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Checkpoints: Controls That Ensure the Order of the Cell Cycle Event
Leland Hartwell and Ted Weinhert
Science : Nov. 3 1989, pp. 629-634
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-G0 Phase (aka quiescent phase): cell arrested in permanent resting state
-Cell spends most time in interphase- not cell division
-During mitotic phase, there is no gene expression occurring
-The chromosomes have became so compacted, (able to see in microscope) can’t get mechanisms (polymerases) down there to even transcribe those genes.
Interphase: G= gap & S = synthesis
-G1- increases cell size, RNA, & proteins
-S- chromosomes doubles (sister chromatids)
-G2- further cell growth &protein synthesis
G1 =46 chromosomes
G2= 92 chromatids
Sister chromatids are joined together at centromere
Kinetochore(protein) outside, microtubule attachment site
-Coined term “Mitosis” (Greek mitos= thread)
Mitotic Spindle Apparatus
Protein units “tubulin” assemble and disassemble.
3 types of Mictotubules
-Asters- (star look) Microtubules going opposite direction of chromatids. Help position centrosomes secure where they need to be
-Kinetochore Microtubules- Grab kinetochore and pull chromatids apart – see microtubule structure fig.
-Polar Microtubule- Doesn’t grab chromatid. Actually goes past sister chromatids and goes to the opposite pole. Pushes poles apart to have separation (oblong shape in division).
Division of Nucleus
Subdivision of Mitosis:
-Nuclear membrane begins to disassociate into smaller vesicles
-Centrosome migrate to opposite poles
-Mitotic spindles can begin to form
-Centrosomes reach the opposite poles (future daughter cells)
-All sister chromatids are captured at kinetochore by microtubules from opposite poles
-Dynamic instability- microtubules growing and shrinking
-Sister chromatids pulled in opposite direction by depolymerizing kinetochore microtubules
-Sister chromatids aligned at plate facing opposite poles
-Polar microtubules are pushing poles apart
-Sisters are pulled apart as can now be called chromosomesDepolymerization of kinetochore microtubules more chromosomes to opposite poles.