The checkpoint is the primary point of the cell cycle that is influenced by external signals.

The G1 or start check point is the primary point of cell cycle that is influenced byexternal signals

Explanation:

Cell cycle check point are processes in eukaryotic cells that allows adequate cell divisions. They serve as potential points along the cell.

There are three types of cell cycle check points and they are;

1.G1 check point also know as restriction or major or start check point.

2. G2 check point.

3. Metaphase or spindle check point. The G1 or start check point is the primary point of cell cycle that is influenced by external signals.