Dr Forsburg's all-purpose Cell Cycle Lecture Notes

These notes were used for my lectures in both BIMM112 (UCSD Division of Biology) and BMS210 (UCSD School of Medicine, Biomedical Sciences Program).

The 2001 Nobel Prize in Physiology or Medicine was awarded to Lee Hartwell, Paul Nurse, and Tim Hunt for their ground-breaking work on cell cycle regulation. Starting in the late 60s, Hartwell used budding yeast to identify mutants that blocked specific stages of cell cycle progression. Nurse, working in fission yeast in the 70s, went on to isolate mutants that could also speed up the cell cycle, thus focussing his attention on the original CDK kinase, cdc2. In the 80s, Hunt identified proteins in sea urchin extracts, the levels of which varied through the cell cycle hence "cyclins". All three have continued to make important advances in cell cycle research including the identification of checkpoints, mechanisms coupling cell morphology to the cell cycle, and identification of additional classes of kinases, cyclins, and inhibitors. For more information about their studies that led to the award, see this BBC brief. You can also visit their web pages: Hartwell, FHCRC Seattle, Nurse (ICRF London), and Hunt, ICRF-Clare Hall. Background on the pombe cell cycle can be found on our site.

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Additional slides used in lecture are available here.
Reading list of some interesting papers that illuminate the principles discussed here. All students: you are responsible for knowing the UCSD Policy on the Integrity of Scholarship.

The Cell Cycle | Evidence for Regulation | Genetic Analysis | Cdc2 Regulation | Cyclins | Inhibitors | Destruction | Mitotic exit | Replication | S to M phase | Checkpoints | Meiosis | Links | What the heck are all these gene names?

Play the cell cycle game

1) The cell cycle

2) Evidence for cell cycle regulation 3) Key work required genetics.

4) isolation of Cdc2 kinase and determining regulation 5) Cyclin/CDK cycles in the cell cycle

6) Inhibitors of cell cycle 7) Regulated destruction
  • SCF in S phase, and APC are related complexes that recognize specific substrates and target for destruction. 8) Regulation of mitotic exit: from one cell cycle to another 9) Regulation of replication onset: downstream of CDK

    10) Dependency of mitosis on S phase

    11) Checkpoints    
    • in budding yeast, things are different.
      • damage checkpoint still regulates mitosis, but by modulating activity of APC and its targets, instead of Cdc2. That is, regulates events in mitosis rather than events pre-mitosis. This may reflect very short G2 in budding yeast (S and M phase normally tightly coupled). Thus, PDS1 is a crucial molecule in the replication and damage checkpoint responses in budding yeast.
      • Also, the MEC1 and RAD53 proteins are essential for viability, indicating an essential role for these proteins. However, there are alleles that are checkpoint defective but viable, indicating the essential and checkpoint functions are genetically separable.

    • In humans, these proteins are also present. Significantly there are additional transducers, including p53 and BRCA tumor suppressor genes, which are substrates of the ATM/ATR (=Rad3) family of kinases.
      • p53 highly unstable, targeted for destruction by binding MDM2. This binding prevented when p53 phosphorylated by checkpoint kinase cascade.
      • p53 induces expression of range of genes including CKIs that block CDK activity
      • Inactivation of CDKs blocks engine of cell cycle, including preventing Rb from activating S phase genes.
      • Visit this site for more about p53.

    • spindle checkpoint: separate mechanism monitors whether cells assemble their chromosomes and spindles correctly. This works by affecting APC activity, in all systems. Don't want to divide in absence of aligned chromosomes, or might lose one. In yeasts this pathway is not essential for life, but in other organisms, it is required. This may indicate that more complex genomes are more prone to problems.
      • pathway bifurcates. One arm: Mad2 protein binds to unattached kinetochores, and prevents APC activation by binding Cdc20 (APC activating complex). Other arm: Bub2 inhibits activation of TEM1 and blocks activity of the Mitotic Exit network (MEN). Once all kinetochores attached and spindle is ready, MAD/BUB release blocks of APC/MEN and mitosis can proceed.
      • Unattached kinetochores are phosphorylated, which may lead to Mad2 binding. Mad1 binds Mad2 and is a substrate for the MPS1 kinase. Mad2 binds CDC20, which prevents APC activation.
      • BUB2 is also a substrate for MPS1. As described above, BUB2 prevents the GTPase TEM1 from being activated and thus blocks activation of the Mitotic Exit Network. This keeps the mitotic cyclins from being degraded, and thus keeps cerevisiae cells in mitosis. In fission yeast, the BUB2 equivalent prevents cells from undergoing cytokinesis. This occurs during G1. Thus, the checkpoint response, like the APC pathways, vary in different organisms although they use conserved proteins.
      • MAD and BUB proteins involved in all these checkpoints in yeast are also involved in human cells. As is the case for DNA metabolism checkpoints, in some cases there are several related proteins where yeast has one. Ability to manipulate spindle dynamics in larger cells allows testing of theories from genetical yeast expts.
    12). Meiosis

    Other sites

    What the heck are all these gene names anyway?
    More than many fields, the cell cycle is particularly complicated because of the plethora of different gene names in different systems. One option in lecture is to use just one generic name--but then you can't read any papers, because everyone in the literature uses different gene names. Here is a table that should help negotiate the different species and different nomenclatures in this lecture.
    Factorwhat is itS. cerevisiaeS. pombemetazoans
    CDKcyclin dependent kinase CDC28 Cdc2 Multiple CDKs: CDK1-6
    G1 cyclinregulatory subunit of CDK for cell cycle entry CLN1,2 and 3 ? Cdk4-cyclinD
    S phase cyclinregulatory subunit of CDK for S phase entry CLB5, 6 Cig2 Cdk2-cyclinE
    late S phase cyclinregulatory subunit of CDK for S phase progression CLB3, 4 ? Cdk2-cyclinA
    M phase cyclinregulatory subunit of CDK for mitosis CLB1, 2Cdc13 Cdc2
    APC Multi-component ubiquitin ligase required for degradation of substrates in mitosis and G1 Many genesMany genes Many genes
    APC specificity factors target the APC towards different substrates CDC20
    Cdc20, fizzy
    Hct1, Fzr
    securinAn APC target, inhibits sister chromatid separation PDS1 Cut2 securin
    separaseThe securin target, a protease that degrades cohesinESP1cut1 separase
    CohesinA complex of proteins that holds sister chromatids togetherSCC1, aka MCD1
    Rad21 aka SCC1
    SCF Multi-component ubiquitin ligase required for degradation of phosphorylated substrates in G1 SKP1
    Pop1, 2
    S is SKP1
    C is cullin
    F is F box protein
    CKIs CDK inhibitors--generally small molecules, not conserved in primary sequenceSIC1Rum1 p16
    ATM/ATRMaster kinase regulators of checkpoint pathways MEC1
    checkpoint sensorComplex of proteins consisting of a clamp loader and a clamp that binds DNA and monitors damage RAD24
    Effector kinasesDownstream of sensor kinase, respond to different challenges CHK1 (damage)
    RAD53 (HU)
    MEN Mitotic exit network, regulates progression out of M phase in S. cerevisiae. Similar proteins in S. pombe regulate septation (called SIN, for septation initiation network). Contains a GTPase, 2-component GTP exchange factor (GEF), and a GAP, upstream of a phosphatase (PPase) Regulated (in S. cerevisiae) by nucleolar localization via protein Net1 TEM1 (GTPase)
    CDC14 (PPase)
    Tumor suppressorsNegative regulators of the cell cycle, which are not found in fungi NONE NONE p53
    Transcription factorsRegulated transcription; the ones here are active for synthesis of S phase genes SWI6
    preRCPre-replication complex, which marks a replication origin as ready to fire ORC1-6
    Cdc7Origin-activating kinase, which may play other roles in maintaining genome integrity. Requires a subunit (DBF4) which does not look like, but acts like, a cyclin CDC7

    The Cell Cycle | Evidence for Regulation | Genetic Analysis | Cdc2 Regulation | Cyclins | Inhibitors | Destruction | Mitotic exit | Replication | S to M phase | Checkpoints | Meiosis

    Created 4/00 Last updated 041502
    text and original drawings © S. L Forsburg

    Made on a Macintosh.