BIOLOGY 2B03 Lecture Notes - Lecture 21: Cyclin, Wee1, Cdc25
18 Jun 2018
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Cell Cycle Regulation: Identifying Cell Cycle Regulators in Yeast
Genetic Screen
genetic screen = unbiased search for genes involved in particular mechanism •
gene mutations created at random - any gene may be mutated •
each mutation can potentially have a different phenotype ◦
a genetic screen looks through all of these phenotypes for those of particular interest •
Temperature-Sensitive Mutant
special class of mutant - codes for temperature-sensitive protein •
this protein folds at permissive temperature (24˚C) but misfolds at restrictive temperature (37˚C) ◦
allows researcher to change temperature and turn on and off protein function •
wild type can grow/divide at both temperatures •
Cell Cycle Mutations - Cdc2 in Fission Yeast
called cycle mutants or cdc mutants - can have 1 of 2 effects on cell: •
elongated cell - cells delayed in G2 and grow instead of going to mitosis ◦
results in longer than normal cells ‣
caused by loss of function mutation in cdc2 gene ‣
wee phenotype - transition into mitosis prematurely ◦
results in smaller than normal cells ‣
caused by going of function mutation in cdc2 gene ‣
essentially, without Cdc2 protein, cell fails to divide •
cdc2 function increased = divides too early/frequently ◦
means cdc2 protein key regulator for entry into mitosis and promotes cell division ◦
Cdc2 protein: •
34kDa protein with kinase activity ◦
required for cell division ◦
forms heterodimer with cyclin, Cdc13 ◦
Cdc13 in Fission Yeast
function loss/gain causes causes same phenotype as Cdc2 •
required for mitosis •
oscillating concentration during cell cycle •
homologous to Xenopus Cyclin B •
Cdc2-Cdc13 heterodimer = MPF or mitotic Cyclin-Cdk •
Cdc25 Mutants
another temperature sensitive mutation in gene coding for cell cycle regulators •
function loss/gain causes same phenotype as Cdc2 and 13 •
dominant cdc25 mutation = wee cell phenotype •
cdc25 is normally an activator of MPR and promotes entry into M-phase •
activates cell cycle thereby activating MPF ◦
Wee1 Mutants
loss of function mutation = wee phenotype •
gain of function mutation = elongated phenotype •
lack of Wee1 causes premature entry into M-phase, so is likely an inhibitor of MPF and delays entry into M-phase •
inactivates cell cycle thereby inactivating MPF ◦
Coordinated Regulation
Wee1 encodes a tyrosine kinase •
