1/25/2012 Genetics

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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




Cell Cycle

-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


Sister Chromatids













-Coined term “Mitosis” (Greek mitos= thread)


Mitotic Spindle Apparatus

Protein units “tubulin” assemble and disassemble.


pastedGraphic_2.pdf   pastedGraphic_3.pdf




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.

Posted on January 25, 2012, in Genetics. Bookmark the permalink. Leave a comment.

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