i have a question .. hope you can answer
through the cell division cycle .M-cdk is going throw phosphorylation and dephosphorylation.. if a mutation happened and affected the activating kinase .. what will happen to the cells?
will they stop in G0 of the cell division cycle? or what ?
If i understand right you're talking about the control of the cell cycle. Right?
Let me explain from the begining.
Cell grows during the G1 phase and the genome replicates during the S phase. During the G2 phase, cells continue to grow and prepare for cells division. During M phase, mitosis occurs. Cytokinesis (C) is the phase when the cytoplasm divides, creating two daughter cells.
^3 princicple checkpoints control the cell cycle in eukaryotes.
- The G1 checkpoint makes the key decision as to whether the cell should divide, delay division or enter a resting stage.
- The G2 checkpoint assess the success of DNA replication and trigers the start of the mitosis (M) phase. If this checkpoint is passes the cell initiates the begining of mitosis.
- The accuracy of mitosis is assessed at the M checkpoint. This checkpoint occurs during metaphase, trigers the exit from mitosis and cytokinesis and the beigining of G1.
At the G2 checkpoint, cyclin-dependant kinases (CDKs) phosphorylate histones and protein that carry the cycle past the checkpoint into mitosis. During G2, the cell gradually accumulates G2 cyclin (also called mitotic cyclin).
The cyclin binds to Cdk to form a complex called MPF (mitosis pomoting factor). When the level of MPF exceeds the threshold necessary to trigger mitosis, the G2 phase ends and mitosis begins. MPF also activates proteins that destroy cyclin.
As mitosis proceeds to the end of metaphase, Cdk levels stay relatively constant but G2 cyclin is degraded, causing less MPF to be availible and initiating the events that end mitosis.
After mitosis, the gradual accumulation of new cyclin starts the next turn of the cell cycle.
The G1 checkpoint is thought to be regulated in a similar fashion. The level of G1 cyclin increases and associates with Cdk. Eventually, a threshold ratio that trigers the next round of DNA replication is reached. The cyclin is degraded and the cycle begins again.
Coming back to your question now. What if there are no Cdk?
Cdk plays a big part at G2 and G1 checkpoint as explained above. Whatever these checkpoints check (see image) won't be regulated anymore.
In normal circumstances the DNA is frequently damaged prior to mitosis, and to prevent transmission of this damage to daughter cells, the cell cycle is arrested via inactivation of the G2 cyclin during G2 checkpoint (via phosphorylation with other protein kinases). Loss of Cdk will cause a marked deficiency in the M phase arrest.
The G 1 checkpoint is where eukaryotes typically arrest the cell cycle if environmental conditions make cell division impossible or if the cell passes into G0 for an extended period. Again this won't happen if there are no Cdk.
Slightly long but i hope this helps. Insh'Allah.
