Now, the cell cycle is not the sort of thing that occurs in a very unchecked manner. There's actually a lot of regulation in play here. In fact, there are two key places that we have extensive regulation of the cell cycle. The first check point is right here between the G1 and the S phase. So, we regulate before we get to the point of DNA replication. The other major checkpoint is right here, between G2 and the step where we jump right to mitosis. And, there are a couple of proteins that regulate this process. Two main ones are called cyclin-dependent kinases, which as you may recall, a kinase is something that adds a phosphate group. So I'll put a plus in parentheses. It will plus a phosphate group. And it will add a phosphate group on other enzymes or proteins to either activate or inactivate them. These cyclin-dependent kinases will work together with a protein you might be able to guess the name of: cyclins! Right? Because what else would these kinases depend on? So an important thing to notice is that these cyclin-dependent kinases, or CDKs, are always present. All the different types are always present in a cell, but their default form, or their default function, is for them to be inactive. And so they need to be activated by these cyclin proteins. And the point of regulation here is that specific cyclins... I'll just write, "spec," are made at specific times. And again, the reason why they're both so important is that when you have a cyclin-dependent kinase, it is only active when it is bound to a specific cyclin. It's at this point, again, that this guy is active, and the CDK is the business-end of this complex. So that's the reason why in G1 you'll see the production of cyclins D and E. >From there you will see CDK-2 bound to your cyclin E, and at the same time you'll also have your CDK-4 bound to your cyclin D. These activated kinases, then, specifically the CDK-4 cyclin D complex, will phosphorylate a protein called, "RB." So I'll draw just a little reaction over here where we add a phosphate group on our RB protein. So when RB is phosphorylated, it can't inhibit DNA replication, like it usually is supposed to do. The phosphate group renders it inactive. And this is sort of the set up we have as we go further on in our cell cycle. In the S phase we have cyclin A produced. Cyclin A will complex, again, with CDK-2 most directly to activate DNA replication, so it helps to activate DNA replication and in a similar way we have cyclin B only produced in the G2 phase, because the cyclin B CDK-1 complex is able to activate what step, do you think? Mitosis, or cell division. So, it's important to recognize that in order to pass these checkpoints, you need to have these cyclin proteins present so that they can go ahead and inhibit proteins that are blocking DNA synthesis or replication from occurring, or so they can promote the production of proteins that are needed for mitosis.