Cell Cycle Checkpoints

Introduction

Mistakes in the duplication or distribution of the chromosomes lead to mutations that may be passed forward to every new cell produced from an abnormal cell
To prevent a compromised cell from continuing to divide, there are internal control mechanisms that operate at three main cell cycle checkpoints
- Cell cycle checkpoints: one of several points in the eukaryotic cell cycle at which the progression of a cell to the next stage in the cycle can be halted until conditions are favorable.
- These checkpoints occur near the end of G1, at the G2/M transition, and during metaphase

The G1 checkpoint: checks for adequate reserves, cell size, and genomic DNA damage
Determines whether all conditions are favorable for cell division to proceed
A cell that does not meet all the requirements will not be allowed to progress into the S phase.
- The cell can halt the cycle and attempt to remedy the problematic condition, or the cell can advance into G0 and await further signals when conditions improve.
A point at which the cell commits to the cell division process

The G2 Checkpoint: checks for cells size, protein reserves, and most importantly that all of the chromosomes have been replicated and that the replicated DNA isn’t damaged
Bars entry into the mitotic phase if certain conditions are not met
If the checkpoint mechanisms detect problems with the DNA, the cell cycle is halted, and the cell attempts to either complete DNA replication or repair the damaged DNA.

The M Checkpoint: determines whether all the sister chromatids are correctly attached to the spindle microtubules
Occurs near the end of the metaphase stage of karyokinesis
Because the separation of the sister chromatids during anaphase is an irreversible step, the cycle will not proceed until the kinetochores of each pair of sister chromatids are firmly anchored to at least two spindle fibers arising from opposite poles of the cell