! Protein kinases
- So the core molecules here are Cdks which are protein kinases & Cdks
stand for cyclin dependent kinase. These are protein kinases & they have
targets that control the cell cycle so they phosphorylate proteins that will
control different stages of the cell cycle.
- One key step is that another protein called a cyclin has to bind to these
kinases to turn them on, so this is one way to turn on these molecules to
pass through the checkpoint.
- Diagram; We’ve got the cyclin present here, here is the Cdk – you need
both of these components, and more as we’ll see in a moment, to activate
! At different stages of the cell cycle
- So to coordinate things in time during this cycle, there are different
cyclin-Cdk checkpoints at different stages of the cell cycle.
- So if we started off here at G1 right here, the cell is growing, things are
favourable, there are lots of resources available, if that’s the case the cell
will then signal to produce this S cyclin, the cyclin that will promote the
synthesis phase. This will then bind to the S-Cdk & this complex can then
phosphorylate machinery involved with DNA replication in the cell so the
cell can dedicate all of its machinery to replicating the DNA.
- Once that job is done, the cell wants to turn off that machinery since the
genome has been replicated, they don’t need all of those enzymes present
anymore to duplicate the DNA so now this cyclin is destroyed. So we have
a very specific cyclin-Cdk complex to promote a specific phase of the cell
cycle & then it’s destroyed after its job is done.
- Now once the cell then passes through S phase, it passes the G2 phase, it
grows again & then it monitors are there is enough resources still and has
the genome been fully replicated? So if both of those things are okay, then
it will produce a different cyclin called an M-cyclin which will interact with
the Cdk, producing a different cyclin-Cdk complex & this complex
promotes the machinery controlling mitosis which we’ll talk about in detail
- Once that is done, so this would be machinery like building the mitotic
spindles out of microtubules, once that’s job is done & the DNA has been
segregated into 2 cells, you don’t need that machinery anymore, so this
cyclin is destroyed, the activity of this cyclin-Cdk complex is turned off so
then this mitosis machinery is turned off.
- So this is a molecular cycle then where the cell can control whether it’s
activating DNA replication machinery or mitosis machinery depending on
which cyclins are available & these are monitoring the availability of
resources & whether the DNA has been duplicated & so on – these are the