The transition from Gap 1 phase (G1) to the Synthesis (S) phase in the cell cycle involves two important checkpoints. The first checkpoint assesses DNA for damage. p53, when active, is a transcription factor for p21, which inhibits G1 and S cyclin-dependent kinases (CDKs), leading to cell cycle arrest or apoptosis. To pass the checkpoint, Mdm2 adds a ubiquitin tag to p53, targeting it to the lysosome for degradation. The “Restriction Checkpoint” is perhaps the most vital checkpoint involved in the G1 to S phase transition. The two main proteins involved are the retinoblastoma protein (Rb) and E2F. The null state involves Rb sequestering E2F in an inhibitory manner. When favorable conditions for cell division are met, G1 CDKs hyperphosphorylate Rb, inhibiting its ability to bind E2F. E2F is a transcription factor, so when active, it is able to go into the nucleus and promote transcription of S-phase cyclins. At this point, the cell has committed to undergoing mitosis. Mutations in either of these checkpoint pathways are quite common in cancer. Common mutations leading to a cancerous phenotype include a loss of function in p53, or a gain of function in E2F.
Comments
suggestion
My suggestion would be to try to add better flow between the sentences. Although, this could be tricky to accomplsih for a specific process like the one you are describing, since it is usually presented with a figure of the cell cycle. So for someone who doesn't know what the cell cycle looks like you might want to add more descriptive sentences.
2nd and 3rd sentences
I would suggest some sort of transition between the second and third sentences. Although it is not the case someone without knowledge on this subject may not realize there is a connection between those 2 sentences.
suggestion
There are a couple of sentences that seemed pretty short maybe you could connect them to previous sentences so it doesn't sound like an abrupt end when reading it.