Final answer:
A cell lacking p21 protein will bypass the G₁ checkpoint and proceed directly to S phase of the cell cycle. This can lead to the accumulation of mutations and potentially tumor growth.
Step-by-step explanation:
A cell that lacks the p21 protein will experience a compromised G₁ checkpoint and proceed directly from G₁ to S phase of the cell cycle, regardless of internal and external conditions. The p21 protein, which is triggered by the p53 protein, acts by binding to and inhibiting the activity of the Cdk/cyclin complexes, preventing cell cycle progression. Without p21, the cell will not effectively halt the cell cycle in response to DNA damage, leading to the accumulation of mutations and potentially resulting in tumor growth.