S phase

S-phase (synthesis phase) is the part of the cell cycle in which DNA is replicated, occurring between G₁ phase and G₂ phase. Precise and accurate DNA replication is necessary to prevent genetic abnormalities which often lead to cell death or disease. Due to the importance, the regulatory pathways that govern this event in eukaryotes are highly conserved. This conservation makes the study of S-phase in model organisms such as Xenopus laevis embryos and budding yeast relevant to higher organisms.

The G1/S transition is a major checkpoint in the regulation of the cell cycle. S Phase: To produce two similar daughter cells, the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase. Depending on levels of nutrients, energy and external factors, cells must decide to enter the cell cycle or move into a non-dividing state known as G₀ phase. This transition, as with all of the major checkpoint transitions in the cell cycle, is signaled by cyclins and cyclin dependent kinase (CDKs). The pulse of G1/S cyclins causes CLN3-Cdk1 to activate Cln1/2, (Start point (yeast)) as well as Clb5/6 at the initiation of S-phase. This pathway contains 2 positive feedback loops, allowing for rapid, unidirectional movement into S-phase. Redundant pathways like this are not uncommon because they allow for tuning the output of the system and often lead to faster genetic evolution.

The major event in S-phase is DNA replication. The goal of this process is to create exactly two identical semi-conserved chromosomes. The cell prevents more than one replication from occurring by loading pre-replication complexes onto the DNA at replication origins during G₁ phase which are dismantled in S-phase as replication begins. In synthesis, the enzyme helicase unwinds the DNA double helix, and the enzyme DNA polymerase re-binds free-floating nucleotides to the separate DNA single strands in accordance with the complementary base pairing rule. DNA synthesis can occur as fast as 2000 nucleotides/second and must create fewer than 2 errors (I.e., wrong bases) in 10¹⁰ nucleotide additions.

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