S phase - wikipedia S phase 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 ) 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. Contents 1 Regulation 2 DNA replication 3 DNA damage 4 See also 5 References Regulation 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. DNA replication 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 G 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 10 nucleotide additions. DNA damage Damage to DNA is detected and fixed during S - phase. When the replication fork comes upon damaged DNA, ATR, a protein kinase is activated. This kinase initiates several complex downstream pathways, which causes a halt in the initiation of new replication origins, prevention of mitosis and replication fork stabilization in order to keep the replication bubble open and DNA polymerase complex attached while the damage is being fixed. See also S phase index ( SPI ) S - fraction or S - phase fraction ( oncology / pathology prognosis ) References Jump up ^ Alberts, Bruce ; Bray, Dennis ; Hopkin, Karen ; Johnson, Alexander ; Lewis, Julian ; Raff, Martin ; Roberts, Keith ; Walter, Peter ( 2014 ). Essential Cell Biology ( Fourth ed. ). New York, New York, USA : Garland Science. p. 642. ISBN 978 - 0 - 8153 - 4454 - 4. access - date = requires url = ( help ) Jump up ^ Bell, S.P. and Dutta, A. : DNA replication in eukaryotic cells. Annu. Rev. Biochem. 2002, 71 : 333 -- 374. Jump up ^ Cooper, S. and Helmstetter, C.E. 1968 DNA synthesis during the division cycle of rapidly growing Escherichia coli. Journal of Molecular Biology, 31 ( 3 ) : 507 -- 518. Jump up ^ Branzei, D and Foiani, M. : The DNA damage response during DNA replication. Curr. Opin. Cell Biol. 2005, 17 : 568 -- 575. Cell cycle proteins Cyclin A ( A1, A2 ) B ( B1, B2, B3 ) D ( D1, D2, D3 ) E ( E1, E2 ) CDK 5 6 7 8 9 10 CDK - activating kinase CDK inhibitor INK4a / ARF ( p14arf / p16, p15, p18, p19 ) cip / kip ( p21, p27, p57 ) P53 p63 p73 family p53 p63 p73 Other Cdc2 Cdc25 Cdc42 Cellular apoptosis susceptibility protein E2F Maturation promoting factor Wee Cullin ( CUL7 ) Phases and checkpoints Interphase G phase S phase G phase M phase Mitosis ( Preprophase Prophase Prometaphase Metaphase Anaphase Telophase ) Cytokinesis Cell cycle checkpoints Restriction point Spindle checkpoint Postreplication checkpoint Other cellular phases Apoptosis G phase Meiosis Retrieved from `` https://en.wikipedia.org/w/index.php?title=S_phase&oldid=841200555 '' Categories : Cell cycle Hidden categories : Pages using citations with accessdate and no URL Articles needing additional references from December 2010 All articles needing additional references All articles with unsourced statements Articles with unsourced statements from May 2012 Talk Contents About Wikipedia Bosanski Čeština فارسی Hrvatski Bahasa Indonesia Italiano Nederlands Polski Português Русский Scots Српски / srpski Srpskohrvatski / српскохрватски Türkçe 中文 7 more Edit links This page was last edited on 14 May 2018, at 15 : 43 ( UTC ). 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Question: when is dna synthesized in the cell cycle
Answer: S phase (synthesis phase)