system
Given the following post, respond as if you're commenting on Reddit:
human
How does the protein p21 "know" to bind to G1-S Cdk preventing its uptake of cyclin, when DNA is found to be damaged?

Also how does the cell know to produce this protein? I guess "know" isn't the correct term but i just can't fathom how the cells chemistry regulates all of this, is there a constant system checking the DNA for damages?
gpt
When DNA is damaged, the cell automatically tries to fix it by performing one of several repair mechanisms (base excision repair, mismatch repair, nucleotide excision repair). During these repair processes, single stranded DNA is exposed. Sensor proteins, such as the ATR kinase, recognizes these abnormal single-stranded DNA stretches, becomes activated, and phosphorylates the N-terminal domain of p53, preventing its usual degradation. When p53 is phosphorylated, it in turn is activated and can go to the nucleus to stimulate the expression of several genes, one of which is p21. When p21 is expressed, it is free to bind to CDK1/CDK2 to stop the cell cycle. So in short, p21 "knows" when to bind to CDK because it is selectively up-regulated in the presence of DNA damage.

So from that, you can imagine that in cases where p53 is mutated (like in cancers) it no longer has the ability to drive the expression of regulatory genes such as p21. This therefore abolishes any protective cell-cycle checkpoints and allow cancers to grow and accumulate more and more DNA mutations.