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Cell Biology Lecture No. 8.docx

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Biology 2382B
Robert Cumming

Cell Biology Lecture No. 8: Mitosis & Cell Cycle Control II th Monday February 4 , 2013 Wee1 & Cdc25 Regulating MPF Activity: • -Wee1 is a kinase that phosphorylates CDK leading to its inactivation, while cdc25 is a phosphatase that removes the inhibitory phosphate group (left by wee1) from CDK and promotes its activation. • In the recessive elongated fission yeast cells, these mutants were long and unable to engage mitosis (increased G2), because their mutation either resulted in an excess of wee1 or a deficit of cdc25 (leading to inappropriately inactive MPF). • In the dominant premature fission yeast cells, these mutants were short and engaged mitosis unprepared (decreased G2), because their mutation either resulted in an excess of cdc25 or a deficit of wee1 (leading to prematurely activated MPF). • When mitotic cyclin binds to the CDK, it initially forms an inactive Mitosis Promoting Factor. • Wee1, an inhibitory kinase, introduces a phosphate group to the tyrosine at position 15 (Y15) on bound CDK, so the MPF complex remains inactive (key to discouraging premature mitosis). • In the next step, CAK (Cyclin Activating Kinase) phosphorylates the threonine at position 161 (T161) on bound CDK, so the MPF complex is somewhat activated (not optimal for engaging mitosis). • It is only when the activating phosphatase cdc25 removes the phosphate on Y15 that the MPF is fully activated and ready to promote mitotic division. Functions of Active MPF: • When the maximal activity of MPF is achieved, there are all sorts of functions that can be carried out with regards to mitosis: o Chromosome condensation (important for early phase), o disassembly of nuclear envelope (lamins, nucleoporins, etc that give nucleus structure are reorganized), o interphase microtubule disassembly/mitotic spindle formation (break apart interphase microtubules for activation of microtubules involved in segregating chromosomes),  An example of interphase microtubule disassembly is observed in the rounding of HeLa cells during mitosis (due to changes in cytoskeleton). o remodelling of Golgi Body and ER (quiet down all the secretory mechanisms that could interfere with mitosis). Exit From Mitosis: • It is important to shut off mitotic activation elements (as we don’t want to mimic the properties of cancer cells). • Thus, the MPF is inactivated during anaphase through the process of ubiquitination (of cyclin specifically). • The key enzyme involved is the APC/C (Anaphase-Promoting Complex/Cyclosome), which is a ubiquitin ligase that recognizes9 highly conserved amino acids (known as the destruction box) found in a number of cyclins. This is crucial for commencing rapid mitotic cyclin degradation. Regulation Of Mitotic Cyclin Levels & Overview Of Cell Cycle: • Note that CDK (cdc2) levels remains constant in the cell, but its activity changes (correlates with high activity of mitotic cyclin at the end of G2). • From G1 to G2 there is an increase in the expression of mitotic cyclin and by late G2, the CDK binds with mitotic cyclin forming an inactive MPF. • Wee1 kinase adds a phos
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