Contents 1 Types 2 Role in cell cycle progression 2.1 CDK partner association 2.2 S phase 2.3 G2 / M phase 3 Regulation 3.1 E2F and pRb 3.2 p53 and p21 4 References 5 External links

Types[edit] Cyclin A was first identified in 1983 in sea urchin embryos.[4] Since its initial discovery, homologues of cyclin A have been identified in numerous eukaryotes including Drosophila,[5] Xenopus, mice, and in humans but has not been found in lower eukaryotes like yeast.[6][7] The protein exists in both an embryonic form and somatic form. A single cyclin A gene has been identified in Drosophila while Xenopus, mice and humans contain two distinct types of cyclin A: A1, the embryonic-specific form, and A2, the somatic form. Cyclin A1 is prevalently expressed during meiosis and early on in embryogenesis. Cyclin A2 is expressed in dividing somatic cells.[7]

Role in cell cycle progression[edit] Cyclin A, along with the other members of the cyclin family, regulates cell cycle progression through physically interacting with cyclin-dependent kinases (CDKs),[8][9] which thereby activates the enzymatic activity of its CDK partner.[1][2][8] CDK partner association[edit] The interaction between the cyclin box, a region conserved across cyclins, and a region of the CDK, called the PSTAIRE, confers the foundation of the cyclin-CDK complex.[10] Cyclin A is the only cyclin that regulates multiple steps of the cell cycle.[7] Cyclin A can regulate multiple cell cycle steps because it associates with, and thereby activates, two distinct CDKs – CDK2 and CDK1.[1] Depending on which CDK partner cyclin A binds, the cell will continue through the S phase or it will transition from G2 to the M phase.[1][3][10] Association of cyclin A with CDK2 is required for passage into S phase while association with CDK1 is required for entry into M phase.[10] S phase[edit] Cyclin A resides in the nucleus during S phase where it is involved in the initiation and completion of DNA replication.[1][6][9] As the cell passes from G1 into S phase, cyclin A associates with CDK2, replacing cyclin E. Cyclin E is responsible for initiating the assembly of the pre-replication complex. This complex makes chromatin capable of replication. When the amount of cyclin A/CDK2 complex reaches a threshold level, it terminates the assembly of the pre-replication complex made by cyclin E/CDK2. As the amount of Cyclin A/CDK2 complex increases, the complex initiates DNA replication.[11] Cyclin A has a second function in S phase, in addition to initiating DNA synthesis, Cyclin A ensures that DNA is replicated once per cell cycle by preventing the assembly of additional replication complexes.[7][11][12] This is thought to occur through the phosphorylation of particular DNA replication machinery components, such as CDC6, by the cyclin A/CDK2 complex.[1][7] Since the action of cyclin A/CDK2 inhibits that of cyclin E/CDK2, the sequential activation of cyclin E followed by the activation of cyclin A is important and tightly regulated in S phase.[7][11] G2 / M phase[edit] In late S phase, cyclin A can also associate with CDK1.[1][2][7] Cyclin A remains associated with CDK1 from late S into late G2 phase when it is replaced by cyclin B. Cyclin A/CDK1 is thought to be involved in the activation and stabilization of cyclin B/CDK1 complex.[7][8] Once cyclin B is activated, cyclin A is no longer needed and is subsequently degraded through the ubiquitin pathway.[3][7] Degradation of cyclin A/CDK1 induces mitotic exit.[7] Cyclin A/CDK2 complex was thought to be restricted to the nucleus and thus exclusively involved in S phase progression. New research has since debunked this assumption, shedding light on cyclin A/CDK2 migration to the centrosomes in late G2.[1][8] Cyclin A binds to the mitotic spindle poles in the centrosome however, the mechanism by which the complex is shuttled to the centrosome is not well understood. It is suspected that the presence of cyclin A/CDK2 at the centrosomes may confer a means of regulating the movement of cyclin B/CDK1 to the centrosome and thus the timing of mitotic events.[1][6][8] A study in 2008[8] provided further evidence of cyclin A/CDK2 complex's role in mitosis. Cells were modified so their CDK2 was inhibited and their cyclin A2 gene was knocked out. These mutants entered mitosis late due to a delayed activation of the cyclin B/CDK1 complex. Coupling of microtubule nucleation in the centrosome with mitotic events in the nucleus was lost in the cyclin A knockout/CDK2 inhibited mutant cells. Cyclin A has been shown to play a crucial role in the G2/M transition in Drosophila and Xenopus embryos.[3][6]

Regulation[edit] Transcription of cyclin A is tightly regulated and synchronized with cell cycle progression.[2][3] Initiation of transcription of cyclin A is coordinated with passage of the R point,[2] a critical transition point that is required for progression from G1 into S phase. Transcription peaks and plateaus mid-S phase and abruptly declines in late G2.[7][12] E2F and pRb[edit] Transcription of cyclin A is predominantly regulated by the transcription factor E2F in a negative feedback loop. E2F is responsible for initiating the transcription of many critical S phase genes.[1][3][6] Cyclin A transcription is off during most of G1 and the begins shortly after the R point.[3][7] The retinoblastoma protein (pRb) is involved in the regulation of cyclin A through its interaction with E2F. It exists in two states: hypophosphorylated pRb and hyperphosphorylated pRb.[2] Hypophosphorylated pRb binds E2F, which prevents transcription of cyclin A. The absence of cyclin A prior to the R point is due to the inhibition of E2F by hypophosphorylated pRb. After the cell passes through the R point, cyclin D/E- complexes phosphorylate pRb. Hyperphosphorylated pRb can no longer bind E2F, E2F is released and cyclin A genes, and other crucial genes for S phase, are transcribed.[2][9][12] E2F initiates transcription of cyclin A by de-repressing the promoter.[7][12] The promoter is bound by a repressor molecule called the cell-cycle-responsive element (CCRE). E2F binds to an E2F binding site on the CCRE, releasing the repressor from the promoter and allowing the transcription of cyclin A.[5][7] Cyclin A/CDK2 will eventually phosphorylate E2F when cyclin A reaches a certain level, completing the negative feedback loop. Phosphorylation of E2F turns the transcription factor off, providing another level of controlling the transcription of cyclin A.[7] p53 and p21[edit] Transcription of cyclin A is indirectly regulated by the tumor suppressor protein p53. P53 is activated by DNA damage and turns on several downstream pathways, including cell cycle arrest. Cell cycle arrest is carried out by the p53-pRb pathway.[13] Activated p53 turns on genes for p21. P21 is a CDK inhibitor that binds to several cyclin/CDK complexes, including cyclin A-CDK2/1 and cyclin D/CDK4, and blocks the kinase activity of CDKs.[9][13] Activated p21 can bind cyclin D/CDK4 and render it incapable of phosphorylating pRb. PRb remains hypophosphorylated and binds E2F. E2F is unable to activate the transcription of cyclins involved in cell cycle progression, such as cyclin A and the cell cycle is arrested at G1.[6][13] Cell cycle arrest allows the cell to repair DNA damage before the cell divides and passes damaged DNA to daughter cells.

References[edit] ^ a b c d e f g h i j k Bendris N, Lemmers B, Blanchard JM, Arsic N (2011). "Cyclin A2 mutagenesis analysis: a new insight into CDK activation and cellular localization requirements". PLoS ONE. 6 (7): e22879. doi:10.1371/journal.pone.0022879. PMC 3145769 . PMID 21829545.  ^ a b c d e f g Weinberg RE (2007). The biology of cancer. New York: Garland Science. ISBN 0-8153-4076-1.  ^ a b c d e f g Henglein B, Chenivesse X, Wang J, Eick D, Bréchot C (June 1994). "Structure and cell cycle-regulated transcription of the human cyclin A gene". Proc. Natl. Acad. Sci. U.S.A. 91 (12): 5490–4. doi:10.1073/pnas.91.12.5490. PMC 44021 . PMID 8202514.  ^ Evans T, Rosenthal ET, Youngblom J, Distel D, Hunt T (June 1983). "Cyclin: a protein specified by maternal mRNA in sea urchin eggs that is destroyed at each cleavage division". Cell. 33 (2): 389–96. doi:10.1016/0092-8674(83)90420-8. PMID 6134587.  ^ a b Huet X, Rech J, Plet A, Vié A, Blanchard JM (July 1996). "Cyclin A expression is under negative transcriptional control during the cell cycle". Mol. Cell. Biol. 16 (7): 3789–98. PMC 231375 . PMID 8668196.  ^ a b c d e f Pagano M, Pepperkok R, Verde F, Ansorge W, Draetta G (March 1992). "Cyclin A is required at two points in the human cell cycle". EMBO J. 11 (3): 961–71. PMC 556537 . PMID 1312467.  ^ a b c d e f g h i j k l m n o Yam CH, Fung TK, Poon RY (August 2002). "Cyclin A in cell cycle control and cancer". Cell. Mol. Life Sci. 59 (8): 1317–26. doi:10.1007/s00018-002-8510-y. PMID 12363035.  ^ a b c d e f De Boer L, Oakes V, Beamish H, Giles N, Stevens F, Somodevilla-Torres M, Desouza C, Gabrielli B (July 2008). "Cyclin A/cdk2 coordinates centrosomal and nuclear mitotic events". Oncogene. 27 (31): 4261–8. doi:10.1038/onc.2008.74. PMID 18372919.  ^ a b c d Soucek T, Pusch O, Hengstschläger-Ottnad E, Adams PD, Hengstschläger M (May 1997). "Deregulated expression of E2F-1 induces cyclin A- and E-associated kinase activities independently from cell cycle position". Oncogene. 14 (19): 2251–7. doi:10.1038/sj.onc.1201061. PMID 9178900.  ^ a b c Jeffrey PD, Russo AA, Polyak K, Gibbs E, Hurwitz J, Massagué J, Pavletich NP (July 1995). "Mechanism of CDK activation revealed by the structure of a cyclinA-CDK2 complex". Nature. 376 (6538): 313–20. doi:10.1038/376313a0. PMID 7630397.  ^ a b c Coverley D, Laman H, Laskey RA (July 2002). "Distinct roles for cyclins E and A during DNA replication complex assembly and activation". Nat. Cell Biol. 4 (7): 523–8. doi:10.1038/ncb813. PMID 12080347.  ^ a b c d Woo RA, Poon RY (2003). "Cyclin-dependent kinases and S phase control in mammalian cells". Cell Cycle. 2 (4): 316–24. doi:10.4161/cc.2.4.468. PMID 12851482.  ^ a b c Levine AJ (February 1997). "p53, the cellular gatekeeper for growth and division". Cell. 88 (3): 323–31. doi:10.1016/S0092-8674(00)81871-1. PMID 9039259. 

External links[edit] Drosophila Cyclin A - The Interactive Fly Cyclin A at the US National Library of Medicine Medical Subject Headings (MeSH) v t e Cell cycle proteins Cyclin A (A1, A2) B (B1, B2, B3) D (D1, D2, D3) E (E1, E2) CDK 1 2 3 4 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 G1 phase S phase G2 phase M phase Mitosis (Preprophase Prophase Prometaphase Metaphase Anaphase Telophase) Cytokinesis Cell cycle checkpoints Restriction point Spindle checkpoint Postreplication checkpoint Other cellular phases Apoptosis G0 phase Meiosis Retrieved from "" Categories: Genes on human chromosome 13Genes on human chromosome 4Cell cycleProteinsHidden categories: Protein pages needing a picture

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CCNA1CCNA1EntrezHuman Genome OrganisationOMIMNational Center For Biotechnology InformationSwiss-ProtLocus (genetics)Chromosome 13 (human)Cyclin A2CCNA2EntrezHuman Genome OrganisationOMIMNational Center For Biotechnology InformationSwiss-ProtLocus (genetics)Chromosome 4 (human)CyclinProteinsCell CycleHomology (biology)DrosophilaXenopusEmbryoSomatic (biology)Cyclin A1Cyclin A2Expression Of Human Cyclins Through The Cell CycleCyclin-dependent KinasesCyclinCyclin-dependent KinaseCDK2CDK1S PhaseG2 PhaseM PhaseDNA ReplicationG1 PhaseCyclin EPre-replication ComplexChromatinPhosphorylationCDC6Cyclin BUbiquitinMitotic ExitCentrosomesMitosisGene KnockoutMicrotubule NucleationTranscription (genetics)R PointE2FNegative Feedback LoopRetinoblastoma ProteinCyclin DDerepressionRepressorTumor Suppressor Protein P53DNA DamageP21Cyclin-dependent Kinase Inhibitor ProteinCDK4Digital Object IdentifierPubMed CentralPubMed IdentifierInternational Standard Book NumberSpecial:BookSources/0-8153-4076-1Digital Object IdentifierPubMed CentralPubMed IdentifierDigital Object IdentifierPubMed IdentifierPubMed CentralPubMed IdentifierPubMed CentralPubMed IdentifierDigital Object IdentifierPubMed IdentifierDigital Object IdentifierPubMed IdentifierDigital Object IdentifierPubMed IdentifierDigital Object IdentifierPubMed IdentifierDigital Object IdentifierPubMed IdentifierDigital Object IdentifierPubMed IdentifierDigital Object IdentifierPubMed IdentifierMedical Subject HeadingsTemplate:Cell Cycle ProteinsTemplate Talk:Cell Cycle ProteinsCell CycleProteinCyclinCyclin A1Cyclin A2Cyclin BCyclin B1Cyclin B2Cyclin DCyclin D1Cyclin D2Cyclin D3Cyclin ECyclin E1Cyclin E2Cyclin-dependent KinaseCyclin-dependent Kinase 1Cyclin-dependent Kinase 2Cyclin-dependent Kinase 3Cyclin-dependent Kinase 4Cyclin-dependent Kinase 5Cyclin-dependent Kinase 6Cyclin-dependent Kinase 7Cyclin-dependent Kinase 8Cyclin-dependent Kinase 9Cyclin-dependent Kinase 10CDK-activating KinaseCyclin-dependent Kinase Inhibitor ProteinCell CycleP14arfP16CDKN2BCDKN2CCDKN2DCell CycleP21CDKN1BCyclin-dependent Kinase Inhibitor 1CP53 P63 P73 FamilyP53TP63P73Cdk1Cdc25CDC42Cellular Apoptosis Susceptibility ProteinE2FMaturation Promoting FactorWee1CullinCUL7InterphaseG1 PhaseS PhaseG2 PhaseCell DivisionMitosisPreprophaseProphasePrometaphaseMetaphaseAnaphaseTelophaseCytokinesisCell Cycle CheckpointRestriction PointSpindle CheckpointPostreplication CheckpointApoptosisG0 PhaseMeiosisHelp:CategoryCategory:Genes On Human Chromosome 13Category:Genes On Human Chromosome 4Category:Cell CycleCategory:ProteinsCategory:Protein Pages Needing A PictureDiscussion About Edits From This IP Address [n]A List Of Edits Made From This IP Address [y]View The Content Page [c]Discussion About The Content Page [t]Edit This Page [e]Visit The Main Page [z]Guides To Browsing WikipediaFeatured Content – The Best Of WikipediaFind Background Information On Current EventsLoad A Random Article [x]Guidance On How To Use And Edit WikipediaFind Out About WikipediaAbout The Project, What You Can Do, Where To Find ThingsA List Of Recent Changes In The Wiki [r]List Of All English Wikipedia Pages Containing Links To This Page [j]Recent Changes In Pages Linked From This Page [k]Upload Files [u]A List Of All Special Pages [q]Wikipedia:AboutWikipedia:General Disclaimer