CELL CYCLE CHECKPOINTS
A checkpoint can be defined as a stage in the cell cycle where the cell examines it’s internal and external factors and decides whether or not to move forward with cell division. Although a cell cycle has several checkpoints the three most important check points are:
- G1 checkpoint at G1/S
- G2 checkpoint at G2/M
- M checkpoint/ spindle checkpoint
The first checkpoint and the primary point at which the cell decides whether or not to proceed depending on the conditions. At G1 checkpoint the cell checks some factors such as
- Nutrient availability
- Molecular signals
- DNA integrity
Only if the cell has the appropriate size and is receiving nutrients and growth factors with no damaged DNA will the cell proceed to divide. Lack in fulfilling any of the above criteria and the cell enters resting phase also known as the G0 phase until the conditions improve.
G2 checkpoint at the G2/M stage checks for DNA integrity, DNA replication. On detecting any errors the cell pauses at G2 until the DNA damage is repaired to enter the mitotic stage. In case the cell is unable to repair the DNA it is then destined to apoptosis.
M checkpoint/ Spindle checkpoint
After entering into the cell division phase, the cell encounters yet another checkpoint, metaphase checkpoint. Here, at this checkpoint, the cell examines whether all the sister chromatids are correctly attached to the spindle microtubules. The cell cycle doesn’t proceed further until it assures the correct attachment and alignment of the spindle fibers.
The life cycle of a dividing non embryonic eukaryotic cell starts with it entering the cell cycle and enter with equal partitioning of the genetic material and cleavage by cytokinesis. Cell cycle can be summed up into 4 phases
G1 – Gap1 phase
S- synthesis phase
G2- gap2 phase
M- mitosis phase
The G1, S and G2 phase together constitute the Interphase. After the mitosis stage the cell may continue with the cell cycle and enter the G1 phase or it may opt out of the cell cycle and diverge into the G0 phase.
CELL CYCLE REGULATORS
Cell cycle regulators are group of proteins and enzymes that interact with each other in varying concentrations deciding the fate of cell cycle. These can be categorized as
- Cyclins( proteins)
- cdks (cyclin dependent kinases are enzymes)
- APC/C (enzyme complex)
Cyclins are a group of related proteins. the four basic types of cyclins found in human and most other eukaryotes are
- G1 cyclins
- G1/S cyclins
- S cyclins
- M cyclins
Each cyclin is associated with a particular phase and helps drive the events of that phase . The concentration of each cyclin increases in the phase it is associated with.. Therefore a typical cyclin is present at low levels for most of the cycle except when its strongly in need during a particular stage of the cell cycle.
Cyclin dependent kinases(cdks)
A cell cycle works with the interaction of cyclins and cdks. Therefore, the smooth functioning of a cell cycle requires a cyclin. Let’s see how.
Cyclins work by activating and inactivating target proteins inside the cell, which help in the events to occur for the successfull completion of a cell cycle. all alone cdks are inactive , it is only when a cyclin binds to it at a specific site does the cdks get activated. These functional enzymes(cdks) then modify the target proteins in the cell to perform their specific set of functions.
What type of enzymes are cdks?
Cdks are kinases, which means that they work by phosphorylating ( adding phosphate groups)their targets.
Maturation promoting factor(MPF)
MPF is a cdks bound to its M cyclin. It is a very good example of how cyclins and cdks work together for a cellcycle transition.MPF complexes add phosphate groups to different targeted proteins in the cell. For example, MPF’s add phosphate group tags to nuclear envelopes ,resulting in is breakdown and also activate targets that promote chromosome condensation and other M phase events.
MPF also triggers its own destruction once the anaphase starts. MPF’s activate APC/C (anaphase promoting complex/cyclosome), these APC/C cause M cyclins to be destroyed as the anaphase starts. Destruction of anaphase disintegrates the MPF and inturn pushes the cell out of mitosis, allowing the new daughter cells to enter G1.
APC/C also cause the destruction of spindle fibers , thus separating the sister chromatids during the anaphase.
APC/C is an enzyme that works by adding protein tags called ubiquitin(Ub) to their targets. Once the target is tagged with ubiquitin, the proteasomes attack and destroy it. In a similar way, APC/C attaches Ub tag to M cyclin, causing them to be chopped by proteasomes, allowing the daughter cells to enter G1 phase.
The role of APC/C in separation of sister chromatids during anaphase
- APC/C first adds the ubiquitin tag to securin ,leading to its destruction be proteasome. Securin normally binds to a protein separase and inactivates it. However, in the absence of securin, separase gets activated.
- The activated separse then chops cohesin that holds sister chromatids together allowing them to seperate.