reading assignment 1 cell cycle .pdf

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Biological Sciences
Connie Potroff

Reading assignment 1 cell cycle January 15, 2013 9:36 PM • The biochemical reactions governing the cell cycle are evolutionaryconserved in eukaryotes • Progressionthrough the cycle is regulated at 3 check points 1. The late G1 phase 2. Late S phase 3. G2/ M boundary • The key enzymes that regulate the cycle are cyclin-dependent protein kinsases or Cdks • Protein kinases are enzymes that phosphorylate proteins using ATP • Most multi cellular eukaryotesuse several protein kinases that are active during different phases of the cell cycle • All depend on regulatory subunits called cyclins for their activities • Three cyclins (A, B, and D) regulate the tobacco cell cycle 1. G1/ S cyclins: Cyclin D active in late G1 2. S cyclins: cyclin A active in late S phase 3. M cyclins: cyclin B active just prior to mitoticphase • The critical restriction point in late G1 commitsthe cell to another round of cell division and is regulated by D type cyclin • Plant hormonesthat promote cell division including cytokinins and brassinosteroids,appear to do so through an increase in cyclin D3 (plant D-type cyclin) • Cdk activity can be regulated in various ways but two of the most important are 1. Cyclin synthesis and degradation 2. Phospho/dephosph o-rylation of key amino acids on Cdk proteins • For the 1st mechanism Cdks are inactive unless they're associated with cyclin • Most cyclins turn over rapidly (synthesized then actively degraded using ATP) • Cyclins are degraded in the cytosolby a large proteolyticcomplex called the 26S proteasome, before being degraded they're marked by addition of a small protein called ubiquitin, ubiquitination is a general mechanism for tagging cell proteins destined for turnover • The 2nd mechanism of phospho/ dephospho-rylationdepends on the fact that Cdks possess two tyrosine phosphorylationsites • One causes activation the other causes in-activation • Specific kinases carry out stimulatory or inhibitory phosphorylations • Similarly protein phosphatases can removephosphates from Cdks stimulating or inhibiting them • Further control of Cdks is exerted by Cdk inhibitors (ICKs) that can influence the G1/ S transition • Mitosis is the process by which previously replicated chromosomesare aligned, separated and distributed to daughter cells in an orderly fashion • Microtubules are an integral part of mitosis. • The period directly before prophase is called preprophase • During it the G2 microtubules are completely re-organized into a preprophase band(the precursor of the cross-wall) • The position of the preprophase band (the underlying cortical division site) and the partition of the cytoplasm the divides central vacuoles determine the plane of division in plant cells, and thus plays a very important part in development plays a very important part in development • At the start of prophase MTs polymerizing on the surface of the nuclear envelope begin to gather at two foci on opposite sides of the nucleus initiating spindle formation • Although not associated with centromeresas they are in animal cells these foci serve the same function in organizing MTs • During the prophase the nuclear envelope remains intact but it breaks down as the cell enters metaphase in a process that involvesre-organization and re-assimilationof the nuclear envelope into the ER • Throughout the division cell division kinases interact with the MTs to help re-organize the spindle by phosphorylating MAPs and kinesins • As the c
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