Cell cycle regulation Basics

An overview of cyclins and CDKs involved in the regulation of cell cycle.

1. CELL CYCLE :- AN OVERVIEW

2. What is cell cycle? The ordered series of events that lead to the division of cell into two daughter cells each containing chromosomes identical to the parent cell. PHASES IN CELL CYCLE G1-S-G2-M-CYTOKINESIS

3. CELL DIVISION CONTROL High fidelity and accuracy are required to ensure that DNA replication is carried out correctly and each daughter cell inherits correct no. of chromosomes. WHAT GOVERNS THE EVENTS IN CELL CYCLE? Checkpoints!!! The surveillance mechanisms that prevent the initiation of each step in cell division until the previous has been completed.

4. CYCLINS Cyclins are so named because their concentrations change during the cell cycle. They bind to and activate CDK They are present only during the cell cycle stage they trigger and are absent in others. They are different from each other in protein sequence, but all of them contain a conserved 100-amino-acid region known as the cyclin box.

5. CDKs CDKs are regulated by :- 1.cyclin binding 2.activating and inhibitory phosphorylation . Together, these regulatory events ensure that CDKs are active only at the appropriate cell cycle stage.

6. G1 CDKs G1 CDKs activate members of E2F transcription factors. During G1 E2Fs are held inactive through their association with retinoblastoma protein ( Rb ) until G1 CDKs activate E2Fs by phosphorylating and inactivating Rb . E2Fs than activates genes encoding many proteins involved in DNA synthesis. Also stimulate the translation of genes encoding G1/S and S phase cyclins . This is one of the events responsible for passage through RESTRICTION POINT. S phase CDKs and M-CDKs maintain Rb protein in the phosphorylated state throughout the S, G2 and early M phase. After ANAPHASE (late M phase) a fall in cyclin -CDK activities leads to dephosphorylation of Rb . Thus, hypo- phosphorylated Rb is available to inhibit E2F during early G1 of the next cycle. Henceforth, G1/S CDKs activity remains low until cell decides to enter a new cell- cycle .

7. S-Phase G1/S CDKs turn off the machinery that degrades S phase cyclins during exit from mitosis and G1 and induce the degradation of a CDK Inhibitor that inhibits S-Phase CDKs.

8. S Phase Regulation When G1/S CDKs levels are high and CKIs of S-CDKs are destroyed it is then S-phase CDKs and a second protein kinase DDK activates MCM HELICASE and recruits DNA polymerases to the sites of ORI (origin of replication). A protein complex known as ORC (origin replication complex) is associated with DNA replication. ORC and two initiation factors cdc6 and cdt1 associate , to load replicative Helicases . MCM helicases can be loaded at low CDK activity during exit from mitosis and early G1. Thus, their activation/ phosphorylation is triggered by S-Phase CDKs.

9. COHESINS ESTABLISH LINKAGES BETWEEN REPLICATED DNA MOLECULES The protein complexes that establish linkages between sister chromatids are called cohesins . They are composed of four subunits: Smc1, Smc3, Scc1 (sometimes called Rad21), and Scc3. Smc1 and Smc3 are members of the SMC protein family, which is characterized by long coiled-coil domains that are flanked by a globular domain containing ATPase activity. The ATPase domains interact with Scc1 and Scc3 and, together, form a ring structure . These rings of cohesin embrace one or both copies of the replicated DNA. When cohesins are inactivated, sister chromatids do not associate properly with each other.

10. M-CDKs Mitotic cyclins are synthesized during S phase and G2 and bind to CDK1 . However, the cyclin - CDK complex is not active because threonine 14 and tyrosine 15 of the CDK1 subunit are phosphorylated by the protein kinase Wee1 . Once DNA replication has been completed, the protein phosphatase Cdc25 is activated and dephosphorylates CDK1 . Active mitotic CDKs further stimulate Cdc25. At the same time, mitotic CDKs inhibit Wee1, the protein kinase that places the inhibitory phosphorylation on the CDK subunit. Ongoing DNA replication inhibits Cdc25 activity.

11. MITOTIC CDKs promote spindle formation Mitotic CDKs induce nuclear envelope breakdown in most eukaryotes by phosphorylating lamins . Centrosome duplication occurs during S phase. Mitotic CDKs induce the separation of the duplicated centrosomes , which initiates mitotic spindle formation. Sister chromatids attach to the mitotic spindle via their kinetochores in a bi-oriented manner, with one sister kinetochore attaching to microtubules emanating from one spindle pole and the other one to microtubules nucleated by the other spindle pole . Cells sense bi-orientation of sister chromatids through a tension-based mechanism. When kinetochores are not under tension, the protein kinase Aurora B phosphorylates the microtubule-binding subunits of the kinetochore , which decreases their microtubule binding affinity. Chromosomes must be compacted for segregation. Condensins , protein complexes that are related to cohesins , facilitate chromosome condensation and are activated by mitotic CDKs.

12. COMPLETION OF MITOSIS Cleavage of cohesin by separase induces chromosome segregation during anaphase. At the onset of anaphase, APC/C is directed by Cdc20 to ubiquitinylate securin , which is subsequently degraded by proteasomes . The degradation of securin activates separase . Exit from mitosis is triggered by mitotic CDK inactivation mainly brought about by mitotic cyclin degradation. Exit from mitosis requires the activity of protein phosphatases such as Cdc14 to remove mitotic phosphates from many different proteins, permitting mitotic spindle disassembly, the decondensation of chromosomes, and the reassembly of the nuclear envelope.