Cell Cycle Checkpoints PDF

Cell Cycle Checkpoints Molecular biology Cycle Checkpoints Intra-S-phase checkpoint The purpose of this checkpoint (SAC) is to prevent that cells enter in mitosis before DNA replication has been completed The key player is ATR, a kinase that phosphorylates CDC25 and thus prevents entry into Mitosis...

Cell Cycle Checkpoints Molecular biology Cycle Checkpoints Intra-S-phase checkpoint The purpose of this checkpoint (SAC) is to prevent that cells enter in mitosis before DNA replication has been completed The key player is ATR, a kinase that phosphorylates CDC25 and thus prevents entry into Mitosis until all DNA has been replicated. Unreplicated DNA ATR CDC25 CDK G2 Mitosis Intra-S-phase checkpoint Unreplicated DNA ATR Spindle assembly checkpoint (SAC) This checkpoint ensures that sister chromatids are not separated before all chromosomes have been attached to the spindle Basically, it ensures that anaphase is not triggered early with unattached chromosomes. Metaphase Anaphase Spindle assembly checkpoint SAC works through a series of proteins (Mad2, BubR1 and Bub3) that binds and inhibits cdc20 and thus prevents cleavage of Cohesin Once all chromosomes are attached, Mad2, BubR1 and Bub3 cease to bind APC/C which now becomes active and degrades Securin. This frees Separase who cleaves cohesion and thus triggers anaphase. Spindle assembly checkpoint Kinetochores are a key player of SAC. Kinetochores are protein complexes that connect centrosomes (DNA) with Tubulin fibers of the spindle. Kinetochores that are not attached to the fiber keep activating Mad2, an APCcdc20 inhibitor. Once the last kinetochore has been attached, Mad2 is no longer turned active and therefore APC/Ccdc20 can cleave securing, allowing Separase to cleave Cohesin and trigger anaphase entry. SAC and Cancer Many SAC components are found mutated in cancer cells WHY? It promotes Aneuploidy, one of the hallmarks of cancer. DNA damage Response (DDR) A checkpoint that gets activates in the presence of DNA damage This DNA damage can come from: – Irradiation – stalled replication forks – Mistakes during DNA replication DNA damage checkpoint The checkpoint can be divided into: – Sensors – Transducers – Effectors The final effect of the chekcpoint is: To STOP cell cycle To activate DNA repair mechanisms To transcribe genes necessary for the Dna damage response If everything fails: to trigger apoptosis ATM and ATR Master regulators of DDR ATM and ATR are kianses that phosphorylate Serines or Threonines followed by Glutamine – SQ or TQ They usually phosphorylate effectors like CHK1, CHK2 and p53 which will drive the DDR ATM and ATR Master regulators of DDR ATR is the main regulator of DDR in the presence of stalled forks – ATR gets activated by the main sensor of replication stall: the 9-1-1 and RFC complex ATM is the main regulator of DDR in the presence of DNA double strand break (DSB) – ATM gets activated by the major sensor of DSB: the MRN complex But remember, there is a lot of redundancy between these two kinases: in the absence of one, the other can take care of its functions. ATR mediated DDR Mostly act during S-phase to activate DDR in the presence of stalled replication forks – Main effector is CHK1 and p53 which will stop replication and prevent entry into mitosis ATM mediated DDR Activated in the presence of DSB and other types of DNA damage Main effectors are p53, CHK1 and CHK2. It can stop cell cycle at G1/S and G2/M as well as activating the Intra-Sphase checkpoint It will also alter the transcription of a myriad of genes and will activate DNA repair mechanisms DDR and the Cell cycle As noted, DDR includes several checkpoints to halt the cells cycle. These DDR cell cycle checkpoints include: – – – Blockage of entry to Sphase Blocking of progress to S-phase Blocking of entry into Mitosis Most of this DDR related checkpoint are controlled by p53 acting over p21. Which checkpoint is activated depends on the type of damage and WHEN this damage is produced