# G1 Checkpoint --- The **G1 checkpoint** or sometimes called **G1/S checkpoint** is a moment during [[cell cycle regulation]] at the end of the [[cell cycle|G1 phase]]. This is where the cell stops and has several mechanisms to make sure everything is good to go before continuing to [[cell cycle#Phases of Interphase|S phase]]. ## Rb & E2F **Rb** or **retinoblastoma protein** is a [[tumor suppressor gene|tumor suppressor protein]] that plays a vital role in the regulation of the cell cycle. **E2F** is a transcription factor that needs to reach the [[nucleus|nucleus]] in order to kick off the [[DNA replication]]. However, if If E2F goes straight into the nucleus, that could cause some problems, because we're not sure everything is in order yet. This is where Rb comes in. Rb, binds to the E2F and prevents it from going anywhwere, which slows the process down enough so the cell can make sure everything is in order before it continues the process. This is where [[cell cycle regulation|cyclin]] and [[cell cycle regulation|Cdk]] come into play. If all systems are go, then Cdk can phosphyrlate Rb, resulting in Rb letting go of E2F. Once free, E2F is free to continue on into the nucleus and trigger [[cell cycle#Phases of Interphase|S phase]]. ![[G1 checkpoint.png]] ## P53 & P21 **P53** is also a [[tumor suppressor gene|tumor suppressor protein]] that acts in the G1/S checkpoint. It's the "guardian of the genome". It functions as a biological sensor, that talks to a lot of other proteins that monitor all sorts of conditions. If all is well, it just stays there inert. If it recieves the signal that something is wrong, it triggers the production of P21 **P21** is a potent Cdk inhibitor. When it is released it will inhibit Cdk, which prevents Cdk from allowing Rb to release E2F, and thereby entering the nucleus. It essentially grinds the wheels of motion in the cell cycle to a halt. ![[P53, P21.png]] ___