The cell cycle, or cell-division cycle, is the series of events that take place in a eukaryotic cell leading to its replication. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called These events can be divided in two brief periods: interphase—during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA—and the mitotic (M) phase, during which the cell splits itself into two distinct cells, often called "daughter cells". Interphase is the phase of the Cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for Cell DNA replication is the process of copying a double-stranded DNA molecule to form two double-stranded molecules Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei 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. For other meanings see Zygote (disambiguation. A zygote (from Greek ζυγωτός zugōtos "joined" or "yoked" Hair is a keratinised protein filament that grows through the epidermis from follicles deep within the Dermis. The skin is the outer covering of living tissue of an animal (or plant A blood cell (also called blood corpuscle) is any cell of any type normally found in Blood.

Phases of the cell cycle

Schematic of the cell cycle. outer ring: I=Interphase, M=Mitosis; inner ring: M=Mitosis, G₁=Gap 1, G₂=Gap 2, S=Synthesis; not in ring: G₀=Gap 0/Resting. Interphase is the phase of the Cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for Cell Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei The G1 phase is a period in the Cell cycle during Interphase, after Cytokinesis and before the S phase. G2 phase is the third final and usually the shortest subphase during interphase within the Cell cycle in which the cell undergoes a period of rapid growth to prepare The S phase, short for synthesis phase, is a period in the Cell cycle during Interphase, between G1 phase and the G2 phase. The G0 phase (G sub 0 is a period in the Cell cycle where cells exist in a quiescent state The duration of mitosis in relation to the other phases has been exaggerated in this diagram.

The cell cycle consists of four distinct phases: G₁ phase, S phase, G₂ phase (collectively known as interphase) and M phase. The G1 phase is a period in the Cell cycle during Interphase, after Cytokinesis and before the S phase. The S phase, short for synthesis phase, is a period in the Cell cycle during Interphase, between G1 phase and the G2 phase. G2 phase is the third final and usually the shortest subphase during interphase within the Cell cycle in which the cell undergoes a period of rapid growth to prepare Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei M phase is itself composed of two tightly coupled processes: mitosis, in which the cell's chromosomes are divided between the two daughter cells, and cytokinesis, in which the cell's cytoplasm divides forming distinct cells. A chromosome is an organized structure of DNA and Protein that is found in cells. Cytokinesis is the process whereby the Cytoplasm of a single Eukaryotic cell is divided to form two daughter cells The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. 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 G₀ phase. The G0 phase (G sub 0 is a period in the Cell cycle where cells exist in a quiescent state

M phase

The relatively brief M phase consists of nuclear division (karyokinesis) and cytoplasmic division (cytokinesis). Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei The cytoplasm is the contents of a cell that is enclosed within the Plasma membrane. Cytokinesis is the process whereby the Cytoplasm of a single Eukaryotic cell is divided to form two daughter cells In plants and algae, cytokinesis is accompanied by the formation of a new cell wall. Plants are living Organisms belonging to the kingdom Plantae. Algae ( sing. alga are a large and diverse group of simple typically Autotrophic organisms ranging from Unicellular to Multicellular forms A cell wall is a tough flexible and sometimes fairly rigid layer surrounding a cell, located external to the Cell membrane, which provides the cell with structural

Interphase

After M phase, the daughter cells each begin interphase of a new cycle. Interphase is the phase of the Cell cycle in which the cell spends the majority of its time and performs the majority of its purposes including preparation for Cell 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.

G₁ phase

The first phase within interphase, from the end of the previous M phase till the beginning of DNA synthesis is called G₁ (G indicating gap or growth). The G1 phase is a period in the Cell cycle during Interphase, after Cytokinesis and before the S phase. During this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. This phase is marked by synthesis of various enzymes that are required in S phase, mainly those needed for DNA replication. Duration of G₁ is highly variable, even among different cells of the same species. ^[1]

S phase

The ensuing S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated, i. The S phase, short for synthesis phase, is a period in the Cell cycle during Interphase, between G1 phase and the G2 phase. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known A chromosome is an organized structure of DNA and Protein that is found in cells. 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. "Haplo" redirects here For the fictional character see The Death Gate Cycle. 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. In Biology, histones are the chief Protein components of Chromatin. ^[2][3] The duration of S phase is relatively constant among cells of the same species. ^[4]

G₂ phase

The cell then enters the G₂ phase, which lasts until the cell enters mitosis. G2 phase is the third final and usually the shortest subphase during interphase within the Cell cycle in which the cell undergoes a period of rapid growth to prepare Again, significant protein synthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of mitosis. Microtubules are one of the components of the Cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers Inhibition of protein synthesis during G₂ phase prevents the cell from undergoing mitosis.

G₀ phase

The term "post-mitotic" is sometimes used to refer to both quiescent and senescent cells. The G0 phase (G sub 0 is a period in the Cell cycle where cells exist in a quiescent state Senescence refers to the biological processes of a living Organism approaching an advanced age (i Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G₀ state from G₁ and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Neurons (ˈnjuːɹɒn also known as neurones and nerve cells) are responsive cells in the Nervous system that process and transmit information This is very common for cells that are fully differentiated. In Developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized Cell type. Cellular senescence is 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. Some cell types in mature organisms, such as parenchymal cells of the liver and kidney, enter the G₀ phase semi-permanently and can only be induced to begin dividing again under very specific circumstances; other types, such as epithelial cells, continue to divide throughout an organism's life. Parenchyma is a term used to describe a bulk of a substance It is used in different ways in Animals and in Plants. In biology and medicine epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body

Regulation of cell cycle

Regulation of cell cycle: Schematic

Regulation of the cell cycle involves steps crucial to the cell, including detecting and repairing genetic damage, and provision of various checks to prevent 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

Two key classes of regulatory molecules, cyclins and cyclin-dependent kinases (CDKs), determine a cell's progress through the cell cycle. Cyclins are a family of Proteins involved in the progression of cells through the Cell cycle. Cyclin-dependent kinases ( CDK) belong to a group of Protein kinases originally discovered as being involved in the regulation of the Cell cycle. ^[5] 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. Leland Harrison (Lee Hartwell (born October 30 1939, in Los Angeles, California) is president and director of the Fred Hutchinson Cancer Sir Richard Timothy "Tim" Hunt, FRS, (born February 19, 1943 in Neston, Cheshire) is an English Biochemist Sir Paul Maxime Nurse, FRS (b January 25, 1949) is a British Biochemist. Year 2001 ( MMI) was a Common year starting on Monday according to the Gregorian calendar. The Nobel Prize in Physiology or Medicine (Nobelpriset i fysiologi eller medicin is awarded once a year by the Swedish Karolinska Institute. ^[6] 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. Conservation refers to a high degree of similarity in orthologous DNA sequences protein sequences, or Protein structures amongst various Many of the relevant genes were first identified by studying yeast, especially Saccharomyces cerevisiae; ^[7] genetic nomenclature in yeast dubs many of these genes cdc (for "cell division cycle") followed by an identifying number, e. Saccharomyces cerevisiae is a Species of Budding Yeast. It is perhaps the most useful Yeast owing to its use since ancient times g. , cdc25. Cdc25 is a dual-specificity Phosphatase first isolated from the yeast Schizosaccharomyces pombe as a Cell cycle defective mutant

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. A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces 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. Phosphorylation is the addition of a Phosphate (PO4 group to a Protein molecule or a small molecule 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. ^[8]

General mechanism of cyclin-CDK interaction

Upon receiving a pro-mitotic extracellular signal, G₁ 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. A cyclin-dependent kinase complex (abbreviated cdkc, also called cyclin-CDK) is a Protein complex formed by the association of Cyclin with In the field of Molecular biology, a transcription factor (sometimes called a sequence-specific DNA binding factor is a Protein that binds to specific sequences DNA replication is the process of copying a double-stranded DNA molecule to form two double-stranded molecules The G₁ cyclin-CDK complexes also promote the degradation of molecules that function as S phase inhibitors by targeting them for ubiquitination. Ubiquitin is a highly-conserved regulatory Protein that is ''ubiquitously'' expressed in Eukaryotes. Once a protein has been ubiquitinated, it is targeted for proteolytic degradation by the proteasome. Proteasomes are large Protein complexes inside all Eukaryotes and Archaea, as well as in some Bacteria. Active S cyclin-CDK complexes phosphorylate proteins that make up the pre-replication complexes assembled during G₁ phase on DNA replication origins. A pre-replication complex (pre-RC is a Protein complex that forms at the Origin of replication during the initiation step of DNA replication. The Origin of replication (also called the replication origin) is a particular sequence in a Genome at which replication is initiated 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. In classical genetics the genome of a Diploid Organism including Eukarya refers to a full set of chromosomes or genes in a Gamete, thereby 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 would also prove deleterious to the daughter cells.

Mitotic cyclin-CDK complexes, which are synthesized but inactivated during S and G₂ phases, promote the initiation of mitosis by stimulating downstream proteins involved in chromosome condensation and mitotic spindle assembly. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei In Cell biology, the spindle apparatus (also called spindle fibers) is the structure that separate the Chromosomes into the daughter cells during 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. A ubiquitin ligase (also called an E3 ubiquitin ligase is a Protein that covalently attaches Ubiquitin to a Lysine on a target protein via an Isopeptide Anaphase-promoting complex ( APC) is a complex of several Proteins which is activated during Mitosis to initiate Anaphase. The kinetochore (pronounced kin et' o core is the protein structure on Chromosomes where the Spindle fibers attach during division to pull the chromosomes apart APC also targets the mitotic cyclins for degradation, ensuring that telophase and cytokinesis can proceed.

Specific action of cyclin-CDK complexes

Cyclin D is the first cyclin produced in the cell cycle, in response to extracellular signals (eg. Cyclin D is a member of the Cyclin family and comprises three closely related Proteins cyclin D1 D2 and D3 growth factors). The term growth factor refers to a naturally occurring Protein capable of stimulating cellular growth proliferation and Cellular differentiation. Cyclin D binds to existing CDK4, forming the active cyclin D-CDK4 complex. Cyclin-dependent kinase 4 is part of the Cyclin-dependent kinase family Cyclin D-CDK4 complex in turn phosphorylates the retinoblastoma susceptibility protein (RB). Retinoblastoma is a Cancer of the Retina. Development of this tumor is initiated by Mutations ref> that inactivate both copies of the RB1 The retinoblastoma protein (abbreviated pRb or Rb) is a Tumor suppressor Protein that is dysfunctional in many types of Cancer 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. E2F is a group of genes that codifies a family of Transcription factors (TF in higher Eukaryotes. Activation of E2F results in transcription of various genes like cyclin E, cyclin A, DNA polymerase, thymidine kinase, etc. Cyclin E is a member of the Cyclin family Cyclin E binds to G1 phase Cdk2 which is required for the transition from G1 to S phase Cyclin A is a member of the Cyclin family Cyclin A binds to S phase Cdk2 and is required for the cell to progress through the S phase. A DNA Polymerase is an Enzyme that assists in DNA replication. Thymidine kinase TK is an Enzyme, a Phosphotransferase (a Kinase) 2'-deoxythymidine kinase ATP-thymidine 5'-phosphotransferase. Cyclin E thus produced binds to CDK2, forming the cyclin E-CDK2 complex, which pushes the cell from G₁ to S phase (G₁/S transition). Cyclin-dependent kinase 2 also known as CDK2 is a human Gene Function The Protein encoded by this gene is a member of the Cyclin-dependent Cyclin A along with CDK2 forms the cyclin A-CDK2 complex, which initiates the G₂/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. Cyclin B is a member of the Cyclin family Cyclin B is a mitotic cyclin The nuclear envelope (NE(also known as the perinuclear envelope, nuclear membrane, nucleolemma or karyotheca) is a double lipid bilayer that Prophase is a stage of Mitosis in which the Chromatin condenses into a highly ordered structure called a Chromosome ^[8]

Cell cycle inhibitors

Two families of genes, the cip/kip family and the INK4a/ARF (Inhibitor of Kinase 4/Alternative Reading Frame) prevent the progression of the cell cycle. Because these genes are instrumental in prevention of tumor formation, they are known as tumor suppressors. See also Cancer A tumor or tumour is the name for a swelling or lesion formed by an abnormal growth of cells (termed neoplastic A tumor suppressor gene, or antioncogene is a Gene that protects a cell from one step on the path to cancer

The cip/kip family includes the genes p21, p27 and p57. Cyclin-dependent kinase inhibitor 1A (p21 Cip1, also known as CDKN1A, is a human Gene. p27Kip1, ( HUGO gene symbol CDKN1B is a Gene which lies on Chromosome 12 in humans and encodes a Protein which belongs to the p57 or p57KIP2 is a Tumor suppressor human Gene on Chromosome 11 (11p15 and belongs to the cip/kip gene family They halt cell cycle in G₁ phase, by binding to, and inactivating, cyclin-CDK complexes. p21 is activated by p53 (which, in turn, is triggered by DNA damage eg. p53 (also known as protein 53 or tumor protein 53) is a Transcription factor encoded by the TP53 gene due to radiation). p27 is activated by Transforming Growth Factor β (TGF β), a growth inhibitor. Transforming growth factor (sometimes referred to as Tumor growth factor, or TGF) is used to describe two classes of Polypeptide Growth factors

The INK4a/ARF family includes p16INK4a, which binds to CDK4 and arrests the cell cycle in G₁ phase, and p14arf which prevents p53 degradation. Cyclin-dependent kinase inhibitor 2A (melanoma p16 inhibits CDK4, also known as CDKN2A, is a human Gene. p14ARF is an alternate reading frame (ARF product of the CDKN2A locus And the amount of chromosomes are able to double at the same rate as in phase 2

Checkpoints

Cell cycle checkpoints are used by the cell to monitor and regulate the progress of the cell cycle. Cell cycle checkpoints are control mechanisms that ensure the fidelity of cell division in Eukaryotic cells. ^[9] Checkpoints prevent cell cycle progression at specific points, allowing verification of necessary phase processes and repair of DNA damage. DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its Genome. The cell cannot proceed to the next phase until checkpoint requirements have been met.

Several checkpoints are designed to ensure that damaged or incomplete DNA is not passed on to daughter cells. Two main checkpoints exist: the G1/S checkpoint and the G2/M checkpoint. Cell cycle checkpoints are control mechanisms that ensure the fidelity of cell division in Eukaryotic cells. Cell cycle checkpoints are control mechanisms that ensure the fidelity of cell division in Eukaryotic cells. G1/S transition is a rate-limiting step in the cell cycle and is also known as restriction point. The restriction point is a G1 phase checkpoint in the Cell cycle of Animal cells Cells that progress through this point ^[8] An alternative model of the cell cycle response to DNA damage has also been proposed, known as the postreplication checkpoint. Postreplication Checkpoint When the genomic DNA of eukaryotic cells becomes damaged by spontaneous processes chemical mutagens or sunlight exposure the replication of damaged

p53 plays an important role in triggering the control mechanisms at both G1/S and G2/M checkpoints. p53 (also known as protein 53 or tumor protein 53) is a Transcription factor encoded by the TP53 gene

Role of cell cycle in tumor formation

A disregulation of the cell cycle components may lead to tumor formation. See also Cancer A tumor or tumour is the name for a swelling or lesion formed by an abnormal growth of cells (termed neoplastic As mentioned above, some genes like the cell cycle inhibitors, RB, p53 etc. p53 (also known as protein 53 or tumor protein 53) is a Transcription factor encoded by the TP53 gene , when they mutate, 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 G0 phase) in tumor cells are much more compared to that in normal cells. 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. Chemotherapy, in its most general sense refers to treatment of disease by chemicals that kill cells specifically those of micro-organisms or Cancer. Radiation therapy (or radiotherapy) is the medical use of Ionizing radiation as part of Cancer treatment to control Malignant 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 G0 to G1 phase (due to increased availability of nutrients, oxygen, growth factors etc. Debulking is the surgical removal of part of a malignant Tumour which cannot be completely excised so as to enhance the effectiveness of radiation or ). Radiation or chemotherapy following the debulking procedure kills these cells which have newly entered the cell cycle. ^[8]

Synchronization of cell cultures

Several methods can be used to synchronise cell cultures by halting the cell cycle at a particular phase. Cell Synchronization. Synchronization literally means to make two or more things happen exactly simultaneously For example, Serum starvation ^[10] and treatment with Thymidine or Aphidicolin ^[11] halt the cell in the G1 phase, Mitotic shake-off, treatment with colchicine ^[12] and treatment with Nocodazole ^[13] halt the cell in M phase and treatment with 5-fluorodeoxyuridine halts the cell in S phase. Thymidine (more precisely called deoxythymidine; can also be labelled deoxyribosylthymine, and thymine deoxyriboside) is a Chemical compound Colchicine is a highly poisonous Natural product and Secondary metabolite, originally extracted from plants of the genus Colchicum ( Autumn Nocodazole is an anti- Neoplastic agent which exerts its effect in cells by interfering with the Polymerization of Microtubules Microtubules are

Observation

There are numerous ways to observe the cell cycle occurring. Onion bulbs or garlic root tips are often used.

A sample of root tip is fixed in a mixture of 99% of 70% aqueous industrial methylated spirit and 1% glacial ethanoic acid for two hours. Acetic acid, also known as ethanoic acid, is an organic chemical compound, giving Vinegar its sour taste Treat the root tips in 1 molar hydrochloric acid at 60°C for 6–7 minutes. In Chemistry, concentration is the measure of how much of a given substance there is mixed with another substance Hydrochloric acid is the Solution of Hydrogen chloride ( H[[Chlorine Cl]] in water Rinse thoroughly with water. Add Schiff's reagent and leave for one hour. The Schiff test invented and named after Hugo Schiff is a Chemical test for the detection of Aldehydes. Rinse again in distilled water. Observe under a microscope.

Mathematical modelling

See cell cycle mathematical model

See also

• List of basic cell biology topics
• Mitosis
• Interphase

References

1. Morgan DO. (2007) The Cell Cycle: Principles of Control. New Science Press: London.
2. Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P. (2003). Molecular Biology of the Cell. Ch 17. Garland Science: New York. 4th ed.
3. Lodish H, Berk A, Matsudaira P, Kaiser CA, Krieger M, Scott MP, Zipursky SL, Darnell J. (2004). Molecular Cell Biology. WH Freeman: New York, NY. 5th ed.
4. Watson JD, Baker TA, Bell SP, Gann A, Levine M, Losick R. (2004). Molecular Biology of the Gene, Ch. 7. Peason Benjamin Cummings; CSHL Press. 5th ed.

External links

• This article contains material from the Science Primer published by the NCBI, which, as a U. The National Center for Biotechnology Information ( NCBI) is part of the United States National Library of Medicine (NLM a branch of the National Institutes S. government publication, is in the public domain. The public domain is a range of abstract materials &ndash commonly referred to as Intellectual property &ndash which are not owned or controlled by anyone
• Transcriptional program of the cell cycle: high-resolution timing
• Cell cycle and metabolic cycle regulated transcription in yeast
• Cell Cycle and Cytokinesis - The Virtual Library of Biochemistry and Cell Biology
• Cell Cycle
• Cell Cycle Portal
• Fucci:Using GFP to visualize the cell-cycle
• Science Creative Quarterly's overview of the cell cycle
• Cells alive
• CCO The Cell-Cycle Ontology
• KEGG - Human Cell Cycle
• Cell cycle modeling