Microtubule organizing centers (aka centrosomes) are composed of asters at each end, with centrosomes spanning between them. The tubules that connect to the chromosome kinetochore are called kinetochore microtubules, while the tubules which interact with each other are polar microtubules (aka non-kinetochore microtubules). Microtubules are composed of α- and β-tubulin monomers polymerized to form hollow tubes. Depolymerization shortens the microtubules, while the chromosomes simultaneously migrate toward the asters, driving apart the spindle poles.

Cycles in M-Phase Promoting Factor (MPF) activity control mitosis. As a protein kinase, MPF likely acts via phosphorylation of the major histone protein H1 and the major nuclear envelope protein lamin. This leads to the degradation of the nuclear envelope and condensation of chromatin into chromosomes in anticipation of mitosis. Found in all organisms, MPF is a heterodimer composed of cyclin-dependent kinase (cdk1) and cyclin B.

Cycles of MPF activity are based on synthesis and degradation of cyclin B. During the M phase (mitosis) the heterodimer is functional and drives cells into metaphase. Cyclin B is degraded, inactivating MPF once cells go into mitosis. During the S phase, newly synthesized Cyclin B from maternal mRNA leads to formation of a new functional MPF heterodimer.

Cytoplasm from mammalian cells arrested in metaphase of mitosis by treatment with drugs that inhibit the polymerization of microtubules (e.g. colchicine or nocodazole) had high oocyte maturation promoting factor (MPF).

When the ctoplasm from Xenopus eggs was injected into the cytoplasm of mammalian cells in G1, it caused the mammalian cells to undergo the events of early mitosis: chomosome condensation and nuclear envelope break down.

Also, when the cytoplasm from mammalian cells arrested in mitotic metaphase by treatment with micrtotubule inhibitors was injected into the cytoplasm of mammalian cells in G1, it caused the mammalian cells to undergo the events of early mitosis: chomosome condensation and nuclear envelope break down.

This is like the mitosis promoting activity first observed in the cell fusion experiments.

So, Xenopus ooctye MATURATION PROMOTING FACTOR = mammalian MITOSIS PROMOTING FACTOR.

(Fortunatley, “MPF” is the abbreviation for both Maturation Promoting Factor (revealed by injection into Xenopus oocytes) and Mitosis Promoting Factor (revealed by fusing an M-phase cell to a cell in G1).

Oocytes remain arrested in G2 until they mature and addition of a sperm nucleus (via fertilization when in vivo). This is a great model for figuring out what the egg must have synthesized beforehand for mitosis to occur.

Polyubiquitination marks eukaryotic proteins for degradation by proteasomes. Three enzymes are required for the Ubiquitin to work: E1, the Ubiquitin-activating enzyme; E2, the Ubiquitin-conjugating enzyme; and E3, the Ubiquitin ligase. SCF and APC/C are ubiquitin protein ligase complexes that that control three major transitions in the cell cycle: onset of S-phase through degradation of Sic1 by SCF; initiation of anaphase via degradation of securin by APC-Cdc20; exit from mitosis via degradation of cyclin B’s by APC-Cdh2. APC has several substrates that must be degraded at different times in the cycle; thus, its activity is directed by specificity factors that bind it. SCF only degrades Sic1 and thus its activity is regulated only by phosphorylation of its substrate.

Cyclin-CDK		Nick-Name	Overview
Mitotic-Cyclin+CDK		MPF	Triggers the formation of mitotic spindle. Promotes mitosis i.e. chromatin condensation. Causes nuclear envelope breakdown by phosphorylating the lamins that form an intermediate filament-type network (nuclear lamina) underlying the inner nuclear membrane. The three lamins present in the nuclear lamina, lamin A,B & C, are phosphorylated by MPF at serine amino residues. This leads to depolymerisation of nuclear lamina & breakdown of nuclear envelope into small vesicles.
Vertebrate	S. cerevisae	S. pombe	Role	Overview
Cyclin B	Cdc28	Cdc13	Mitotic Cyclin	Mitotic cyclins must be degraded by APC/C-Cdh2 for chromosomes to decondense; however, this degradation is not needed for sister chromatids to separate during anaphase. Mitotic cyclins all have a conserved sequence at their N-terminus (Arg-X-X-Leu-Gly-X-Ile-Gly-X) that is recognized by APC/C-Cdh2; without this sequence, mitotic cyclins are not degraded. Cyclin-B+CDK1 is MPF in Xenopus and S. pombe.
CDK1	Cdc28	Cdc2	Mitotic CDK	MPF is Cyclin-B+CDK1 in Xenopus and S. pombe.
Cyclin-CDK		Nick-Name	Overview
Mid-G1-cyclin+CDK		SPF	Known as the s-phase promoting factor, as it is required to initiate DNA synthesis. Activates transcription factors for genes encoding the S phase cyclins and Sic1, and for genes needed for dNTP and DNA synthesis. Also, the G1 cyclin-CDK inactivates Cdh2, which would have destructed the S phase and mitotis cyclins.
Vertebrate	S. cerevisae	S. pombe	Role	Overview
Cyclin D	Cln3	Cdc13	Mid G1 Cyclin	Transcription “growth” factors activating Cyclin D and CDK4/6 are encoded by delayed response genes.
CDK4 & CDK6			Mid G1 CDK	Transcription “growth” factors activating Cyclin D and CDK4/6 are encoded by delayed response genes.

A cyclin-CDK consists of a regulatory subunit (the cyclin) and a kinase subunit (the cyclin-dependent kinase, aka CDK). CDKs trigger cell cycle events and regulate transcription and mRNA processing (except CDK9, which has a totally unrelated function). CDKs phosphorylate Serine and Threonine residues, but have little or no activity unless bound by a cyclin (hence their name). Cdc28p (the predominant yeast cyclin-dependent kinase involved in cell cycle control) is highly homologous to Cdk2.

High Copy Suppressor Experiment

Cdc28 (found in budding yeast) is highly homologous to Cdc2, a CDK found in S. pombe (fission yeast). Thus, it was theorized that Cdc28 is a CDK itself; thus, it must have a corresponding cyclin. If Cdc28 is a CDK, then in wild-type cells there must be a G₁ cyclin that bound Cdc28 to form an S-phase promoting factor (aka SPF).