2009 Group 5 Project
The Cell Cycle
German Pathologist, Rudolf Ludwig Karl Virchow’s realization in 1885 that "cells only arise from pre-existing cells" sparked the beginning of an intense investigation into the cell cycle, which is still as active and dynamic today.  Over the years, a great number of discoveries and advances have been made in bettering our understanding of this cycle.
Early scientists described the cytology of the cell division  and more recently, scientists from an array of disciplines, spanning embryologists to microbiologists have collaborated and begun to understand the underlining mechanisms and workings involved in the cell cycle. 
- Since 1953, when James Watson and Francis Crick described the double-helical base-paired structure of DNA,  a lot of studies on cell cycle has been focused on DNA synthesis and replication.
- In the 1970s, general control mechanisms that regulate the onset of each phases of the cell cycle were established.  Also, the Gap 0 phase (G0) was identified, and Pardee (1974) studied the regulatory mechanisms and how cells switch between proliferative and quiescent (G0) states in the cell cycle. 
- During the 1980s, the function of centromeres in segragating chromosomes were studied and a functional centromere was isolated from a budding yeast.  Also, the precise control mechanisms between the phases, and the changes in levels of 'cyclins' - "proteins that showed cell-cycle-oscillatory behaviour"  were discovered and studied in more details. 
- During the late 1980s, replication origins in cells were identified and the importance of 'checkpoint controls' that arrest cells following DNA damage was discovered.  These help to avoid improper transitions between phases, and provided important clues to the nature of cell cycle regulation. Furthermore, the importance of tumour suppressor genes such as retinoblastoma and p53 were also discovered. 
- In the 1990s, a new class of proteins, known as cyclin-dependent kinase (CDK) was identified. Their interaction with cyclins were studied in detail and has lead to the discovery of many more new classes of cyclins such as cyclins C, D and E.  In 1993, CDK inhibitors (CKIs) were identified and their interaction with CDKs was studied in detail. 
Throughout the history, five Nobel prizes for Physiology or Medicine have been awarded in the development and understanding of the cell cycle. A lot of information have been collaborated to present, but there still exists areas that are not fully understood. Current research is now centred on the use of cell-cycle-specific treatments in cancer and other cell cycle related diseases. Cancer therapy may greatly benefit from a better understanding of the connenctions between cell cycle and apoptosis. Major advances have been recently made in the discovery of `cell cycle drugs', and their use as anticancer drugs is being extensively investigated.  The discovery of cell cycle regulators in the brain of Alzheimer's disease patients leaves us with the idea that cell cycle studies, initially supported by the antitumour purpose, may have applications in quite unexpected fields.
The Cell Cycle
The cell cycle is a series of changes a cell undergoes from the time it forms until it reproduces itself.  Depending on what text you read, this cycle can be divided into two or four major phases. We will be discussing the division as four phases, as the prescribed text; Essentials in Cell Biology: 2nd Ed.  defines.
- Gap 1 phase (G1)
- Synthetic phase (S phase)
- Gap 2 phase (G2) and
- Mitotic phase (M phase) (see Figure 1).
It is important to understand that the timing, or length, of the cell cycle differs from cell to cell and from organism to organism. For instance, a small single celled organism can quickly reproduce, whereas a more complex multicelluar organism takes longer.  From here on in, we will be discussing the phases of the cycle for that of a typical fast dividing mammalian cell, with an approximate cell cycle of twenty-four hours. 
Gap 1 Phase (G1)
G1 is the interval between the completion of the M phase and the beginning of the S phase.  It is typically the longest and most variable phase of the cell cycle (see Figure 2).
In part, this lengthy time frame can be explained by the fact that G1 ensues the M phase, a phase in which cell division takes place.  The first opportunity for newly formed cells from the M phase to grow and develop is in G1, a phase of rest and growth. In addition to this function, G1 (along with the other gap phase G2) provides a time for cells to monitor their surrounding environment, both inside and outside of the cell, to ensure that conditions are appropriate and preparations are complete before committing to the next phase, the S phase. 
There is another sub phase within G1, known as the Gap zero phase (G0) (see Figure 3). When conditions are unfavorable, cells delay progress through G1 and enters G0. In this specialized phase, cells can remain in a rest form of periods from days to even years,  before returning to a proliferating state.
Some cells, such as cardiac muscle cells and nerves cells are unable to divide. That is, they have lost their ability to replicate themselves and as a consequence never leave G0 . Other cells like the red blood cells are continually dividing throughout our life. Some cells will only divide under certain circumstance, for instance if part of the cell population is damaged or dies.
The synthetic phase or S phase of the cell cycle occurs between the gap phases of the cycle. It is a short phase consisting of few hours in which the entire nuclear content of the cell is replicated.  Replication of DNA molecule is undertaken in 3 steps; where a bidierectional mechanism is initiated to facilitate multiple replication sites along a chromosome.  This is subsequent to a preparation period where all hereditary material of the parent DNA molecule is portioned prior to cellular division.  A parent chromosome is composed of 2 complementary strands of DNA, where every base on one strand is paired with a specific partner on the other strand. That is, Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C). 
The process of DNA Synthesis:
• The initial step of DNA replication in vertebrate cells involves the separation of the double stranded parent molecule of DNA, where hydrogen bonds linking base pairs are broken.  Separation of the DNA molecule is facilitated via an unwinding mechanism and leads to the formation of a 'v' by the two single stranded molecules.
• As the molecule unwinds, DNA synthesis occurs at this location of the 'v' which is also referred to as the replication fork.  The process of DNA synthesis in eukaryotes is semi-conservative and occurs in a bi-directional manner. Consequently, there is a 'leading strand' and a 'lagging strand' formed. The leading strand replicates in the same direction as the proceeding replication fork, and is therefore faster. The lagging strand copies the molecule in the opposite direction, and therefore produces copied fragments of DNA known as Okazaki fragments, which will be joined together to form a single strand.  This is illustrated in Figure 4, which shows the process of replication in a double stranded molecule.
• As a result of separation of the double-stranded molecule and the synthesis of the single strands of DNA, new nucleotides are able to bind to the exposed bases with the assistance of a DNA polymerase enzyme.  Each strand is then used as a template for determining the order of base pairs in the new complementary strand. This results in the synthesis of a new strand that is complementary to the pre-existing template strand.  This then leads to the formation of duplicate daughter strands in an anti-parallel orientation, as shown in Figure 5.
• The final step of replication and synthesis produces one parent and one new strand of DNA. At this point, the cycle progresses into a phase of rapid protein synthesis to ensure cellular growth. Different cyclin-dependent kinase complexes trigger the process of DNA synthesis.  There are 3 main trigger complexes of the S phase, which are classified into a group known as the S phase promoting factors (SPF).  There is firstly an accumulation of cyclin E/cdk2 during late G1 phase, initiating the S phase.  Cyclin B/cdk2 is then synthesised, but phosphorylation of the complex occurs at Thr14 -Tyr15 and renders it inactive.  Lastly, cyclin A/cdk2, which is considered the most prominant factor in the group, enters the cell nucleus and prepares for cellular duplication.  It accumulates during S phase and its activation triggers the transition to G2 of the cycle.
Gap 2 Phase (G2)
G2 is the third stage of the cell cycle, preparing the cell for entry into mitosis (M phase). It is the second shortest phase of the cell cycle after mitosis, lasting approximately four to five hours in a 24-hour cell cycle.  It is a period of rapid cell growth and production of new proteins, similar to G1 and includes key mechanisms that control the completion of DNA and chromosomal replication following DNA synthesis during the S phase. 
In G2, the nucleus bound by a nuclear envelope is well defined, and includes at least one nucleolus. However, the replicated chromosomes occur as loosely packed chromatin fibres and cannot be distinguished individually.  One key process that occurs during G2 is the duplication of the centrosomes resulting with two centrosomes by the end of G2.  This centrosome replication is necessary in order to guide the chromosomes during the M phase that follows.
Cyclins and cyclin dependent kinases (CDKs) are a family of proteins which regulate the distinct stages of the cell cycle.  During the G2 phase, cyclins A and B coupled to CDK-1 drives the cell up to the M phase. 
The successful transition from G2 to the M phase is controlled by a key checkpoint at the end of this gap phase. This ‘DNA structure checkpoint’ has the important role of providing a quality check ahead of mitosis.  If DNA replication is incomplete or if there are mistakes in the copies of DNA, the G2 checkpoint prevents the transition to M phase and cells will undergo repair or apoptosis (programmed cell death). 
p53 is a protein which can block the cell cycle if the DNA is damaged. By blocking the cell cycle, it provides more time for DNA repair and increased p53 levels are present in damaged cells.  More information on p53 can be found here.
Mitosis is the division of a mother cell into two identical daughter cells, in which a eukaryotic cell separates the chromosomes in its cell nucleus.  Eukaryotic mitosis can be divided into five key stages. These are;
- * prophase
- * prometaphase
- * metaphase
- * anaphase and
- * telophase
The prophase stage is signified when the DNA has been replicated and the resulting chromosomes become condensed. Each chromosome is made up of two chromatids, these pairs of chromatids are linked by a structure called centromere. . Near the nucleus are structures called centrosomes, made from pairs of centrioles, which coordinate the cell's microtubules. The centrioles also start moving to opposite poles of the cell, allowing the mitotic spindle to develop in-between. Microtubules disassemble into their tubulin components which will be used to form the mitotic spindle. 
As the nuclear envelope deteriorates, the cell enters the prometaphase stage. In this stage, the centrioles move to opposite poles of the cell and the chromosomes attach to the spindle fibres via the kinetochores located at the centromere.  Prometaphase is sometimes considered part of prophase.
Once the chromosomes have completely bound to the spindle fibres, the chromosomes align to the metaphase plate located at the centre of the cell, signalling that the cell is at metaphase. 
Following metaphase, anaphase is characterised by the shortening kinetochore microtubules and the separation of the chromatid pairs. The chromosomes move towards opposite poles of the cell as they follow along the mitotic spindle.  This chromosomal movement and separation is referred to as anaphase A, while the separation of the spindle poles is called anaphase B. 
The final stage of mitosis is telophase whereby new nuclear envelopes develop on both sides of the cell.  The chromosomes beginning to decondense into chromatin, and the breakdown of the mitotic spindle also begins.  Also during this time the mother cell is completely divided creating two daughter cells by the process of cytokinesis. These daughter cells then enter interphase at G1, restarting the cell cycle.
Progression through the cell cycle is monitored at various checkpoints, where inhibition or activation of the cell cycle is possible depending on the formation of cyclin/CDK complexes and their binding activity.  In the cell cycle, 3 major checkpoints are elicited and these are:
- G1/S checkpoint (restriction point)
- G2/M checkpoint and
- M-phase checkpoint.
1. The first checkpoint is the G1 checkpoint, also known as the restriction point, which occurs midway through the cell cycle's G1 phase. At this point, the cell can decide whether it should divide, delay division, or enter a resting period (G0).  This restriction point is mainly controlled by the actions of Cdk inhibitors.  Activation at the restriction point is a result of appropriate cyclin-cdk complex forming and will trigger the phosphorylation of proteins, which in tern activates M-phase cyclins, thereby leading to the transition into G2/M checkpoint.  Conversely should an error occur during DNA replication, G1/S cyclin-cdk complexes (at the restriction point) will not bind and hence inhibition of cycle progression to G2/M checkpoint will occur. 
2. What takes place at the G2 checkpoint?
- The cell is checked to make sure that DNA is replicated successfully and that two identical sets of the genome are now present and intact. 
- Proof reading of both sets of the genome takes place and a check for molecular damage decides on whether the genome will be maintained, repaired or rejected. 
- If DNA repair is required, the cell cycle is delayed for the duration of the DNA repair. 
- If DNA is badly damaged and rejected, apoptosis (programmed cell death) will be triggered.
- Overall competence of the cell to enter mitosis is determined. If the cell is ready, mitosis promoting factor (MPF) promotes the entrance into mitosis. 
3. Also known as the mitotic spindle checkpoint, it controls the entry into anaphase from metaphase. At this checkpoint, the cell senses the tension of the bipolar attachment that is created by the correct alignment of chromosomes at the mitotic plate.  If this checkpoint is satisified, it results in the degradation of cyclin B, which usually blocks the entry to anaphase by inhibiting the anaphase promoting complex.  The cell can now progress and continue the cell cycle.
Presence of checkpoints in the cell cycle ensure duplication of genomic material occurs without error such that abnormalities resulting from genomic instability can be avoided. 
When a cell starts dividing uncontrollably producing more and more cells, it will eventually produce a mass of cell known as a tumor. This tumor might them begin to interfere with normal body function causing complications and leading to other diseases. Although not fully understood, it has been identified that the formation of a tumor is the direct result of an abnormal regulation of the cell cycle.
Cellular abnormalities may also be a result of errors incurred during DNA replication wherein discrepancies in nucleotide sequences of the chromosome may be found. Such errors within a genome have the potential to cause cellular malfunctioning and mutation of the gene. Accumulation of mutated genetic materials will lead to the other complications, including the development of cancer.
- Centriole: Cylindrical structures composed of groupings of microtubules. Assist in the assembly of microtubules during cell division.
- Centromere: A condensed and constricted region of a chromosome, to which the spindle fiber is attached during mitosis.
- Chromatid: One of the two strands of the chromosome pair undergoing cell division.
- Chromosome: A Strand of DNA associated with proteins in the nucleus of eukaryotic cells. It contains the hereditary material of the organism required for cellular function and life.
- Cyclin: A class of proteins that vary in abundance during specific points of the cell cyclin and are responsible for its regulation by binding to kinases.
- Cytokinesis: The phase of cell division in which the cytoplasm divides.
- Cytology: Division of biology that is associated with the formation, structure, and function of cells.
- Kinetochore: A large multi-protein complex which binds to the centromeres of the microtubules of the mitotic spindle in the cell cycle during metaphase.
- Proliferation: Term used to describe the growth or rapid multiplication of cellular material and tissues.
- Replication fork: The point at which the two strands of DNA are separated to allow replication of each strand.
- Retinoblastoma: A malignant tumor of the retina that occurs predominantly in young children.
- Learn more about Cyclin B/CDK1
- Learn more about Cyclin D
- Learn more about Cyclin E/CDK2
- Learn more about p53
- Search "cell cycle" on Entrez all databases
- Online textbooks on "cell cycle" | Molecular Biology of the Cell | Molecular Cell Biology | The Cell- A molecular Approach | Bookshelf
- Alberts B, Bray D, Hopkin K, Johnson A, Lewis J, Raff M, Roberts K, Walter P. Essentials in Cell Biology: 2nd Edition. Garland Science Textbooks. 2004.
- Singer S. A Short History of Biology. Clarendon, Oxford. 1931.
- Boye E, Nordström K. Coupling the cell cycle to cell growth. EMBO Rep. 2003 Aug;4(8):757-60. PMID 12897798
- Tanaka S, Nojima H. Nik1: a Nim1-like protein kinase of S. cerevisiae interacts with the Cdc28 complex and regulates cell cycle progression. Genes Cells. 1996 Oct;1(10):905-21. PMID 9077450
- Watson JD, Crick FH. Molecular structure of nucleic acids; a structure for deoxyribose nucleic acid. Nature. 1953 Apr 25;171(4356):737-8. PMID 13054692
- Rao PN, Johnson RT. Mammalian cell fusion: studies on the regulation of DNA synthesis and mitosis. Nature. 1970 Jan 10;225(5228):159-64. PMID 5409962
- Pardee AB. A restriction point for control of normal animal cell proliferation. Proc Natl Acad Sci U S A. 1974 Apr;71(4):1286-90. PMID 4524638 Cite error: Invalid
<ref>tag; name "Pardee" defined multiple times with different content
- Clarke L, Carbon J. Isolation of a yeast centromere and construction of functional small circular chromosomes. Nature. 1980 Oct 9;287(5782):504-9. PMID 6999364
- Evans T, Rosenthal ET, Youngblom J, Distel D, Hunt T. Cyclin: a protein specified by maternal mRNA in sea urchin eggs that is destroyed at each cleavage division. Cell. 1983 Jun;33(2):389-96. PMID 6134587
- Murray AW, Kirschner MW. Cyclin synthesis drives the early embryonic cell cycle. Nature. 1989 May 25;339(6222):275-80. PMID 2566917
- Murray AW, Solomon MJ, Kirschner MW. The role of cyclin synthesis and degradation in the control of maturation promoting factor activity. Nature. 1989 May 25;339(6222):280-6. PMID 2566918
- Hartwell LH, Weinert TA. Checkpoints: controls that ensure the order of cell cycle events. Science. 1989 Nov 3;246(4930):629-34. PMID 2683079
- Baker SJ, Fearon ER, Nigro JM, Hamilton SR, Preisinger AC, Jessup JM, vanTuinen P, Ledbetter DH, Barker DF, Nakamura Y, White R, Vogelstein B. Chromosome 17 deletions and p53 gene mutations in colorectal carcinomas. Science. 1989 Apr 14;244(4901):217-21. PMID 2649981
- Matsushime H, Roussel MF, Ashmun RA, Sherr CJ. Colony-stimulating factor 1 regulates novel cyclins during the G1 phase of the cell cycle. Cell. 1991 May 17;65(4):701-13. PMID 1827757
- Koff A, Cross F, Fisher A, Schumacher J, Leguellec K, Philippe M, Roberts JM. Human cyclin E, a new cyclin that interacts with two members of the CDC2 gene family. Cell. 1991 Sep 20;66(6):1217-28. PMID 1833068
- Xiong Y, Hannon GJ, Zhang H, Casso D, Kobayashi R, Beach D. p21 is a universal inhibitor of cyclin kinases. Nature. 1993 Dec 16;366(6456):701-4. PMID 8259214
- Harper JW, Adami GR, Wei N, Keyomarsi K, Elledge SJ. The p21 Cdk-interacting protein Cip1 is a potent inhibitor of G1 cyclin-dependent kinases. Cell. 1993 Nov 19;75(4):805-16. PMID 8242751
- Serrano M, Hannon GJ, Beach D. A new regulatory motif in cell-cycle control causing specific inhibition of cyclin D/CDK4. Nature. 1993 Dec 16;366(6456):704-7. PMID 8259215
- Polyak K, Kato JY, Solomon MJ, Sherr CJ, Massague J, Roberts JM, Koff A. p27Kip1, a cyclin-Cdk inhibitor, links transforming growth factor-beta and contact inhibition to cell cycle arrest. Genes Dev. 1994 Jan;8(1):9-22. PMID 8288131
- Meijer L, Jézéquel A, Ducommun B. (Eds.) Progress in Cell Cycle Research. Volume 4. Springer:Toulouse, France. 2000. Link
- Foundation N. The Cell Cycle Development. WileyBlackwell. 2001.
- Copper GM, Hausman RE. The Cell: A Molecular Approach, 4th Ed. 2006 ASM Press and Sinauer Associates, Inc. 2006.
- Smith JA, Martin L. Do cells cycle? Proc Natl Acad Sci U S A. 1973 Apr;70(4):1263-7. PMID 4515625
- Shackelford RE, Kaufmann WK, Paules RS. Cell cycle control, checkpoint mechanisms, and genotoxic stress. Environ Health Perspect. 1999 Feb;107 Suppl 1:5-24. PMID 10229703
- Sherr CJ. Cancer cell cycles. Science. 1996 Dec 6;274(5293):1672-7. PMID 8939849
- Kornberg A. DNA replication. J Biol Chem. 1988 Jan 5;263(1):1-4. PMID 3275635
- Champe P, Harvey R, Ferrier D. Biochemistry, 4th Edition. In Lippincott's Illustrated Reviews. C H I P S. 2008;Chapter 29:pp.395-400
- Israels ED, Israels LG. The cell cycle. Oncologist. 2000;5(6):510-3. PMID 11110604
- Dulić V, Lees E, Reed SI. Association of human cyclin E with a periodic G1-S phase protein kinase. Science. 1992 Sep 25;257(5078):1958-61. PMID 1329201
- Galderisi U, Jori FP, Giordano A. Cell cycle regulation and neural differentiation. Oncogene. 2003 Aug 11;22(33):5208-19. PMID 12910258
- Copper GM, Hausman RE. The Cell: A Molecular Approach, 4th ed. 2006 ASM Press and Sinauer Associates, Inc. 2006
- Bell SP, Dutta A. DNA replication in eukaryotic cells. Annu Rev Biochem. 2002;71:333-74. PMID 12045100
- Johnson DG, Walker CL. Cyclins and cell cycle checkpoints. Annu Rev Pharmacol Toxicol. 1999;39:295-312. PMID 10331086
- Kaufmann WK, Paules RS. DNA damage and cell cycle checkpoints. FASEB J. 1996 Feb;10(2):238-47. PMID 8641557
- Taylor WR, Stark GR. Regulation of the G2/M transition by p53. Oncogene. 2001 Apr 5;20(15):1803-15. PMID 10331086
- Pickett-Heaps J. The evolution of mitosis and the eukaryotic condition. Biosystems. 1974 Jul;6(1):37-48. PMID 4157379
- Stanfield CL, Germann WJ. Principles of Human Physiology, 2nd edition. Pearson Benjamin Cummings, San Francisco. 2005.
- Ribeiro KC, Pereira-Neves A, Benchimol M. The mitotic spindle and associated membranes in the closed mitosis of trichomonads. Biol Cell. 2002 Jun;94(3):157-72. PMID 12206655
- Chan GK, Liu ST, Yen TJ. Kinetochore structure and function. Trends Cell Biol. 2005 Nov;15(11):589-98. PMID 16214339
- De Souza CP, Osmani SA. Mitosis, not just open or closed. Eukaryot Cell. 2007 Sep;6(9):1521-7. PMID 17660363
- Draviam VM, Xie S, Sorger PK. Chromosome segregation and genomic stability. Curr Opin Genet Dev. 2004 Apr;14(2):120-5. PMID 15196457
- Elledge SJ. Cell cycle checkpoints: preventing an identity crisis. Science. 1996 Dec 6;274(5293):1664-72. PMID 8939848
- O'Connell MJ, Walworth NC, Carr AM. The G2-phase DNA-damage checkpoint. Trends Cell Biol. 2000 Jul;10(7):296-303. PMID 10856933
- Allocate sections to group members (See above)
- Research topic and headings
- Write and format Wiki
- Citation and reference
- Use 'edit' and 'discussion' to make comments/ explanations
- Review and edit project page as allocated
- Monitor and review all the edits
- Write group reflection
- Download all the edits made during the 'edit' period
- Submit final wiki version by WEDNESDAY
2009 Group Projects
Group 1 Meiosis | Group 2 Cell Death - Apoptosis | Group 3 Cell Division | Group 4 Trk Receptors | Group 5 The Cell Cycle | Group 6 Golgi Apparatus | Group 7 Mitochondria | Group 8 Cell Death - Necrosis | Group 9 Nucleus | Group 10 Cell Shape