Download Cell cycle - Instructure

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