortment of genetic material that occurs in the production of gametes during meiosis.,Visible evidence of crossing o
3、ver,Comparison of the mechanisms of chromosome alignment (at metaphase) and separation (at anaphase) in meiotic division I and meiotic division II.,Meiotic chromosome pairing culminates in the formation of the synaptonemal complex,Influence of Sry on gonad development.,The stages of oogenesis and sp
4、ermatogenesis,5. The cell-cycle control system,A. The cell-cycle control system triggers the major processes of the cell cycle,B. The control system can arrest the cell cycle at specific checkpoints,C. The cell cycle control system is based on cyclically actived protein kinases-cyclin-dependent kina
5、ses (Cdks). Engine molecules for cell cycle,G1checkpoint: fission yeast cells: Start point； Animal cells: Restriction Point,D. MPF (Maturation-promoting factor， Mitosis-promoting factor),Lab. of Toronto Univ and Yale Univ. maturation promoting factor, MPF Purdue Univ.: Behavior of MPF,The fusion of
6、M-phase HeLa cell with Ptk cell (G1、S、G2) inducing PCC. Exp. Demonstration that cells contain factors that stimulate entry into mitosis.；,The prematurely condensed chromosome (PCC),MPF(M-Cdk):p34cdc2cyclinB,Two subunits: Catalytic subunit transfers P from ATP to Ser and Thr; Regulatory subunit calle
7、d cyclin (Marine Biological Lab.at Woods Hole and UC Berkeley Univ. of Oxford-yeast),E. A simplified view of the core of the cell-cycle conreol system,Cdk associates successively with different cyclins to trigger the different events of the cycle. Cdk activity is usually terminated by cyclin degrada
8、tion.,Tree of cyclins are required in all eucaryotic 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 and are required for the initiation of DNA replication. M-cyclins promote the events of mitosis.,The structural basis of Cdk
9、activation,The human Cdk2 is shown in three states: (A) In the inactive, without cyclin bound, the active site is blocked by a region of the protein called the T-loop; (B) The binding of cyclin causes the T-loop to move out of the active site, resulting in partial activition of Cdk2; (C) Phosphoryla
10、tion of Cdk2 by CAK at a threonine residue in the T-loop, fully active.,Cdk activity can be suppressed both by inhibitory phosphorylation (pp586:14.6) and by inhibitory proteins (CKIs).,P27Cdk inhibitor proteins(CKIs),The cell-cycle control system depends on cyclical proteolysis-The control of prote
11、olysis by SCF and APC during the cell cycle.,(A)The phosphorylation of a target protein(CKI),allows the CKI to be recognized by SCF (constitutive active), E1and E2(two additional proteins), SCF serves as a ubiquitin ligase.The ubiquitylated CKI is then immediately recognized and degraded in a protea
12、some. SCF mediate the destruction of G1-cyclins, Cdk inhibitor,and other cell cycle proteins. (B)M-cyclin ubiquitylation is performed by APC.,In G1 and S phase,In metaphase/anaphase,M-cyclins,F. Intracellular control of cell-cycle events,S-phase Cyclin-Cdk complexes (S-Cdks) initiate DNA replication
13、 once per cycle,Evidence from cell-fusion exp. For a replication block.,The initiation of DNA replication once per cell cycle.,ORC: origin recognition complex; Cdc6: regulatory protein, it is present at low levels during most of the cell cycle but increases transiently in early G1,where it is requir
14、ed for the binding of a complex composed of a group of closely related proteins,the Mcm proteins.,The activation of M-phase cyclin-Cdk complexes (M-Cdks) triggers entry into mitosis,The activation of M-Cdk,The DNA replication checkpoint: Entry into mitosis is blocked by incomplete DNA replication,Th
15、e experiments of DNA replication checkpoint in the mammalian cells in culture were treated with caffeine and hydroxyurea.,The spindle-attachment checkpoint: Unattached chromosomes block sister-chromatid separation The sister chromatid separation is triggered by proteolysis,The triggering of sister-c
16、hromatid separation by the APC. APC: anaphase-promoting complex The destruction of Securin allows separase to cleave the cohesin complex. Securin(后期抑制因子 ) separase cohesin,Mad2 is normally localized at the kinetochores of prometaphase and misaligned metaphase chromosomes. Mad2 provides a “wait” sign
17、al that delays a cells progression into anaphase. The cell that possess mutant Mad2 fail to arrest at metaphase when their chromosomes are misaligned.Mad2 bind to Cdc20, inhibiting its activation of the APC, an event that is required for the metaphase-to-anaphase transition.It is only after the Mad2 is absent from all of the chromosomes that APC activation can occur and anaphase can begin. 2. The cell that contains Monoattached chromosome delays the onset of anaphase until the chromosome becomes a biattached chromosome and aligned at the equator.,
