<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2010) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2010) (Learn how and when to remove this template message) </Td> </Tr> <P> S phase (synthesis phase) is the part of the cell cycle in which DNA is replicated, occurring between G phase and G 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 . </P> <P> The G1 / S transition is a major checkpoint in the regulation of the cell cycle . To produce two similar daughter cells, the centrosome and 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 G 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), 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 . </P>

When is dna synthesized in the cell cycle