Which of the following is the normal cell cycle primarily regulated at checkpoints

The normal cell cycle is primarily regulated at checkpoints. These checkpoints ensure that each stage of the cell cycle is completed accurately before the next stage begins. There are three main checkpoints in the cell cycle:

1. G1 Checkpoint (Restriction Point): This checkpoint determines whether the conditions are favorable for the cell to proceed to the S phase and replicate its DNA. Factors such as cell size, nutrient availability, DNA damage, and growth factors are assessed here.

2. G2 Checkpoint: This checkpoint occurs at the end of the G2 phase, just before entry into mitosis (M phase). It verifies whether DNA replication in the S phase was completed accurately and checks for DNA damage.

3. M Checkpoint (Spindle Checkpoint): This checkpoint ensures that all the chromosomes are properly aligned at the metaphase plate before the onset of anaphase. It monitors the attachment of spindle fibers to the kinetochores of the chromosomes.

These checkpoints are crucial for maintaining genomic integrity and preventing the proliferation of damaged or abnormal cells, which could lead to diseases like cancer.