Is the restriction point a checkpoint?

Yes, the restriction point is also known as the G1/S checkpoint in the cell cycle. The restriction point acts as a decision-making point that determines whether or not the cell will continue with cell division and enter the S phase, or exit the cell and enter the G0 phase. At the restriction point, the cell goes through a number of assessments and integrates internal and external cues before deciding to commit to division. Factors such as cell size, nutrient availability, growth factors, and DNA integrity are all assessed by the checkpoint to ensure a cell is ready to proceed. The cell must reach a certain size to ensure that it has enough cellular components and energy for a full division; it also must have enough nutrients necessary for cell growth. The presence or absence of growth factors are also assessed, as the presence of sufficient growth factors promotes favorable conditions for cell division. The absence of growth factors may delay or stop cell cycle progression. Lastly, DNA integrity is verified to ensure the DNA is not significantly damaged or mutations have occurred that may affect its ability to successfully replicate.