S-phase-promoting factor

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

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]