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.
