Touchback - wikipedia Touchback Jump to : navigation , search This article is about the football term . For the 2011 film , see Touchback ( film ) . In American football , a touchback is a ruling which is made and signaled by an official when the ball becomes dead on or behind a team 's own goal line ( i.e. , in an end zone ) and the opposing team gave the ball the momentum , or impetus , to travel over or across the goal line . Such impetus may be imparted by a kick , pass , fumble , or in certain instances by batting the ball . A touchback is not a play , but a result of events that may occur during a play . A touchback is the opposite of a safety with regard to impetus since a safety is scored when the defending team is responsible for the ball becoming dead on or behind its own goal line . Examples of instances where a touchback would be awarded include when : A kickoff or punt enters the end zone and is downed by the receiving team without the ball being advanced beyond the goal line . Thus , a player on the receiving team could attempt to advance the ball out of his own end zone , but the original impetus from the kick remains as long as the ball does not completely cross the goal line into the field of play . If a kick is fielded by the receiving team in its end zone , is advanced beyond the goal line , and then the ball carrier retreats back into his own end zone where the ball is downed , it is a safety . If a member of the kicking team recovers a kick - off in the end zone , the play is ruled as a touchdown . In high school football , any kicked ball that crosses the plane of the goal line , unless it is a successful field goal . A kickoff or punt touches the ground in the receiving team 's end zone before being touched by a player of the receiving team . However , in the NFL , a kickoff is considered live until it is downed by the receiving team . If a kicked - off ball in the NFL goes untouched into the end zone and then is recovered by a member of the kicking team , it 's a touchdown for the kicking team . A kickoff or punt goes out of bounds behind or over the receiving team 's goal line or touches the goal posts or crossbar ( and does not score a field goal ) . A ball carrier fumbles the ball within the field of play forward into his opponents end zone and the loose ball then goes out of bounds behind or above his opponent 's goal line , is recovered and downed by an opposing player in the end zone , or touches the pylon . The opposing team would be awarded the touchback . A defensive player intercepts a forward pass in his own end zone and the ball becomes dead behind or over the goal line . Like the instance of a kickoff or punt fielded in an end zone , the intercepting player can attempt to advance the ball but it is still a touchback as long as the ball never completely crosses the goal line into the field of play before it is downed . A blocked punt goes back into the end zone and the defensive team intentionally bats or kicks the ball out the back of the end zone . Offense must decline penalty . Contents ( hide ) 1 American football 2 Canadian football 3 Six - and eight - man football 4 Differences 5 References American football ( edit ) In standard outdoor American football , the team awarded the touchback receives possession of the ball at its own 20 - yard line in college football , and the 20 yard - line for professional football , on kickoffs and free kicks after a safety as of the 2012 season . In arena football , and other indoor football games , a touchback results in the team awarded the touchback receiving the football at its own 3 - yard line . This can result from any of the above events except for punting , which is not a part of arena football . ( In arena football , a kicked ball usually bounces back into play off of the rebound nets , but the above can still occur when the ball lands in the slack nets behind the goalposts after a kickoff , passes under the rebound nets and out of play , or in the event of fumbles and interceptions . ) On March 23 , 2016 , the NFL announced that it would award a touchback on kickoffs at the 25 - yard line instead of the previous 20 - yard line and much discussion and analysis on the impact of this change has emerged . This new rule was to be re-evaluated after the 2016 NFL season . If a defensive player gains possession of the ball during a play between his own five - yard line and goal line and the player 's original momentum carries him into the end zone , there is no touchback . Instead , the ball is dead at the point where possession changed . In the National Football League , this rule applies regardless of whether possession is gained inside the five - yard line . In July 2016 , the NCAA announced that it had approved the Ivy League to test an experimental kickoff rule in conference play in the 2016 season . Under this rule , kickoffs will be taken from the 40 - yard line instead of the 35 , and touchbacks will be marked at the 20 - yard line instead of the 25 . The rule will be re-evaluated in February 2017 . Canadian football ( edit ) In Canadian football the term touchback is not used . The failure to advance a kicked ball out of the goal area results in a single point being scored by the kickers , as well as possession by the receivers at their 35 - yard line or at the point the ball was kicked from . A turn - over by fumble or interception in the defense 's goal area that is not advanced back into the field of play , or a kickoff untouched out of bounds in the end zone , or a kick that touches the goal posts , crossbar , or goal assembly results in a scrimmage on the 25 - yard line with no points awarded . If a player 's momentum causes the ball to be in the end zone , the ball is treated as if it was recovered in the end zone . Six - and eight - man football ( edit ) For high schools which play six - man football and eight - man football on an 30 - yard long field , a touchback is brought out to the 15 - yard line . Differences ( edit ) A special rule applies in college football and the NFL with regard to field goal attempts . If a missed field goal occurs in these leagues , where the other team receives possession of the ball depends on the spot from which the ball has been kicked . In NCAA football , the ball will be placed either on the twenty or the line of scrimmage of the play in which the attempt was made ; in the NFL , either the twenty or the place from which the ball was kicked . ( In either case , the ball goes to the spot which is further from the goal line . ) The purpose of this rule is to discourage low - percentage , long - range field goal attempts and to deemphasize the advantage which can accrue when only one team has a kicker who has a reasonable possibility of success from a great distance . In American high school football ( except in Massachusetts and Texas , which use college football rules ) , the missed field goal , regardless of where attempted on the field , results in a touchback as long as the attempt breaks the plane of the goal line , and in arena football , the field goal is treated as if it were a punt . References ( edit ) Jump up ^ National Football League . 2012 Official Playing Rules and Casebook . Retrieved September 12 , 2013 . Jump up ^ The anatomy of the new touchback rule -- https://412sportsanalytics.wordpress.com/2016/05/04/the-anatomy-of-the-new-touchback-rule/ Jump up ^ NFL passes automatic ejection rule for 2016 season -- http://www.nfl.com/news/story/0ap3000000646837/article/nfl-passes-automatic-ejection-rule-for-2016-season Jump up ^ `` Ivy League to move kickoffs to 40 - yard line '' ( Press release ) . NCAA . July 20 , 2016 . Retrieved July 23 , 2016 . ( hide ) Gridiron football plays Play from scrimmage Play calling system Offense Rush Keeper Bootleg Sneak Dive Power Counter Off - tackle End Option Sweep Toss Trap Veer Zone Pass Incomplete Checkdown Screen Drop - back Hail Mary Alley - oop Defense Man - to - man Zone Blitz Zone blitz Special teams Touchback Kickoff Kickoff return Punt Punt return Drop kick Place kick Fair catch Fair catch kick Onside kick Squib kick Other Trick play Quick kick Retrieved from `` https://en.wikipedia.org/w/index.php?title=Touchback&oldid=818499844 '' Categories : American football plays American football terminology Canadian football terminology Talk Contents About Wikipedia Deutsch Español Français Italiano עברית Polski Português Edit links This page was last edited on 3 January 2018 , at 22 : 47 . Text is available under the Creative Commons Attribution - ShareAlike License ; additional terms may apply . By using this site , you agree to the Terms of Use and Privacy Policy . 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Question: what is the new touchback rule in the nfl

a touchback on kickoffs at the 25-yard line instead of the previous 20-yard line


Cell cycle - wikipedia Cell cycle Jump to : navigation , search For the separation of chromosomes that occurs as part of the cell cycle , see mitosis . For the Academic journal , see Cell Cycle ( journal ) . See also : Cell division Life Cycle of the cell Onion ( Allium ) cells in different phases of the cell cycle . Growth in an ' organism ' is carefully controlled by regulating the cell cycle . The cell cycle or cell - division cycle is the series of events that take place in a cell leading to its division and duplication of its DNA ( DNA replication ) to produce two daughter cells . In bacteria , which lack a cell nucleus , the cell cycle is divided into the B , C , and D periods . The B period extends from the end of cell division to the beginning of DNA replication . DNA replication occurs during the C period . The D period refers to the stage between the end of DNA replication and the splitting of the bacterial cell into two daughter cells . In cells with a nucleus , as in eukaryotes , the cell cycle is also divided into three periods : interphase , the mitotic ( M ) phase , and cytokinesis . During interphase , the cell grows , accumulating nutrients needed for mitosis , preparing it for cell division and duplicating its DNA . During the mitotic phase , the chromosomes separate . During the final stage , cytokinesis , the chromosomes and cytoplasm separate into two new daughter cells . To ensure the proper division of the cell , there are control mechanisms known as cell cycle checkpoints . The cell - division cycle is a vital process by which a single - celled fertilized egg develops into a mature organism , as well as the process by which hair , skin , blood cells , and some internal organs are renewed . After cell division , each of the daughter cells begin the interphase of a new cycle . Although the various stages of interphase are not usually morphologically distinguishable , each phase of the cell cycle has a distinct set of specialized biochemical processes that prepare the cell for initiation of cell divisions . Contents ( hide ) 1 Phases 1.1 G phase ( quiescence ) 1.2 Interphase ( intermitosis ) 1.2. 1 G phase ( First growth phase or Post mitotic gap phase ) 1.2. 2 S phase ( DNA replication ) 1.2. 3 G phase ( growth ) 1.3 Mitotic phase ( chromosome separation ) 1.4 Cytokinesis phase ( separation of all cell components ) 2 Regulation of eukaryotic cell cycle 2.1 Role of cyclins and CDKs 2.1. 1 General mechanism of cyclin - CDK interaction 2.1. 2 Specific action of cyclin - CDK complexes 2.2 Inhibitors 2.3 Transcriptional regulatory network 2.4 DNA replication and DNA replication origin activity 3 Checkpoints 4 Fluorescence imaging of the cell cycle 5 Role in tumor formation 6 See also 7 References 8 Further reading 9 External links Phases ( edit ) The cell cycle consists of four distinct phases : G phase , S phase ( synthesis ) , G phase ( collectively known as interphase ) and M phase ( mitosis ) . M phase is itself composed of two tightly coupled processes : karyokinesis , in which the cell 's chromosomes are divided , and cytokinesis , in which the cell 's cytoplasm divides forming two daughter cells . Activation of each phase is dependent on the proper progression and completion of the previous one . Cells that have temporarily or reversibly stopped dividing are said to have entered a state of quiescence called G phase . Schematic of the cell cycle . outer ring : I = Interphase , M = Mitosis ; inner ring : M = Mitosis , G = Gap 1 , G = Gap 2 , S = Synthesis ; not in ring : G = Gap 0 / Resting . State Phase Abbreviation Description Resting Gap 0 A phase where the cell has left the cycle and has stopped dividing . Interphase Gap 1 Cells increase in size in Gap 1 . The G checkpoint control mechanism ensures that everything is ready for DNA synthesis . Synthesis DNA replication occurs during this phase . Gap 2 During the gap between DNA synthesis and mitosis , the cell will continue to grow . The G checkpoint control mechanism ensures that everything is ready to enter the M ( mitosis ) phase and divide . Cell division Mitosis Cell growth stops at this stage and cellular energy is focused on the orderly division into two daughter cells . A checkpoint in the middle of mitosis ( Metaphase Checkpoint ) ensures that the cell is ready to complete cell division . After cell division , each of the daughter cells begin the interphase of a new cycle . Although the various stages of interphase are not usually morphologically distinguishable , each phase of the cell cycle has a distinct set of specialized biochemical processes that prepare the cell for initiation of cell division . G phase ( quiescence ) ( edit ) Plant cell cycle Animal cell cycle G is a resting phase where the cell has left the cycle and has stopped dividing . The cell cycle starts with this phase . The word `` post-mitotic '' is sometimes used to refer to both quiescent and senescent cells . Non-proliferative ( non-dividing ) cells in multicellular eukaryotes generally enter the quiescent G state from G and may remain quiescent for long periods of time , possibly indefinitely ( as is often the case for neurons ) . This is very common for cells that are fully differentiated . Cellular senescence occurs in response to DNA damage and external stress and usually constitutes an arrest in G. Some cells enter the G phase semi-permanently and are considered post-mitotic , e.g. , some liver , kidney , and stomach cells . Many cells do not enter G and continue to divide throughout an organism 's life , e.g. , epithelial cells . Cellular senescence is also a state that occurs in response to DNA damage or degradation that would make a cell 's progeny nonviable ; it is often a biochemical alternative to the self - destruction of such a damaged cell by apoptosis . Interphase ( intermitosis ) ( edit ) Before a cell can enter cell division , it needs to take in nutrients . All of the preparations are done during interphase . Interphase is a series of changes that takes place in a newly formed cell and its nucleus , before it becomes capable of division again . It is also called preparatory phase or intermitosis . Previously it was called resting stage because there is no apparent activity related to cell division. Typically interphase lasts for at least 91 % of the total time required for the cell cycle . Interphase proceeds in three stages , G , S , and G , followed by the cycle of mitosis and cytokinesis . The cell 's nuclear DNA contents are duplicated during S phase but may continue till G in case of heterochromatin . G phase ( first growth phase or post mitotic Gap phase ) ( edit ) The first phase within interphase , from the end of the previous M phase until the beginning of DNA synthesis , is called G ( G indicating gap ) . It is also called the growth phase . During this phase , the biosynthetic activities of the cell , which are considerably slowed down during M phase , resume at a high rate . The duration of G is highly variable , even among different cells of the same species . In this phase , the cell increases its supply of proteins , increases the number of organelles ( such as mitochondria , ribosomes ) , and grows in size . In G phase , a cell has three options . ( 1 ) To continue cell cycle and enter S phase ( 2 ) Stop cell cycle and enter G phase for undergoing differentiation. ( 3 ) Get arrested in G phase hence it may enter G phase or re-enter cell cycle . The deciding factor is availability of nitrogens and storage of energy rich compounds at the deciding point called check point . This check point is called G cyclin or C. It causes transition of G to S phase . Once the check point of G phase is crossed , cell cycle will go uninterrupted till it is completed . S phase ( DNA replication ) ( edit ) The ensuing S phase starts when DNA synthesis commences ; when it is complete , all of the chromosomes have been replicated , i.e. , each chromosome has two ( sister ) chromatids . Thus , during this phase , the amount of DNA in the cell has effectively doubled , though the ploidy of the cell remains the same . Rates of RNA transcription and protein synthesis are very low during this phase . An exception to this is histone production , most of which occurs during the S phase . G phase ( growth ) ( edit ) G2 phase occurs after DNA replication and is a period of protein synthesis and rapid cell growth to prepare the cell for mitosis . During this phase microtubules begin to reorganize to form a spindle . Mitotic phase ( chromosome separation ) ( edit ) Main article : Mitosis The relatively brief M phase consists of nuclear division ( karyokinesis ) . It is a relatively short period of the cell cycle . M phase is complex and highly regulated . The sequence of events is divided into phases , corresponding to the completion of one set of activities and the start of the next . These phases are sequentially known as : prophase , metaphase , anaphase , telophase Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei . During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell . Mitosis occurs exclusively in eukaryotic cells , but occurs in different ways in different species . For example , animals undergo an `` open '' mitosis , where the nuclear envelope breaks down before the chromosomes separate , while fungi such as Aspergillus nidulans and Saccharomyces cerevisiae ( yeast ) undergo a `` closed '' mitosis , where chromosomes divide within an intact cell nucleus . Prokaryotic cells , which lack a nucleus , divide by a process called binary fission . Cytokinesis phase ( separation of all cell components ) ( edit ) Main article : Cytokinesis Mitosis is immediately followed by cytokinesis , which divides the nuclei , cytoplasm , organelles and cell membrane into two cells containing roughly equal shares of these cellular components . Mitosis and cytokinesis together define the division of the mother cell into two daughter cells , genetically identical to each other and to their parent cell . This accounts for approximately 10 % of the cell cycle . Because cytokinesis usually occurs in conjunction with mitosis , `` mitosis '' is often used interchangeably with `` M phase '' . However , there are many cells where mitosis and cytokinesis occur separately , forming single cells with multiple nuclei in a process called endoreplication . This occurs most notably among the fungi and slime moulds , but is found in various groups . Even in animals , cytokinesis and mitosis may occur independently , for instance during certain stages of fruit fly embryonic development . Errors in mitosis can either kill a cell through apoptosis or cause mutations that may lead to cancer . The process of mitosis is complex and highly regulated . The sequence of events is divided into phases , corresponding to the completion of one set of activities and the start of the next . These stages are prophase , prometaphase , metaphase , anaphase and telophase . During the process of mitosis the pairs of chromosomes condense and attach to fibers that pull the sister chromatids to opposite sides of the cell . The cell then divides in cytokinesis , to produce two identical daughter cells . Regulation of eukaryotic cell cycle ( edit ) Regulation of the cell cycle involves processes crucial to the survival of a cell , including the detection and repair of genetic damage as well as the prevention of uncontrolled cell division . The molecular events that control the cell cycle are ordered and directional ; that is , each process occurs in a sequential fashion and it is impossible to `` reverse '' the cycle . Role of cyclins and CDKs ( edit ) Nobel Laureate Paul Nurse Nobel Laureate Tim Hunt Two key classes of regulatory molecules , cyclins and cyclin - dependent kinases ( CDKs ) , determine a cell 's progress through the cell cycle . Leland H. Hartwell , R. Timothy Hunt , and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these central molecules . Many of the genes encoding cyclins and CDKs are conserved among all eukaryotes , but in general more complex organisms have more elaborate cell cycle control systems that incorporate more individual components . Many of the relevant genes were first identified by studying yeast , especially Saccharomyces cerevisiae ; genetic nomenclature in yeast dubs many of these genes cdc ( for `` cell division cycle '' ) followed by an identifying number , e.g. cdc25 or cdc20 . Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activated heterodimer ; cyclins have no catalytic activity and CDKs are inactive in the absence of a partner cyclin . When activated by a bound cyclin , CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle . Different cyclin - CDK combinations determine the downstream proteins targeted . CDKs are constitutively expressed in cells whereas cyclins are synthesised at specific stages of the cell cycle , in response to various molecular signals . General mechanism of cyclin - CDK interaction ( edit ) Upon receiving a pro-mitotic extracellular signal , G cyclin - CDK complexes become active to prepare the cell for S phase , promoting the expression of transcription factors that in turn promote the expression of S cyclins and of enzymes required for DNA replication . The G cyclin - CDK complexes also promote the degradation of molecules that function as S phase inhibitors by targeting them for ubiquitination . Once a protein has been ubiquitinated , it is targeted for proteolytic degradation by the proteasome . However , results from a recent study of E2F transcriptional dynamics at the single - cell level argue that the role of G1 cyclin - CDK activities , in particular cyclin D - CDK4 / 6 , is to tune the timing rather than the commitment of cell cycle entry . Active S cyclin - CDK complexes phosphorylate proteins that make up the pre-replication complexes assembled during G phase on DNA replication origins . The phosphorylation serves two purposes : to activate each already - assembled pre-replication complex , and to prevent new complexes from forming . This ensures that every portion of the cell 's genome will be replicated once and only once . The reason for prevention of gaps in replication is fairly clear , because daughter cells that are missing all or part of crucial genes will die . However , for reasons related to gene copy number effects , possession of extra copies of certain genes is also deleterious to the daughter cells . Mitotic cyclin - CDK complexes , which are synthesized but inactivated during S and G phases , promote the initiation of mitosis by stimulating downstream proteins involved in chromosome condensation and mitotic spindle assembly . A critical complex activated during this process is a ubiquitin ligase known as the anaphase - promoting complex ( APC ) , which promotes degradation of structural proteins associated with the chromosomal kinetochore . APC also targets the mitotic cyclins for degradation , ensuring that telophase and cytokinesis can proceed . Specific action of cyclin - CDK complexes ( edit ) Cyclin D is the first cyclin produced in the cell cycle , in response to extracellular signals ( e.g. growth factors ) . Cyclin D binds to existing CDK4 , forming the active cyclin D - CDK4 complex . Cyclin D - CDK4 complex in turn phosphorylates the retinoblastoma susceptibility protein ( Rb ) . The hyperphosphorylated Rb dissociates from the E2F / DP1 / Rb complex ( which was bound to the E2F responsive genes , effectively `` blocking '' them from transcription ) , activating E2F . Activation of E2F results in transcription of various genes like cyclin E , cyclin A , DNA polymerase , thymidine kinase , etc . Cyclin E thus produced binds to CDK2 , forming the cyclin E-CDK2 complex , which pushes the cell from G to S phase ( G / S , which initiates the G / M transition ) . Cyclin B - cdk1 complex activation causes breakdown of nuclear envelope and initiation of prophase , and subsequently , its deactivation causes the cell to exit mitosis . A quantitative study of E2F transcriptional dynamics at the single - cell level by using engineered fluorescent reporter cells provided a quantitative framework for understanding the control logic of cell cycle entry , challenging the canonical textbook model . Genes that regulate the amplitude of E2F accumulation , such as Myc , determine the commitment in cell cycle and S phase entry . G1 cyclin - CDK activities are not the driver of cell cycle entry . Instead , they primarily tune the timing of E2F increase , thereby modulating the pace of cell cycle progression . Inhibitors ( edit ) Overview of signal transduction pathways involved in apoptosis , also known as `` programmed cell death '' . Two families of genes , the cip / kip ( CDK interacting protein / Kinase inhibitory protein ) family and the INK4a / ARF ( Inhibitor of Kinase 4 / Alternative Reading Frame ) family , prevent the progression of the cell cycle . Because these genes are instrumental in prevention of tumor formation , they are known as tumor suppressors . The cip / kip family includes the genes p21 , p27 and p57 . They halt cell cycle in G phase , by binding to , and inactivating , cyclin - CDK complexes . p21 is activated by p53 ( which , in turn , is triggered by DNA damage e.g. due to radiation ) . p27 is activated by Transforming Growth Factor of β ( TGF β ) , a growth inhibitor . The INK4a / ARF family includes p16 , which binds to CDK4 and arrests the cell cycle in G phase , and p14 which prevents p53 degradation . Synthetic inhibitors of Cdc25 could also be useful for the arrest of cell cycle and therefore be useful as antineoplastic and anticancer agents . Transcriptional regulatory network ( edit ) Current evidence suggests that a semi-autonomous transcriptional network acts in concert with the CDK - cyclin machinery to regulate the cell cycle . Several gene expression studies in Saccharomyces cerevisiae have identified 800 -- 1200 genes that change expression over the course of the cell cycle . They are transcribed at high levels at specific points in the cell cycle , and remain at lower levels throughout the rest of the cycle . While the set of identified genes differs between studies due to the computational methods and criteria used to identify them , each study indicates that a large portion of yeast genes are temporally regulated . Many periodically expressed genes are driven by transcription factors that are also periodically expressed . One screen of single - gene knockouts identified 48 transcription factors ( about 20 % of all non-essential transcription factors ) that show cell cycle progression defects . Genome - wide studies using high throughput technologies have identified the transcription factors that bind to the promoters of yeast genes , and correlating these findings with temporal expression patterns have allowed the identification of transcription factors that drive phase - specific gene expression . The expression profiles of these transcription factors are driven by the transcription factors that peak in the prior phase , and computational models have shown that a CDK - autonomous network of these transcription factors is sufficient to produce steady - state oscillations in gene expression ) . Experimental evidence also suggests that gene expression can oscillate with the period seen in dividing wild - type cells independently of the CDK machinery . Orlando et al. used microarrays to measure the expression of a set of 1,271 genes that they identified as periodic in both wild type cells and cells lacking all S - phase and mitotic cyclins ( clb1 , 2 , 3 , 4 , 5 , 6 ) . Of the 1,271 genes assayed , 882 continued to be expressed in the cyclin - deficient cells at the same time as in the wild type cells , despite the fact that the cyclin - deficient cells arrest at the border between G and S phase . However , 833 of the genes assayed changed behavior between the wild type and mutant cells , indicating that these genes are likely directly or indirectly regulated by the CDK - cyclin machinery . Some genes that continued to be expressed on time in the mutant cells were also expressed at different levels in the mutant and wild type cells . These findings suggest that while the transcriptional network may oscillate independently of the CDK - cyclin oscillator , they are coupled in a manner that requires both to ensure the proper timing of cell cycle events . Other work indicates that phosphorylation , a post-translational modification , of cell cycle transcription factors by Cdk1 may alter the localization or activity of the transcription factors in order to tightly control timing of target genes . While oscillatory transcription plays a key role in the progression of the yeast cell cycle , the CDK - cyclin machinery operates independently in the early embryonic cell cycle . Before the midblastula transition , zygotic transcription does not occur and all needed proteins , such as the B - type cyclins , are translated from maternally loaded mRNA . DNA replication and DNA replication origin activity ( edit ) Analyses of synchronized cultures of Saccharomyces cerevisiae under conditions that prevent DNA replication initiation without delaying cell cycle progression showed that origin licensing decreases the expression of genes with origins near their 3 ' ends , revealing that downstream origins can regulate the expression of upstream genes . This confirms previous predictions from mathematical modeling of a global causal coordination between DNA replication origin activity and mRNA expression , and shows that mathematical modeling of DNA microarray data can be used to correctly predict previously unknown biological modes of regulation . Checkpoints ( edit ) Main article : Cell cycle checkpoint Cell cycle checkpoints are used by the cell to monitor and regulate the progress of the cell cycle . Checkpoints prevent cell cycle progression at specific points , allowing verification of necessary phase processes and repair of DNA damage . The cell can not proceed to the next phase until checkpoint requirements have been met . Checkpoints typically consist of a network of regulatory proteins that monitor and dictate the progression of the cell through the different stages of the cell cycle . There are several checkpoints to ensure that damaged or incomplete DNA is not passed on to daughter cells . Three main checkpoints exist : the G / S checkpoint , the G / M checkpoint and the metaphase ( mitotic ) checkpoint . G / S transition is a rate - limiting step in the cell cycle and is also known as restriction point . This is where the cell checks whether it has enough raw materials to fully replicate its DNA ( nucleotide bases , DNA synthase , chromatin , etc . ) . An unhealthy or malnourished cell will get stuck at this checkpoint . The G / M checkpoint is where the cell ensures that it has enough cytoplasm and phospholipids for two daughter cells . But sometimes more importantly , it checks to see if it is the right time to replicate . There are some situations where many cells need to all replicate simultaneously ( for example , a growing embryo should have a symmetric cell distribution until it reaches the mid-blastula transition ) . This is done by controlling the G / M checkpoint . The metaphase checkpoint is a fairly minor checkpoint , in that once a cell is in metaphase , it has committed to undergoing mitosis . However that 's not to say it is n't important . In this checkpoint , the cell checks to ensure that the spindle has formed and that all of the chromosomes are aligned at the spindle equator before anaphase begins . While these are the three `` main '' checkpoints , not all cells have to pass through each of these checkpoints in this order to replicate . Many types of cancer are caused by mutations that allow the cells to speed through the various checkpoints or even skip them altogether . Going from S to M to S phase almost consecutively . Because these cells have lost their checkpoints , any DNA mutations that may have occurred are disregarded and passed on to the daughter cells . This is one reason why cancer cells have a tendency to exponentially accrue mutations . Aside from cancer cells , many fully differentiated cell types no longer replicate so they leave the cell cycle and stay in G until their death . Thus removing the need for cellular checkpoints . An alternative model of the cell cycle response to DNA damage has also been proposed , known as the postreplication checkpoint . Checkpoint regulation plays an important role in an organism 's development . In sexual reproduction , when egg fertilization occurs , when the sperm binds to the egg , it releases signalling factors that notify the egg that it has been fertilized . Among other things , this induces the now fertilized oocyte to return from its previously dormant , G , state back into the cell cycle and on to mitotic replication and division . p53 plays an important role in triggering the control mechanisms at both G / S and G / M checkpoints . In addition to p53 , checkpoint regulators are being heavily researched for their roles in cancer growth and proliferation . Fluorescence imaging of the cell cycle ( edit ) Fluorescent proteins visualize the cell cycle progression . IFP2. 0 - hGem ( 1 / 110 ) fluorescence is shown in green and highlights the S / G / M phases . smURFP - hCdtI ( 30 / 120 ) fluorescence is shown in red and highlights the G / G phases . Pioneering work by Atsushi Miyawaki and coworkers developed the fluorescent ubiquitination - based cell cycle indicator ( FUCCI ) , which enables fluorescence imaging of the cell cycle . Originally , a green fluorescent protein , mAG , was fused to hGem ( 1 / 110 ) and an orange fluorescent protein ( mKO ) was fused to hCdt1 ( 30 / 120 ) . Note , these fusions are fragments that contain a nuclear localization signal and ubiquitination sites for degradation , but are not functional proteins . The green fluorescent protein is made during the S , G , or M phase and degraded during the G or G phase , while the orange fluorescent protein is made during the G or G phase and destroyed during the S , G , or M phase . A far - red and near - infrared FUCCI was developed using a cyanobacteria - derived fluorescent protein ( smURFP ) and a bacteriophytochrome - derived fluorescent protein ( movie found at this link ) . Role in tumor formation ( edit ) A disregulation of the cell cycle components may lead to tumor formation . As mentioned above , when some genes like the cell cycle inhibitors , RB , p53 etc . mutate , they may cause the cell to multiply uncontrollably , forming a tumor . Although the duration of cell cycle in tumor cells is equal to or longer than that of normal cell cycle , the proportion of cells that are in active cell division ( versus quiescent cells in G phase ) in tumors is much higher than that in normal tissue . Thus there is a net increase in cell number as the number of cells that die by apoptosis or senescence remains the same . The cells which are actively undergoing cell cycle are targeted in cancer therapy as the DNA is relatively exposed during cell division and hence susceptible to damage by drugs or radiation . This fact is made use of in cancer treatment ; by a process known as debulking , a significant mass of the tumor is removed which pushes a significant number of the remaining tumor cells from G to G phase ( due to increased availability of nutrients , oxygen , growth factors etc . ) . Radiation or chemotherapy following the debulking procedure kills these cells which have newly entered the cell cycle . The fastest cycling mammalian cells in culture , crypt cells in the intestinal epithelium , have a cycle time as short as 9 to 10 hours . Stem cells in resting mouse skin may have a cycle time of more than 200 hours . Most of this difference is due to the varying length of G , the most variable phase of the cycle . M and S do not vary much . In general , cells are most radiosensitive in late M and G phases and most resistant in late S phase . For cells with a longer cell cycle time and a significantly long G phase , there is a second peak of resistance late in G . The pattern of resistance and sensitivity correlates with the level of sulfhydryl compounds in the cell . Sulfhydryls are natural substances that protect cells from radiation damage and tend to be at their highest levels in S and at their lowest near mitosis . See also ( edit ) Cellular model Synchronous culture -- synchronization of cell cultures References ( edit ) Jump up ^ Wang JD , Levin PA ( November 2009 ) . `` Metabolism , cell growth and the bacterial cell cycle '' . Nature Reviews . Microbiology. 7 ( 11 ) : 822 -- 7 . doi : 10.1038 / nrmicro2202 . PMC 2887316 . PMID 19806155 . 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Watson JD , Baker TA , Bell SP , Gann A , Levine M , Losick R ( 2004 ) . `` Chapter 7 '' . Molecular biology of the gene ( 5th ed . ) . San Francisco : Pearson / Benjamin Cummings . ISBN 0 - 8053 - 4642 - 2 . External links ( edit ) Wikimedia Commons has media related to Cell cycle . This article incorporates public domain material from the NCBI document `` Science Primer '' . David Morgan 's seminar : Controlling the Cell Cycle The cell cycle & Cell death Transcriptional program of the cell cycle : high - resolution timing Cell cycle and metabolic cycle regulated transcription in yeast Cell Cycle Animation 1Lec.com Cell Cycle Fucci : Using GFP to visualize the cell - cycle Science Creative Quarterly 's overview of the cell cycle KEGG -- Human Cell Cycle ( hide ) 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 NDL : 00576703 Evolutionary biology portal Molecular and cellular biology portal Science portal Retrieved from `` https://en.wikipedia.org/w/index.php?title=Cell_cycle&oldid=838170339 '' Categories : Cell cycle Cellular processes Cellular senescence Hidden categories : All articles with unsourced statements Articles with unsourced statements from February 2016 Wikipedia articles incorporating text from the United States National Library of Medicine Use dmy dates from July 2011 Talk Contents About Wikipedia Башҡортса Български Bosanski Brezhoneg Català Čeština Deutsch Eesti Español Euskara فارسی Français Galego 한국어 Հայերեն हिन्दी Hrvatski Bahasa Indonesia Íslenska Italiano עברית Basa Jawa Қазақша Kreyòl ayisyen Magyar Монгол Nederlands 日本 語 Norsk Norsk nynorsk Polski Português Română Русский Scots Slovenčina Slovenščina کوردی Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska தமிழ் ไทย Українська Tiếng Việt 中文 39 more Edit links This page was last edited on 25 April 2018 , at 10 : 28 . 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Question: the resting stage of the cell cycle is