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= S-phase-promoting factor =


 * 1) Control of S-phase-promoting factor:
 * 2) The S-phase-promoting factor is controlled by regulating cyclins levels, and by inhibitors seen in the other phases, such as G1( Kapuy, 2009). One specific inhibitor seen in G1 is known as stoichiometric inhibitors, and causes the inhibition of cdk/cyclin complexes. 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)( Kapuy, 2009). This controls the rate of production of cyclin, and regulates cyclin levels and controls the S-phase-promoting factor(Kapuy, 2009).
 * 3) S-phase:
 * 4) During cell replication when DNA is replicated, and is initiated by the S-phase-promoting factor(SPF) cyclin complexes(Nougarede, 2009). The DNA replication takes place, due to the increase in SPF during the switching from G1 to S phase in the cell cycle(Nougarede, 2000). 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 (Nougarede, 2000).
 * 5) Cyclins:
 * 6) There are a variety of cyclins that can be found, and vary based on the type of eukaryotic cell. However, there are two that are found in all eukaryotes(Nougaredel, 2000).. The presence of cyclin-CDK is crucial for the replication of DNA to occur in the S-phase(Nougarede, 2000).
 * 7) Through different studies done on the effects and contributions to DNA replication, it is clear that certain cyclins hold significant influence over SPF activity(Strausfeld, 1996). For instance, there was a particular study done on the activity of Xenopus(frog) eggs(Strausfeld, 1996). 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(Strausfeld, 1996). 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(Strausfeld, 1996).  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(Strausfeld,1996).

The table conveys different eukaryotes, and Cyclin-CDK complexes needed for the species to initiate DNA replication, which occurs in the S-phases(Nougarede, 2000).

Works Cited

Nougarède, R, et al. “Hierarchy of S-Phase-Promoting Factors: Yeast Dbf4-Cdc7 Kinase Requires Prior S-Phase Cyclin-Dependent Kinase Activation.” Molecular and Cellular Biology, American Society for Microbiology, June 2000, www.ncbi.nlm.nih.gov/pmc/articles/PMC85702/.

Strausfeld, U.p., et al. “Both Cyclin A and Cyclin E Have S-Phase Promoting (SPF) Activity in Xenopus Egg Extracts.” Journal of Cell Science, vol. 109, no. 6, 1996, pp. 1555–1563., doi:10.1242/jcs.109.6.1555.

Kapuy, O., He, E., López-Avilés, S., Uhlmann, F., Tyson, J. J., & Novák, B. (2009). System-level feedbacks control cell cycle progression. FEBS Letters, 583(24), 3992–3998. https://doi.org/10.1016/j.febslet.2009.08.023