S-phase-promoting factor

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#Introduction: ##S-phase-promoting factor(SPF) is varying Cdk/cyclin complexes in eukaryotes that initiates the S-phase in the cell cycle.<ref name=":2" /> SPF is at its peak when the cell cycle is transiting from G1 phase to the S-phase.<ref name=":2" /> The SPF is at its lowest during the cell cycle once the cyclin subunits are used up, and broken down.<ref name=":2" /> Therefore, everything that happens during mitosis is irreversible, which is why there are many steps within the cell cycle.<ref name=":2" /> However, these steps are irreversible because one is needed in order for the next step to occur.<ref name=":2" /> #Control of S-phase-promoting factor: ## The S-phase-promoting factor is controlled by regulating cyclins levels, and by inhibitors seen in the other phases, such as G1.<ref name=":2" /> One specific inhibitor seen in G1 is known as stoichiometric inhibitors, and causes the inhibition of cdk/cyclin complexes.<ref name=":2" /> 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).<ref name=":2" /> This controls the rate of production of cyclin, and regulates cyclin levels and controls the S-phase-promoting factor.<ref name=":2" /> # S-phase: ## During cell replication when DNA is replicated, and is initiated by the S-phase-promoting factor(SPF) cyclin complexes.<ref name=":0" /> The DNA replication takes place, due to the increase in SPF during the switching from G1 to S phase in the cell cycle.<ref name=":0" /> 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.<ref name=":0" /> # Cyclins: ## 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.<ref name=":0" /> The presence of cyclin-CDK is crucial for the replication of DNA to occur in the S-phase.<ref name=":0" /> ## Through different studies done on the effects and contributions to DNA replication, it is clear that certain cyclins hold significant influences over SPF activity.<ref name=":1" /> For instance, there was a particular study done on the activity of Xenopus eggs.<ref name=":1" /> 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.<ref name=":1" /> 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.<ref name=":1" /> 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.<ref name=":1" />

{| class="wikitable" |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.<ref name=":0" />

==References== <references>

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== See also == * Restriction point

Category:Cell cycle Category:DNA replication Category:Enzymes