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Cell cycle By KK Sahu Sir
The document explores the cell cycle, detailing its phases, checkpoints, and control mechanisms across different organisms, particularly in yeast and multicellular beings. It explains how cells regulate their growth and division through various parameters and checkpoints to ensure proper DNA replication and cell division. The importance of the cell cycle in producing new life and maintaining cellular health is emphasized, along with references for further reading.
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Cell cycle By KK Sahu Sir
- 1. CELL CYCLE By KAUSHAL KUMARSAHU Assistant Professor (Ad Hoc) Department of Biotechnology Govt. Digvijay Autonomous P. G. College Raj-Nandgaon ( C. G. )
- 2. CONTENT INTRODUCTION CELLCYCLE CELL CYCLE PARAMETERS PHASES OF CELL CYCLE CHECKPOINTS CELL CYCLE CONTROL SYSTEM CELL CYCLE IN YEAST CONCLUSION REFRENCES
- 3. INTRODUCTION “Where acell arises, there must be a previous cell, just as animals can only arise from animals and plants from plants”. Rudolf Virchow – 1858 All living organisms from unicellular bacterium to the multicellular mammal are products of repeated rounds of cell growth and division extending back in time to the beginning of life over 3 billion years ago. This cycle of duplication and division is known as CELL CYCLE.
- 4. CELL CYCLE “Thecell cycle is the series of events that takes place in a cell leading to its division and duplication”. PROKARYOTES EUKARYOTES
- 5. CELL CYCLE PARAMETERS The details of cell cycle vary from organism to organism. Like, a single celled yeast can divide in every 90-120 mins. Whereas a mammalian liver cell divides on average less than once a year.
- 6. CELL TYPE CELLCYCLE TIMES Early frog embryo cells 30 minutes Yeast cells 1.5-3 hours Intestinal epithelial cells ~12 hours Mammalian fibroblasts in culture ~20 hours Human liver cells ~1 year
- 7. Major sites ofcell division in plants and animals. PLANTS Root apex Shoot apex Vascular cambium ANIMALS Blood cells Intestinal epithelium Skin
- 8. PHASES OF CELLCYCLE INTERPHASE •G1 phase •S phase •G2 phase •G0 phase MITOSIS PHASE •Prophase •Metaphase •Anaphase •Telophase •Cytokinensis
- 9. I. INTERPHASE • Beforea cell can enter division, it needs to take in nutrients. All of the preparations are done during interphase. • Cell increases in size. • Cell continues to transcribe genes. • Synthesize protiens. • Grow in mass.
- 10. (a) G1 phase:- • Growth phase / Gap phase. • First growth stage. • Cells increases in size and prepare itself to copy its DNA. • Biosynthetic activities increases. • Synthesis of various enzymes which are required in S-phase.
- 11. (b) S phase:- • Synthetic phase. • Cell replicates its DNA. • Amount of DNA. is doubled in cell but ploidy remains same. • Rates of RNA transcription and protien synthesis is very low.
- 12. (c) G2 phase:- • This phase lasts until it enters M phase. • Production of microtubules takes place. • Inhibition of protien synthesis takes place which prevents the cell from undergoing mitosis.
- 13. During bothG1 and G2 phase, the cell monitors the internal and external environment to ensure that conditions are suitable and preparations are complete before it commit itself to the major disorder of S phase and mitosis. At particular points in G1 and G2 phase, the cell decides whether to proceed to the next phase or pause to allow more time to prepare. G1 and G2 phases provides additional time for the cell to grow and duplicate its cytoplasmic organelles.
- 14. Go phase:- Restingstage / Quiescent stage. G0 phase is viewed as either an extended G1 phase, where the cell is neither dividing nor preparing to divide, or as a distinct quiescent stage that occurs outside of the cell cycle. G0 phase is sometimes referred to as a "post-mitotic" state since cells in G0 phase are in a non-dividing state.
- 15. II. Mitosis phase •Mitosis is the process by which cell seperates the chromosomes in its cell nucleus into two identical sets in two nuclei. • The first visible sign that a cell is about to enter M phase is the progressive condensation of its chromosomes. These condensed chromosomes are visible under light microscope. • M phase consists of karyokinensis which is followed by cytokinenesis.
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- 19. Checkpoints Checkpoints areused by the cell to monitor and regulate the progress of cell cycle. It is the surveillance mechanism that halt the cell cycle if DNA is damaged or if cell cycle processes have not been completed. The cell cannot proceed to next phase until checkpoint requirements have been met.
- 20. There are mainly3 categories of checkpoints. They are as follows : SENSORS • Detect abnormalities and emit signal. TRANSMITTORS • Send signal along proper pathway. EFFECTORS • Inhibit cell cycle machinery.
- 21. Checkpoints ensurethat all the phases in the cycle must run in a sequential pattern. If DNA synthesis is slowed or stopped then, mitosis is also delayed. Similarly if DNA is damaged then, cycle must arrest in either G1 or G2 so that cell can repair the damage. The cell cycle control system achieves all of this by means of molecular brakes that can stop the cycle at various checkpoints, thus control system does not trigger next step unless cell is prepared.
- 22. There are3 important checkpoints occur during cell cycle. 1. G1 checkpoint :- • It allows the cell to confirm that environment is favourable for cell proliferation and DNA is complete before committing to S phase. • It depends on nutrients and specific signal molecules in the extracellular environment. • If extracellular conditions are unfavourable, cell can delay progress and may enter a resting stage (G0 phase).
- 23. 2. G2 checkpoint:- Ensures that cell do not enter mitosis phase until damaged DNA is repaired and DNA replication is complete. 3. M checkpoint:- Ensures whether cell division is complete or not ,then only it proceeds to the process of cytokinenesis.
- 25. Cell cycle controlsystem To ensure correct progression through the cell cycle, cells have evolved a complex network of regulatory protien, known as cell cycle control system. To coordinate these activities the system responds to various signals from inside and outside the cells. The system have biochemical switches that control events of cycle including DNA replication and segregation of duplicated chromosomes.
- 26. The cell cyclecontrol system depends on cyclically activated protien kinases • Entry of cell in m phase is initiated by a protien kinase known as maturation promoting factor (MPF). • The protien kinases are core part of cell cycle control system which is present in proliferating cells throughout the cell cycle. • They are activated only at appropriate times. thus, activity of each of these kinases rises and falls in a cyclical fashion. • For e.g.- some of protien kinases become active toward the end of G1 phase and are responsible for driving the cell into S phase; another kinase becomes active just before m phase and is responsible for driving the cell into mitosis.
- 27. Switching thesekinases on and off at the appropriate times is partly the responsibility of a second set of protien components of control system- the cyclins. When cyclin concentration is low, the kinase lacks the cyclin subunit and as a result, is inactive. When the cyclin concentration rises, the kinase is activated, causing the cell to enter M-phase. These protien kinases depend on cyclin for their activity, thus they also known as Cyclin dependent kinases (Cdks).
- 28. Cyclin dependent kinases(Cdks) 11 members of Cdks family are known. Among these Cdk 1,2,3,4,and 6 are known to play important roles in cell cycle. Cdk 5 activity is restricted to nervous system. Cdk 7 activity plays indirect role in cell cycle by activating Cdks involved in cell cycle. Cdks 7,8,9 are regulators of transcription. Not much is known about functions of Cdk 10 and 11.
- 29. Cyclins 15 cyclinshave been identified so far. Cyclin A and E activate with Cdks. Cyclin D1, D2 and D3 associate with cdk4 and 6. Cyclin A and B with Cdk1 and cyclin H with Cdk7.
- 30. Cell cycle inSchizosaccharomyces pombe It is also known as "fission yeast", is a species of yeast. It is used as a model organism in molecular and cell biology. It is a unicellular eukaryote whose cells are rod-shaped. It is a single-celled fungus with simple, fully characterized genome and a rapid growth rate. Fission yeast governs mitosis by mechanisms that are similar to those in multicellular animals.
- 32. CONCLUSION For bacteriaor yeast, which are single-celled organisms, this cell division will produce a new and complete organism. In a multicellular organism (like human beings) , a fertilized single- celled egg requires many cell divisions to make a new individual. In either case it is the completion of the cell cycle that produces new organisms, a process that can go on throughout life. Our survival requires a production of millions of cells a second. This fact was illustrated by the exposure of organisms to massive doses of x-rays that stop all cell division and cause an individual to die within a few days. In the end, it is the cell cycle that ensures life will always be able to produce more life in an organized fashion.
- 33. REFERENCES BOOKS P.K.Gupta Karp INTERNET www.wikipedia.com www.tutorvista.com