1. Introduction:
    1. S-phase-promoting factor(SPF) is varying Cdk/cyclin complexes in eukaryotes that initiates the S-phase in the cell cycle.[1] SPF is at its peak when the cell cycle is transiting from G1 phase to the S-phase.[1] The SPF is at its lowest during the cell cycle once the cyclin subunits are used up, and broken down.[1] Therefore, everything that happens during mitosis is irreversible, which is why there are many steps within the cell cycle.[1] However, these steps are irreversible because one is needed in order for the next step to occur.[1]
  2. Control of S-phase-promoting factor:
    1. The S-phase-promoting factor is controlled by regulating cyclins levels, and by inhibitors seen in the other phases, such as G1.[1] One specific inhibitor seen in G1 is known as stoichiometric inhibitors, and causes the inhibition of cdk/cyclin complexes.[1] Regulating cyclin levels is done by the production and destruction of cyclin, which is done through the phosphorylation and dephosphorylation of anaphase-promoting complex (APC).[1] This controls the rate of production of cyclin, and regulates cyclin levels and controls the S-phase-promoting factor.[1]
  3. S-phase:
    1. During cell replication when DNA is replicated, and is initiated by the S-phase-promoting factor(SPF) cyclin complexes.[2] The DNA replication takes place, due to the increase in SPF during the switching from G1 to S phase in the cell cycle.[2] SPF is also used to inhibit double replication of chromosomes  in the cell cycle, which is important for not allowing a duplication of our genome to occur.[2]
  4. Cyclins:
    1. There are a variety of cyclins that can be found, and vary based on the type of eukaryotic cell. However, there are two cyclins that are found in all eukaryotes.[2] The presence of cyclin-CDK is crucial for the replication of DNA to occur in the S-phase.[2]
    2. Through different studies done on the effects and contributions to DNA replication, it is clear that certain cyclins hold significant influences over SPF activity.[3] For instance, there was a particular study done on the activity of Xenopus eggs.[3] This research indicated the importance of cyclins A, E and B in regards to the activity of SPF. It was concluded that there was more influence over the activity of SPF with different combinations of cyclins A and E, whereas there was not for cyline B.[3] Specifically, different concentrations contributed to the activity of SPF, which affects DNA replication. Having high concentration of cyclin A within the cell cycle causes mitosis to occur, which directly affects DNA replication by being inhibited.[3]  Therefore, the type of cyclins and their concentrations have a direct effect on the activity of SPF when in S-phase, which has an effect on DNA replication.[3]
Cyclins Species
cyclin E (cycE)-Cdk2, cycA-Cdk2, and cycA-Cdc2 kinases Humans, frogs and flies
Clb5p-Cdc28p, Clb6p-Cdc28p, and cig2-cdc2 Yeast

The table conveys different eukaryotes, and Cyclin-CDK complexes needed for the species to initiate DNA replication, which occurs in the S-phases.[2]

References

  1. 1 2 3 4 5 6 7 8 9 Kapuy, Orsolya; He, Enuo; López-Avilés, Sandra; Uhlmann, Frank; Tyson, John J.; Novák, Béla (2009-12-17). "System-level feedbacks control cell cycle progression". FEBS Letters. 583 (24): 3992–3998. doi:10.1016/j.febslet.2009.08.023. PMC 3811919. PMID 19703449.
  2. 1 2 3 4 5 6 Nougarède, Romain; Della Seta, Flavio; Zarzov, Patrick; Schwob, Etienne (June 2000). "Hierarchy of S-Phase-Promoting Factors: Yeast Dbf4-Cdc7 Kinase Requires Prior S-Phase Cyclin-Dependent Kinase Activation". Molecular and Cellular Biology. 20 (11): 3795–3806. doi:10.1128/mcb.20.11.3795-3806.2000. ISSN 0270-7306. PMC 85702. PMID 10805723.
  3. 1 2 3 4 5 Strausfeld, U.P.; Howell, M.; Descombes, P.; Chevalier, S.; Rempel, R.E.; Adamczewski, J.; Maller, J.L.; Hunt, T.; Blow, J.J. (1996-06-01). "Both cyclin A and cyclin E have S-phase promoting (SPF) activity in Xenopus egg extracts". Journal of Cell Science. 109 (6): 1555–1563. doi:10.1242/jcs.109.6.1555. ISSN 1477-9137. PMID 8799842.

See also


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