#Introduction: ##S-phase-promoting factor(SPF) is varying Cdk/

cyclin Cyclin is a family of proteins that controls the progression of a cell through the cell cycle by activating cyclin-dependent kinase (CDK) enzymes or group of enzymes required for synthesis of cell cycle. Etymology Cyclins were originally disco ...

complexes in

eukaryotes Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bact ...

that initiates the

S-phase S phase (Synthesis Phase) is the phase of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Since accurate duplication of the genome is critical to successful cell division, the processes that occur during ...

in the cell cycle. SPF is at its peak when the cell cycle is transiting from

G1 phase The G1 phase, gap 1 phase, or growth 1 phase, is the first of four phases of the cell cycle that takes place in eukaryotic cell division. In this part of interphase, the cell synthesizes mRNA and proteins in preparation for subsequent steps lea ...

to the

. The SPF is at its lowest during the cell cycle once the

subunits are used up, and broken down. Therefore, everything that happens during

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maint ...

is irreversible, which is why there are many steps within the

cell cycle The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA ( DNA replication) and some of its organelles, and sub ...

. However, these steps are irreversible because one is needed in order for the next step to occur. #Control of S-phase-promoting factor: ## The S-phase-promoting factor is controlled by regulating

cyclins Cyclin is a family of proteins that controls the progression of a cell through the cell cycle by activating cyclin-dependent kinase (CDK) enzymes or group of enzymes required for synthesis of cell cycle. Etymology Cyclins were originally di ...

levels, and by inhibitors seen in the other phases, such as G1. One specific inhibitor seen in G1 is known as

stoichiometric inhibitors Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equals ...

, and causes the inhibition of cdk/cyclin complexes. Regulating

levels is done by the production and destruction of cyclin, which is done through the

phosphorylation In chemistry, phosphorylation is the attachment of a phosphate group to a molecule or an ion. This process and its inverse, dephosphorylation, are common in biology and could be driven by natural selection. Text was copied from this source, ...

and

dephosphorylation In biochemistry, dephosphorylation is the removal of a phosphate (PO43−) group from an organic compound by hydrolysis. It is a reversible post-translational modification. Dephosphorylation and its counterpart, phosphorylation, activate and deact ...

anaphase-promoting complex Anaphase-promoting complex (also called the cyclosome or APC/C) is an E3 ubiquitin ligase that marks target cell cycle proteins for degradation by the 26S proteasome. The APC/C is a large complex of 11–13 subunit proteins, including a cul ...

(APC). This controls the rate of production of

, and regulates cyclin levels and controls the S-phase-promoting factor. # S-phase: ## During cell replication when DNA is replicated, and is initiated by the S-phase-promoting factor(SPF) cyclin complexes. The DNA replication takes place, due to the increase in SPF during the switching from G1 to

S phase S phase (Synthesis Phase) is the phase of the cell cycle in which DNA is replicated, occurring between G1 phase and G2 phase. Since accurate duplication of the genome is critical to successful cell division, the processes that occur during ...

in the cell cycle. 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 In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...

to occur. # Cyclins: ## There are a variety of cyclins that can be found, and vary based on the type of

eukaryotic cell Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...

. However, there are two

that are found in all

. The presence of cyclin-CDK is crucial for the replication of DNA to occur in the

. ## Through different studies done on the effects and contributions to DNA replication, it is clear that certain

hold significant influences over SPF activity. For instance, there was a particular study done on the activity of

Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-kno ...

eggs. This research indicated the importance of

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. Specifically, different concentrations contributed to the activity of SPF, which affects DNA replication. Having high concentration of cyclin A within the cell cycle causes

to occur, which directly affects DNA replication by being inhibited. Therefore, the type of

and their concentrations have a direct effect on the activity of SPF when in S-phase, which has an effect on DNA replication. The table conveys different

, and Cyclin-CDK complexes needed for the species to initiate DNA replication, which occurs in the S-phases.

References

See also