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Transcript
Lecture 16/17: Cell cycle 2 The budding/brewer's/baker's yeast Saccharomyces cerevisiae rules cell cycle genetics Divides every 90 min Replicates sexually or asexually Best developed genetics of any organism (including E coli) Budding yeast reproduce by budding, with asymmetric division, and no nuclear breakdown Surrounded by 100 nm thick, polysaccharide > protein cell wall Microtubule organizing center (MTOC) is a spindle pole body, not a centrosome We study essential processes by isolating conditional mutants in a genetic screen Most often, these are temperature sensitive (ts): grow at 20˚C, dead at 37˚C You can estimate how many mutagenized cells you want to screen Typically geneticists aim for an average of 5 mutants in a gene since e-5 = 0.007 Cell division cycle mutants (cdc) Arrest the cell cycle at a discrete point Screening1,500,000 colonies yields cdc mutants in 32 genes Combining these mutants produces a logical map of the cell cycle Some events depend on each other Mitosis depends on finishing replication and spindle pole body duplication Mitosis and DNA replication don't depend on cytokinesis or budding One mutation (cdc28) blocks multiple events The budding yeast life cycle Haploids come in two mating types, called a and α Haploids can reproduce indefinitely or fuse with each other to make diploids Diploids reproduce indefinitely, unless starvation induces them to form haploid spores Sex requires pheromone-mediated communication Pheromones arrest cells between the end of mitosis and bud formation They can be used experimentally to arrest the cell cycle and synchronize cells Cells can adopt four fates early in the cell cycle: commit to S phase and mitosis, mate (haploids), sporulate (diploids), arrest (starvation) These choices and the ability of cdc28 mutants to block all early cell cycle events define Start If cells grow in size exponentially, the time it take to double in mass is independent of birth mass Start is delayed in daughters (small) to let them grow to approach the size of their mothers Fission yeast Separated by about 500M yrs of evolution from budding yeast, divides symmetrically Cells can replicate DNA, segregate chromosomes, and divide faster than doubling their size Even if cells divide symmetrically, small errors will eventually cause problems Fission yeast mutants alter cell size: interactions suggest Wee1 and Cdc25 regulate Cdc2 Mutants that alter the critical size for cell division can produce unusually short cell cycles Fission yeast Cdc2 and budding yeast Cdc28 are the "same" protein MPF is a complex of Cdc2/Cdc28 and mitotic cyclin or cycin B The complex is a protein kinase, hence Cdc2/28 referred to as Cdk1 (cyclin dependent kinase 1) Cyclin synthesis and destruction controls the kinase activity of Cdk1 and the cell cycle Cyclin is tagged for destruction by adding chains of ubiqutin, a 76 amino acid protein The ubiquitin chain directs the cyclin to the proteasome, a complex proteolytic machine The regulated enzyme is the anaphase promoting complex (APC); catalyzes ubituitin attachment The somatic cell cycle has two more cyclin classes: G1 cyclins, S phase cyclins The cell cycle engine integrates a variety of external and internal signals to regulate the cell cycle Optional reading Murray A. & Hunt T. The cell cycle, an introduction, 1993 Chapter 3: Genetic analysis of the cell cycle Posted on course website