There are other, higher levels of regulation that occur here
and I started to talk about them when I mentioned our
RB protein. So, control of the cell cycle
occurs on a higher level with a couple of
key proteins in addition to RB.
One of the main proteins we talk about that
regulate on a very high level is p53.
It's even got a nickname: it's the "guardian of the genome."
It's so important, actually, that Science Magazine called it,
"the molecule of the year" in 1993.
Theobromine, the main molecule that's in chocolate,
hasn't even gotten this honor yet. So it shows you
how important p53 is. It's more important than chocolate.
P53 will bind DNA directly to produce proteins that block
the progression of the cell cycle.
One of those proteins include p21.
P21 will function to inhibit CDK.
So the CDK will not be able to activate
DNA replication or activate mitosis.
RB is another protein that is associated
with the function of P53, and these proteins are considered
tumor suppressor genes, so RB is a protein that is
produced from a tumor suppressor gene, just like p53,
so I'll write that down here. These proteins are considered
tumor suppressor genes... They're made from
tumor suppressor genes I should rather say.
These proteins are made from tumor suppressor genes,
which are important to have because
if they are defected, or if they have a mutation in them
that makes them have loss of function,
so that's an important term,
(repeats) defected or if they have a loss of function,
what ends up happening is that you tend to get cancer.
Cancer, which I think you and I can both agree is not
a good thing to have. So, it is very important
to have tumor suppressor genes.
To illustrate how important that is, if you look at
p53, greater than 50% of tumors have a defect in p53.
RB got its name because a defect in RB would lead to a
tumor of the eye, known as retino-that's where you got the R-
retinoblastoma, which is why these two proteins are
considered tumor suppressor genes.
P21 is very unusual in that it doesn't actually lead to
cancer when it's defected. Instead,
scientists have found that mice that are without p21
have the ability to regenerate their limbs.
How weird is that?!
P21 lack causes the ability to regenerate their limbs,
their arms and legs.
So we're all still trying to figure out exactly
how some of these key proteins work.
But it's important to understand that
these tumor suppressor genes
are essential for making sure that we regulate
the cell cycle so we don't just go to the
divide, divide, divide phase.
We want to make sure that we only divide when it's appropriate,
when a cell is ready to, or if it's even a cell that
is supposed to divide in the first place.
Because, as we'll see in our next couple of videos,
mitosis, or cell division, requires a lot of energy
and protein machinery to happen.
So we need to make sure it's the right time to divide.