Cell cycle

Cell cycle.

1. By Dr.N.Sreekanth DNB-RT CELL CYCLE

2. 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 to produce daughter cells. There are two types of cell division. Mitosis Growth and repair Produces diploid cells identical to each other and the parent cell Meiosis Sexual reproduction Produces haploid cells different to each other and the parent cell. Somatic cells Germ cells

3. Eukaryotic Cell Populations Germ cells - Unlimited proliferation by meiotic division. (Unlike cancer cells, these cells form immortal cell lines through meiotic division) Stem cells - These cells have two functions. Proliferation & Differentiation (Unlike cancer cells, these cells can only pass through a limited number of cell cycles) Partially differentiated cells - These have a limited capacity for proliferation, and their daughter cells are fully differentiated with no proliferative ability. Fully differentiated cells - These cells never proliferate.

4. MITOSIS

5. Prophase The nuclear membrane and endoplasmic reticulum disappear. The chromosomes shorten and thicken. Centrosomes move towards opposite poles. The nucleolus disappears. Spindle cells form from the poles to the center.

6. Metaphase The chromosomes shorten and thicken further. Sister chromatids are kept together using centromeres. The chromosomes are arranged side-by-side in a row in the equatorial plane. The chromosomes hold on to spindle cells with their centromeres.

7. Anaphase The contraction and relaxation movements of spindle cells break the centromeres that lock the chromatids together. The sister chromatids are separated from each other and are moved to opposite poles.

8. Telophase The chromosomes stop moving. The chromosomes unwind their helices and become chromatins. The nucleolus reappears. RNA and protein syntheses start. Spindle cells disappear. The nuclear membrane forms, and the endoplasmic reticulum takes on a shape again. Vital events restart in the cell. Cytogenesis occurs, and division finishes.

9. Interphase The interphase is the preparation phase for the redivision of a cell. It is the longest phase of the eukaryotic cell cycle. The interphase is divided into THREE stages. (G0/G1, S, G2)

10. G0 phase Most cells in an adult are not in the process of cell division. They enter an inactive period called G0, a phase outside of the cell cycle. Mitogens or growth factors can, however, induce cells in G0 to re-enter the cell cycle.

11. G1 Phase (Gap1) This occurs just after cytogenesis. This is the stage where matter transportation , synthesis , lysis reactions , organelle production , RNA synthesis and tissue functions continue at their highest levels. It is the longest stage.

12. S Phase (Synthesis) DNA is duplicated and the number of chromatins doubles (Replication). The most intense protein synthesis is performed at this stage. The order of centromere duplication is observed.

13. G2 Phase (Gap2) Enzymes related to division are synthesized. The number of organelles increases. DNA synthesis finishes, but RNA synthesis continues. Centrosome synthesis finishes, and these centrosomes start moving towards opposite poles.

14. The most radiosensitive stages during the cell cycle are the early G2 and M stages The radiosensitivity of a cell is four-fold greater during the mitotic phase than during the interphase . Radioresistance is high in the S, late G1 and G0 phases. The resistance of the S phase is due to the large amounts of synthesis enzymes present, which have the ability to rapidly repair DNA.

15. CHECK POINTS Cell cycle checkpoints, a series of biochemical signaling pathways that sense and induce a cellular response to DNA damage, are important for maintaining the integrity of the genome. There are 3 important check points G1 check point G2 check point M check point

16. The G1 checkpoint leads to the arrest of the cell cycle in response to DNA damage, ensuring that DNA damage is not replicated during S phase. The G2 checkpoint leads to the arrest of the cell cycle in response to damaged and/or unreplicated DNA to ensure proper completion of S phase. The M checkpoint leads to the arrest of chromosomal segregation in response to misalignment on the mitotic spindle.

17. Tim Hunt Paul Nurse Lee Hartwell Nobel Prize in Physiology or Medicine in 2001 The prize was received for studies on the regulation of the cell cycle during which the cyclins were discovered.

18. The components of the checkpoints are proteins that act as DNA damage sensors, signal transducers, or effectors. Disruption of checkpoint function leads to genomic and chromosomal instability leading to mutations that can induce carcinogenesis.

19. The passage of the cell through the different phases of the cell cycle is coordinated and regulated by a set of proteins called cyclins and their associated cyclin - dependent kinases ( cdks ). Cyclins are regulatory subunits of their cdks .

20. Cyclins were so named because of the cyclical changes in their concentrations that occur over a series of cell divisions. The concentration of cyclin protein is dependent on the transcription of its gene and by subsequent regulated protein degradation.

21. The cyclin – cdk complexes exert their effect by phosphorylating target proteins Binding of a cyclin to its cdk partner, cyclin induces a conformational change in the catalytic subunit of the cdk revealing its active site. The concentration of cdks does not fluctuate during the cell cycle.

22. The pairing of cyclins to the cdks is highly specific. Cyclin D - cdks 4/6 C yclin E - cdk2 Cyclin A - cdk2 Cyclins A,B - cdk1

23. Cyclin D is the first cyclin to be synthesized. Cyclin D - cdks 4/6  drives through G1. Cyclin E - cdk2  G1 to S phase transition. Cyclin A–cdk2  S phase progression. Cyclins A,B–cdk1  G2 & G2 to M phase transition.

24. G1 Check point The retinoblastoma (RB) protein is an important target of cyclin D– cdks 4/6 and a key regulator of the G1 to S phase transition. RB exerts its effects by protein–protein interactions with the E2F transcription factor and HDACs. Hypophosphorylated RB inactivates E2F and recruits HDACs.

25. The activity of RB is regulated by phosphorylation via different cyclins – cdks . Signaling pathway for the growth factor EGF results in the transcriptional activation of the cyclin D gene and allows progression through the restriction point. Repression is relieved for some genes such as cyclin E

26. Phosphorylated RB releases E2F leading to Expression of its target genes, such as cyclin A, thymidylate synthase, and dihydrofolate reductase , that are important for S phase

27. G2 Check point The G2 checkpoint blocks entry into M phase in cells that have incurred DNA damage in previous phases or have not correctly completed S phase. The G2 checkpoint is induced by DNA damage and aberrant DNA synthesis and blocks entry into M phase. Specific Cdc25 s (type B and C) are important in the G2–M phase transition.

28. There is also a decatenation G2 checkpoint that is involved chromatid separation during anaphase of mitosis. Topoisomerase II, an enzyme that can release torsional stress by making double-strand DNA breaks to allow unwinding, is key in the decatenation G2 checkpoint.

29. Mitotic Check point The mitotic checkpoint (also known as the spindle assembly checkpoint) is a signaling cascade that ensures correct chromosomal segregation during mitosis and the production of two genetically identical nuclei . ( Prevents mis -segregation of chromosomes during anaphase ) The Aurora kinases (A, B, and C) regulate important aspects of mitosis, including chromosome segregation and the spindle checkpoint. They are serine/threonine kinases that phosphorylate target proteins, which play a role in chromosome structure and spindle assembly.

30. Mechanisms of cdk regulation Cdks are regulated by association with cyclins , inhibitors, and by activating and inhibitory phosphorylation.

31. Association with cyclins

32. Association with inhibitors

33. Regulation by phosphorylation The tyrosine kinase, wee1, phosphorylates Thr14 and Tyr15.

34. Regulation by phosphorylation

35. THANK YOU

36. Four stages: G1, S phase, G2, and M phase. G1, S, and G2 make up the part of the cycle called interphase. The genetic material of a cell is replicated in S phase (DNA synthesis). M phase involves the partitioning of the cell to produce two daughter cells and includes mitosis and cytokinesis. G1 and G2 are ‘gaps’ preceding the S and M phases during which time the cell prepares for the next phase.

37. Aurora kinase A Localizes to centrosomes during interphase Upregulated at the beginning of mitosis and relocates to the spindle poles and spindle microtubules Role in centrosome maturation and assembly of the spindle apparatus Aurora kinase B activity is highest later in mitosis. Role in bipolar spindle attachment to chromosomal centromeres, and the spindle checkpoint and monitoring of chromosomal segregation and cytokinesis. Aurora kinase C is active during late mitosis and localizes to spindle poles .