Checkpoints
Checkpoints
- at various stages of the cell cycle, the cell checks whether everything supposed to have been done has been done and whether cell is in good shape to continue
- if not, cycle stops
o can then restart if problem is solved
o can do apoptosis if problem does not get resolved
Checkpoints
- G1-to-S
o Checks for DNA damage
- G2-to-M
o Checks for DNA damage and unreplicated DNA
- Spindle assembly checkpoint
o Anaphase of mitosis
G1 to S checkpoint
- DNA damage by any of a number of mechanisms
o Think of a nick in the DNA
o Now replicate that
o One strand is OK, the other gets replicated in two different pieces
§ Chromosome rearrangements result
- ATR/ATM
o These are two kinases with overlapping functions that are activated by DNA damage
o They phosphorylate a protein called p53
- This protein is a transcription factor, normally very unstable and so at low levels in the cell, but stabilized by phosphorylation
- Induces transcription of the p21 CIP gene (CIP = cdk inhibitor protein)
- p21 inihibits all cyclin/cdks and arrests cells at G1
- When damage is repaired, phosphoryation levels drop, p53 dephosphorylated, and p21 CIP levels fall
- Cell cycle continues
- If damage not repaired, p53 stays phosphorylated and directs cells to do apoptosis
o p53 mutated in many (>50%) of all human cancers
o mutations that inactivate allow cell to continue cell cycle even in presence of DNA damage
o leads to mutation and cancer
G2 to M checkpoint
- checks for DNA damage and complete replication of DNA
- involves RAD9 protein
o no RAD9p, cells with abnormal DNA allowed into mitosis
§ not good
Spindle assembly checkpoint
- checks that all chromosomes are attached to the spindle
o if not, get aneuploidy (abnormal chromosome number)
- proteins involved in this still mysterious
Signalling and the cell cycle
Cells talk to each other with chemical signals
- signaling cell makes a signal molecule
o hormones (endocrine signaling
o growth factors (usually small peptides)
- target cell
o expresses receptor (around 1543 in H sapiens genome predicted)
o ligand is what binds receptor (that is, the ligand is the hormone or growth factor normally)
o receptor-ligand interaction is very specific
Receptor tyrosine kinases and the MAP kinase pathway
- RTKs are receptors that are also tyrosine kinases
- Bind a growth factor
o This growth factor is supposed to propel the cell into mitosis – like nerve growth factor
- Binding of growth factor to receptor activates the tyrosine kinase
o
o Phosphorylates Ras and causes Ras to drop GDP and bind GTP and be activated
- Ras binds Raf and activates it (by a not yet understood series of kinases and phosphorylations)
o Raf is Ser/Thr kinase
- Hydrolysis of GTP to GDP+P releases activated Raf
- Raf phosphorylates and activates Mek
o MEK is a kinase
- MEK phosphorylates MAP kinase
o MAP is a kinase (mitogen activated protein kinase)
- One thing MAP kinase does
o Phosphorylates p90RSK
o Both activated MAP kinase and p90 enter nucleus
o Phosphorylate transcription factors TCF (ternary complex factor) and SRF (serum response factor)
o Bind an enhancer called SRE (serum response element)
§ Lots of genes have this
§ Including c-Fos, a transcriptional activator
§ Influences txn of other genes necessary for progression through cell cycle and mitosis
· Like Myc!
- Ras often found to be constitutively active in tumors (maybe ¼ of all human tumors)
o These cells proliferation is independent of external growth factors
- Colon, lung, bladder, melanoma
- Raf proteins missing N-terminal domain (regulatory, binds to Ras) are constitutively active ((oncogene)) and turn on the whole rest of the cascade even when no growth factor present
o Found in 66% of all human melanomas
Questions about section 9.1 that I liked….
2, 5, 7, 8, 9, 10, 11