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Good morning, class. Nice
to be here with you again

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after a long hiatus. So,
we're going to talk today about

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the cell cycle. That is
to say the life of a cell,

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how a cell grows and
divides. And, as it turns out,

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this is one of our first
entrees into cell biology.

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That is to say we're move,
beginning to move away from the

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molecules inside cells and beginning
to look at a, at the next higher

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level of complexity which
is how cells proliferate.

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It turns out this is a very critical
issue in a number of different

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disease areas, including
notably cancer which we'll

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start talking about next time.
And this is also the transition

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point, the inflection point in the
semester when we begin to apply some

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of the things we've learned earlier
to learning about specific disease

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processes. So, you'll
sense more and more

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introduction of discussions of human
disease processes as the semester

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goes from here on to the
end. And here, in fact,

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is what we call the cell cycle.
And I'm going to get into the

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details of it quite shortly,
but the cell cycle is effectively

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the life cycle of a cell from the
moment it is born to the moment that

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it, in turn, becomes the
mother of two daughter cells.

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And this process is divided up,
as you can see, into four different

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periods here, or as they are called
phases. These are cell cycle phases.

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The first phase is
called M or mitosis.

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The second phase is called G1 phase.
The third is called the S or the

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synthetic phase, and that
refers to the fact that

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this is the time when
cells replicate their DNA.

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And, as you can see, it
actually takes a long time

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because this entire cycle in
cultured mammalian cells normally

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takes as much as a day for
a mammalian cell to divide,

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grow and divide. Contrast that,
by the way, with many bacterial

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cells which can grow and
divide in 30, 30 minutes.

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So, our cells take a lot
longer to divide, many of them,

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but there are exceptions to that.
After cells have gone through G,

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through S phase they go into G2.
And these two Gs are, are called Gs

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because of the gaps they have.
Here's the gap between S phase and

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M phase, or mitosis. G1 is
the gap between mitosis and,

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and the, the subsequent
S phase. And, obviously,

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the most important parts of the
cell cycle are the time when the cell

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divides, which is mitosis, and
the time it replicates its DNA

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which is in S phase. One
reason why it's so important is

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that one has to be sure that cells
that arise from the process of

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growth and division,
which is what this is,

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actually end up with a full genome's
worth of DNA and chromosomes.

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And, therefore, during S
phase there has to be a precise

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replication of all the 3. billion
bases in the haploid genome,

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or 6.4 billion in
the diploid genome.

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And during M phase there has to be
a precise allocation of the resulting

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replicated DNA in equal,
exactly equal parts to the two

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daughter cells. And
this is no mean feat.

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It is an extraordinarily
challenging process,

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which cells, it turns out, with
very high efficiency through a

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variety of complex control
mechanisms. And with this outline

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in mind, I just want to go into some
of its details to illustrate to you

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why we think that the scheme
is organized the way it is.

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The fact of the matter is that the,
that the M phase, or the mitosis is

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actually organized into a series
of sub phases. And you may remember

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that from high school biology but
I'll just go into it in some detail,

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not because we're going to dwell on
it but just because I want to give

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you a feeling for what it's composed
of. It's composed of a series of,

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of sub, sub phases I just mentioned.
And, importantly, we start out with

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an interphase cell. And
interphase refers to the

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entirety of the cell cycle
outside of M. That is the entirety,

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entirety outside of the M phase
thereby including G1S and G2.

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And you notice that what happens
in prophase is, first of all,

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the chromosomes condense. And
that's very important because prior

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to that condensation the chromatin,
and chromatin represents the

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material out of which the
chromosomes is assembled consisting

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largely of DNA and proteins with
a big of RNA, the chromatin is

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dispersed through the nucleus.
It's not condensed together.

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And as such, as you can see
here, one doesn't see any specific

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chromosomes because they're
just scattered about the whole

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chromosomes in a very dilute form.
But once the condensations happen,

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the condensation of chromosomes
happen then one begins to appreciate

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that there are chromosomes.
Indeed, if it were not for the case

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that this, this condensation would
happen, it would have taken a very

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long time before we even realized
at the beginning of the 20th century

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that there were distinct
chromosomes themselves.

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Because when you look in the
nucleus like this, and as you,

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one might do, be doing here
stains for DNA, one doesn't see any

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distinct structures within the
nucleus. Here this condensation

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causes a delineation of
these structures in humans,

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as you know this being 46.
And, by the way, this, this,

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this sequence of events is, is
basically standard hardware for all

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eukaryote cells. It's
not as if we invented this

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recently. This whole sequence of
events has largely been around for 1.

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billion years. One thing
you begin to notice here

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already as we go into the Metaphase,
this is the beginning of metaphase,

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it's sometimes called prometaphase
if one wants to split it up even

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further, is the beginning of
the disappearance of the nuclear

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membrane. So, here we are
in a situation where the

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chromosomes are no longer physically
separated from the cytoplasm by the

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nuclear membrane. The
membrane, in effect,

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disappears. It dissolves.
Obviously, at another point it's

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going to have to reassemble when
one starts making daughter cells.

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And at this point one begins
to see quite distinctly the,

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that, that each chromosome
is composed of two chromatids.

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A chromatid is each one of these
arms. And the fact that there are

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two of them is a direct consequence
of the previous DNA replication

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which made from one
chromatid two chromatids.

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So, these two chromatids are
identical, one to the other.

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They're the consequence
of the previous S phase of,

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the previous DNA replication.
And if all things have gone well

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then these two chromatids have
identical DNA sequences because

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they're the descendants of a
single chromatid that existed,

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actually a chromosome as
it's called at that point,

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existed at the beginning of S
phase. In fact, the fidelity of DNA

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replication, that is to say
the accuracy with which DNA is

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replicated is so good that often
one can demonstrate that the mistakes

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that are made are less than
one in ten to the ninth bases.

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That's stunning. One
in ten to the ninth,

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only one in ten to the ninth basis
actually is, ends up being miscopies.

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In fact, we now realize that the
reason why there's such enormous

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fidelity is that the DNA polymerases
that previously begin to replicate

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the DNA to generate these chromatids
make a mistake of about one in ten

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to the minus five bases. So,
they're much more error prone.

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But there are subsequent
proofreading mechanisms.

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Just the way you would copy
edit text, there are proofreading

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mechanism which erase those
errors and reduce the end result of

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miscopying, of misreplication down
in one, down to one and ten to the

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minus ninth. And given that
the genome itself is only,

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the haploid genome is only 3.
billion, that is 3.2 times ten to

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the ninth basis of DNA, that
means that there are only about,

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roughly speaking, three
mistakes made in the entire

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genome each time a cell goes through
a cell cycle in the haploid genome.

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Now, what we begin to notice here
in prometaphase is also the beginning

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of the assembly of the machinery
that will eventually divide those

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two chromatids from one another.
And here, in the midst of metaphase,

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as you see in the top left, now
the chromosomes line up in what

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is effectively a two-dimensional
plane in the middle.

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It's called an equatorial
plane in the middle of the cell.

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Where, as you can see here in
this schematic, each chrome,

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each chromosome is, here's
once again the two chromatids,

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and these chromid, the places
where these two chromatids are held

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together are lined up in this plane
with the arms of the chromosomes

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kind of flopping around,
as you can see here. What's

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critical here is the points
where these, these two chrome,

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chromatids are joint. And
those, those points of joining,

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as I'll mention in a moment,
those are called centromeres.

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Here, here is a more schematized
view of what's about to happen,

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just to give you a feeling for
what this really looks like.

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And here, as, as we begin to
move later into the mitosis,

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we begin to see that each of the
chromatids is joined here by a

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centromere and that there, and
that there are mitotic spindles

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which are reaching from these paired
chromosomes to centrioles at either

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side of the cell. So,
these two centrioles,

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and the whole apparatus
itself is called a centrosome,

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have microtubules that reach
from, from the centro, from the

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centromeres here in the chromosomes,
so the, so the structure in the

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middle of the chromosome
is called a centromere.

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Via microtubules that reach all the
way over to one pole or the other.

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So, you see, one effect
has two poles here. And if,

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and if we begin to develop this
further, we, we've just gotten into

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serious metaphase here. And
now what we see in anaphase is

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that the two chromatids
are being separated apart,

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they're being pulled apart
by these microtubule spindles.

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These are called spindle fibers,
which reach from each of those

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centrosomes over here back to
the chromosomes, and literally are

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involved in the mechanical pulling
them apart, one from the other.

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And to state the obvious, if we're
talking about the fidelity of DNA

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replication, here we need also to
talk about the fact that when one

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has a set of paired chromatids,
one of the chromatids has to go to

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one side and one to the other.
That, in itself, is in principle

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also an error-prone mechanism,
or an error-prone process.

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Mistakes could be made. And
indeed in cancer cells where

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many of these processes break down
sometimes one has both chromatids

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going to one or the other side.
And as you can intuit from that,

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that leads obviously to a disruption
of the normal makeup of the

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chromosomes that are eventually
allocated to each of the two

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daughter cells. In fact,
this allocation happens

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here at telophase, the
last of the four major sub

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phases of mitosis. Here you
see that what's happened is

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that the, these chromatids
have now been pulled apart.

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And the moment that they're pulled
apart and no longer paired up one

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with the other they now
are recognized, once again,

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to be card-carrying chromosomes.
So, now here, here you see the

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nomenclature has changed. Now
they're chromosomes. They're

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being pulled, pulled apart.
And, obviously, one wants an

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identical set of chromosomes
in both daughter cells.

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And once they're allocated to the
two future daughter cells the entire

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mitotic spindle dissolve.
And, as you can imagine,

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there is a reformation, as is
indicated here schematically and not

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very clearly, in the nuclear
membrane. So now the whole cell is

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reconstructed and now each of
the two daughter cells is able,

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in principle, to split off one from
the other and to go off on its merry

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way. And this looks like a
really neat process. And in some

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eukaryotic cells it happens much
more rapidly than the 24 hours that

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I just indicated to you. There
are indications that in some

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lymphocyte populations it
only takes three or four hours.

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And in some early embryonic cell
populations, where there's an

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absolutely frenetic pace
of cell growth and division,

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it may happen every 30 minutes
rather than the 24 hours I just

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talked about. And you'll say, so
what, that, who cares about that.

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But 30 minutes for replicating an
entire 6.4 billion bases of DNA and

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then allocating them to two
daughter cells is actually quite

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an achievement. And we
don't really understand how

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that happens. I've talked
implicitly about the process of cell

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growth and division. And
what I mean to say in more

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detail about that is the following.
After one gets these two initial

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daughter cells they're obviously
only half the size of the previously

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existing mother cell. And,
therefore, what must happen in

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the subsequent cell cycle is that
each of these daughter cells must

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actually physically grow in size
so that it once again becomes a big

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mama over here after
the next cell cycle.

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And so that growth begins
immediately. And much of it occurs

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just through the generation
of new sets of ribosomes,

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new proteins, new membranes
in the cell. All of the complex

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constituents of a cell need to be
duplicated for the next cell cycle

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including, obviously,
all of the organelles,

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the small structures in
the cytoplasm and including,

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indeed, the mitochondria. So,
imagine now the complexity of

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this because not only is the
eukaryotic cell so complex but its

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entire contents must be faithfully
replicated during the subsequent

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cell cycle. And that obviously
implies the intervention,

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the involvement of very complex
control mechanisms about which we

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understand almost nothing.
Now, how, let's get back to this

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for a moment, because what I've been
talking about over the last six to

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eight minutes is simply
mitosis, the M phase here.

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In fact, in many of these
early embryonic cell cycles,

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00:14:37,000 --> 00:14:41,000
what happens is that cells go from M
phase directly into S phase and from

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00:14:41,000 --> 00:14:45,000
S phase directly into M. In
other words, they don't indulge

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00:14:45,000 --> 00:14:49,000
themselves in a period over here,
a long period, and in some cultured

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00:14:49,000 --> 00:14:53,000
mammalian cells this can be
12 or 14 hours of getting set,

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00:14:53,000 --> 00:14:58,000
getting prepared
for DNA replication.

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00:14:58,000 --> 00:15:01,000
Similarly, after cells
have replicated their DNA,

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00:15:01,000 --> 00:15:05,000
and as we just said, for example,
converted one chromosome into two

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00:15:05,000 --> 00:15:09,000
paired chromatids, after
they get through S phase

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00:15:09,000 --> 00:15:13,000
they'll also wait another four or
five hours, most cultured mammalian

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00:15:13,000 --> 00:15:16,000
cells before they go back into
M phase, these two gap periods.

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00:15:16,000 --> 00:15:20,000
Well, how do we actually know
how long each of these gaps takes?

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00:15:20,000 --> 00:15:24,000
Well, as is usual, I'm
glad I asked that question.

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00:15:24,000 --> 00:15:28,000
And one way we can do
so is the following.

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00:15:28,000 --> 00:15:32,000
What we can do is we can take cells
and we can block them in mitosis.

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00:15:32,000 --> 00:15:37,000
How can we block them in mitosis?
We can use a microtubule antagonist.

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00:15:37,000 --> 00:15:42,000
So, I told you before that the
spindle fibers that pull the

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00:15:42,000 --> 00:15:47,000
chromosomes apart are made of
microtubules. Microtubules happen

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00:15:47,000 --> 00:15:52,000
to be one of the constituents of the
cytoskeleton that gives structure to

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00:15:52,000 --> 00:15:57,000
the cell, and these microtubules are
used as the fibers for pulling apart

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00:15:57,000 --> 00:16:02,000
the two chromatids during
anaphase, as I just mentioned.

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00:16:02,000 --> 00:16:06,000
And we can use a microtubule
antagonist. And in biochemistry,

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00:16:06,000 --> 00:16:10,000
when you want to antagonize
something often you use a symbol

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00:16:10,000 --> 00:16:15,000
like this. And, for
instance, here's a drug called

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00:16:15,000 --> 00:16:19,000
colcemid, and colcemid is
a microtubule antagonist.

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00:16:19,000 --> 00:16:23,000
And even though it has a bit of an
effect on the overall cytoskeleton,

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00:16:23,000 --> 00:16:28,000
if you put colcemid on cells then
they'll go into mitosis like this

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00:16:28,000 --> 00:16:32,000
and they'll go into metaphase,
as we indicated before, but the

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00:16:32,000 --> 00:16:37,000
subsequent sub phases of mitosis
will, will grind to a halt.

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00:16:37,000 --> 00:16:41,000
Why? Because the microtubule
spindle fibers will be unable to

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00:16:41,000 --> 00:16:46,000
assemble properly because of this,
now, I forget whether there's an E

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00:16:46,000 --> 00:16:50,000
here or not. Probably not.
OK. Typo. The colcemid prevents

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00:16:50,000 --> 00:16:55,000
the spindle fibers from forming,
so therefore cells get hung up, they

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00:16:55,000 --> 00:17:00,000
get blocked in M phase. In
fact, if you're interested in

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00:17:00,000 --> 00:17:04,000
looking at the, at the
array of chromosomes in a

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00:17:04,000 --> 00:17:08,000
cell, the simplest way of
doing it with a mammalian,

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00:17:08,000 --> 00:17:12,000
with a human cell is to put colcemid
in it, and the colcemid will block

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00:17:12,000 --> 00:17:16,000
them in, in M phase,
they'll block them in,

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00:17:16,000 --> 00:17:20,000
in metaphase, they can't get out of
metaphase simply because these pink

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00:17:20,000 --> 00:17:24,000
fibers here are unable to assemble
properly, and therefore they get

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00:17:24,000 --> 00:17:29,000
hung up right here while the
chromosomes are condensed.

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00:17:29,000 --> 00:17:33,000
And this allows you to visualize
the chromosomes under the microscope.

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00:17:33,000 --> 00:17:37,000
Why do need to do that? Well, in
the absence of blocking cells and

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00:17:37,000 --> 00:17:41,000
trapping them in, in
metaphase, cells will just spin

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00:17:41,000 --> 00:17:45,000
right around the cell cycle and
they'll only be in metaphase for

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00:17:45,000 --> 00:17:50,000
maybe only 10 or 15 minutes
out of the 24 hours. And,

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00:17:50,000 --> 00:17:54,000
therefore, in a population of cells
which is moving around the cell

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00:17:54,000 --> 00:17:58,000
cycle willy-nilly very rapidly,
only a minute fraction of the cells

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00:17:58,000 --> 00:18:02,000
at any one point in time will, by
chance, be in metaphase when you

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00:18:02,000 --> 00:18:07,000
want to study their chromosomes.
So, the best thing to do is to try

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00:18:07,000 --> 00:18:12,000
to block them in metaphase.
Now, what you can do, in fact,

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00:18:12,000 --> 00:18:16,000
is put cells in metaphase, and I'm
going to draw the cell cycle around

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00:18:16,000 --> 00:18:21,000
here again. Here's M, here's
S phase, slightly different

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00:18:21,000 --> 00:18:26,000
than what we just saw before,
here's G1, and here is G2. So,

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00:18:26,000 --> 00:18:31,000
now we're going to block cells with
colcemid right here in metaphase.

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00:18:31,000 --> 00:18:35,000
And now what happens is that
we put metaphase in. Ideally,

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00:18:35,000 --> 00:18:39,000
if we put it in for a day we'd start
out with a population of cells which

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00:18:39,000 --> 00:18:43,000
we will call asynchronous.
Asynchronous means that at any one

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00:18:43,000 --> 00:18:47,000
point in time these cells will be
distributed randomly around the cell

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00:18:47,000 --> 00:18:51,000
cycle. They'll be all
over the place, obviously,

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00:18:51,000 --> 00:18:55,000
because there's nothing to cause
them to move synchronously through

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00:18:55,000 --> 00:18:59,000
the cell cycle. A random
population of cells will be

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00:18:59,000 --> 00:19:03,000
scattered all over the cells cycle
like this. We put in colcemid,

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00:19:03,000 --> 00:19:07,000
and what's gradually going to happen
is that cells are moving around the

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00:19:07,000 --> 00:19:11,000
cell cycle and when they get
to here they'll start piling up.

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00:19:11,000 --> 00:19:15,000
In principle, if all the cells are
scattered around the cell cycle at

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00:19:15,000 --> 00:19:19,000
the moment you put in the colcemid,
and it takes them 24 hours to get

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00:19:19,000 --> 00:19:23,000
all the way around the cell cycle,
then after a day's worth of colcemid

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00:19:23,000 --> 00:19:27,000
treatment all the cells should
have piled up right here.

287
00:19:27,000 --> 00:19:31,000
In fact, you can begin to look.
What, what's the kinetics with which

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00:19:31,000 --> 00:19:35,000
cells pile up into the M phase?
And it looks like this. They go

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00:19:35,000 --> 00:19:40,000
around, they go on piling up, and
then at the end of 24 hours they

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00:19:40,000 --> 00:19:44,000
plateau. Why do they plateau at
24 hours? Because by 24 hours the

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00:19:44,000 --> 00:19:48,000
stragglers, which started out right
here, will have had time to go all

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00:19:48,000 --> 00:19:53,000
the way around the cell cycle
and get trapped in M phase.

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00:19:53,000 --> 00:19:57,000
And that gives you a feeling for
how long the entire cell cycle time

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00:19:57,000 --> 00:20:02,000
is, now, or this cycle which
I called growth and division.

295
00:20:02,000 --> 00:20:05,000
And, again, to emphasize,
growth is literally the physical

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00:20:05,000 --> 00:20:09,000
growth of the cell. Division
is the production of two

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00:20:09,000 --> 00:20:12,000
daughter cells at mitosis. Now,
in principle, we can relieve

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00:20:12,000 --> 00:20:16,000
this metaphase block by
getting rid of the colcemid.

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00:20:16,000 --> 00:20:19,000
And now, interestingly enough,
we have a population of cells that

300
00:20:19,000 --> 00:20:23,000
is emerging from
metaphase synchronously.

301
00:20:23,000 --> 00:20:27,000
What do I mean by that? I mean
to indicate that they're all

302
00:20:27,000 --> 00:20:30,000
moving in lockstep. They're
all moving at the same time

303
00:20:30,000 --> 00:20:34,000
out of the cell cycle, so
now there's a whole cohort of

304
00:20:34,000 --> 00:20:37,000
cells which starts out right here.
They've all left metaphase at the

305
00:20:37,000 --> 00:20:41,000
same time. This is not
an asynchronous culture.

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00:20:41,000 --> 00:20:45,000
And they start moving around,
advancing around the cell cycle

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00:20:45,000 --> 00:20:48,000
together. In principle you'd say,
well, they should remain synchronous

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00:20:48,000 --> 00:20:52,000
forever. But the fact is all of
these phases of the cell cycle,

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00:20:52,000 --> 00:20:55,000
the times are quasi-stochastic,
sometimes go, some cells go through

310
00:20:55,000 --> 00:20:59,000
G1 in ten hours, some
cells go through cells in

311
00:20:59,000 --> 00:21:03,000
eleven hours, some
through in eight hours.

312
00:21:03,000 --> 00:21:07,000
And, as a consequence, as
they move further and further

313
00:21:07,000 --> 00:21:11,000
around the cell cycle, the
population of cells becomes

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00:21:11,000 --> 00:21:15,000
progressively more asynchronous
and they won't really enter into the

315
00:21:15,000 --> 00:21:20,000
next M phase totally synchronously.
But now we can ask the following

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00:21:20,000 --> 00:21:24,000
question, how long is G1? And
the way we can do that is to do

317
00:21:24,000 --> 00:21:28,000
another kind of experiment where
what we do is we release cells,

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00:21:28,000 --> 00:21:32,000
a population of cells from M
phase by removing the colcemid,

319
00:21:32,000 --> 00:21:37,000
for example. And
now we treat cells,

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00:21:37,000 --> 00:21:41,000
we, we treat aliquots of this
synchronously advancing population.

321
00:21:41,000 --> 00:21:46,000
Each hour we pluck out some of
these cells and we expose them for,

322
00:21:46,000 --> 00:21:51,000
let's say, the next hour thereafter
to some tritiated thymidine.

323
00:21:51,000 --> 00:21:55,000
Tritiated thymidine, as you
may recall, H3 thymidine as it's

324
00:21:55,000 --> 00:22:00,000
sometimes denoted, is
obviously a radiolabeled

325
00:22:00,000 --> 00:22:05,000
precursor of DNA. And it's
going to get incorporated

326
00:22:05,000 --> 00:22:09,000
into the DNA and, and to
no other macromolecules in

327
00:22:09,000 --> 00:22:13,000
the cell. And so the question is if
we expose cells for an hour here to

328
00:22:13,000 --> 00:22:17,000
tritiated thymidine, how
much tritiated thymidine is

329
00:22:17,000 --> 00:22:21,000
going to be incorporated into
their DNA? How do we know what's

330
00:22:21,000 --> 00:22:25,000
incorporated and how much tritiated
thymidine remains unincorporated?

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00:22:25,000 --> 00:22:30,000
We extract the DNA from the cells
and we precipitate it in an acid.

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00:22:30,000 --> 00:22:33,000
And when that happens the
macromolecules of DNA go to the

333
00:22:33,000 --> 00:22:37,000
bottom, they precipitate, and
the unincorporated tritiated

334
00:22:37,000 --> 00:22:41,000
thymidine, which is still soluble,
it has not yet been polymerized into

335
00:22:41,000 --> 00:22:45,000
DNA molecules, remains
in the acid solution.

336
00:22:45,000 --> 00:22:49,000
So, we asked how much
acid-precipitable thymidine counts

337
00:22:49,000 --> 00:22:53,000
are there here? How
many in the next,

338
00:22:53,000 --> 00:22:57,000
are made in the next hour
with another aliquot of cells?

339
00:22:57,000 --> 00:23:01,000
How many in the next hour and
so forth? And when we do such an

340
00:23:01,000 --> 00:23:05,000
experiment we find that we get
a, a curve which looks like this.

341
00:23:05,000 --> 00:23:08,000
All of a sudden, here
we got no sales labeling,

342
00:23:08,000 --> 00:23:12,000
in the second hour none, third,
fourth, fifth, sixth, in the seventh

343
00:23:12,000 --> 00:23:16,000
hour there are some cells which make
DNA, in the eighth hour there are

344
00:23:16,000 --> 00:23:19,000
some cells, even more cells making
DNA, and by the time they're in the

345
00:23:19,000 --> 00:23:23,000
ninth or the tenth hour then we
find a high rate of DNA synthesis.

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00:23:23,000 --> 00:23:27,000
Now, in principle we could continue
this experiment if we wanted to,

347
00:23:27,000 --> 00:23:31,000
if we had enough money for tritiated
thymidine, which one does because

348
00:23:31,000 --> 00:23:34,000
it's very cheap. And what
would find is if we started

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00:23:34,000 --> 00:23:38,000
labeling over here with, for
another hour pulse, when I say

350
00:23:38,000 --> 00:23:41,000
an hour pulse I mean we just put
the tritiated thymidine in at the

351
00:23:41,000 --> 00:23:44,000
beginning of this one-hour
period way over here,

352
00:23:44,000 --> 00:23:48,000
and an hour later we take out the
cells and extract their DNA and

353
00:23:48,000 --> 00:23:51,000
measure how much DNA has been
incorporated in that interval.

354
00:23:51,000 --> 00:23:54,000
And if we did a tritiated
thymidine pulse over here,

355
00:23:54,000 --> 00:23:58,000
we'd begin to see that the rate of
DNA synthesis was actually lower,

356
00:23:58,000 --> 00:24:01,000
and eventually it would go down,
back down to this as the cells were

357
00:24:01,000 --> 00:24:05,000
moving asynchronously
through the culture.

358
00:24:05,000 --> 00:24:09,000
What does that mean?
Well, this is the time,

359
00:24:09,000 --> 00:24:13,000
obviously these cells are in S
phase, these cells were in G1,

360
00:24:13,000 --> 00:24:18,000
and these cells have now emerged
from S phase into G2 and are no

361
00:24:18,000 --> 00:24:22,000
longer making DNA. And on
that basis we can actually

362
00:24:22,000 --> 00:24:27,000
calculate, we can determine how
many hours it takes for cells to move

363
00:24:27,000 --> 00:24:31,000
through G1. We can do a similar
kind of experiment to figure out how

364
00:24:31,000 --> 00:24:36,000
long G2 is, the gap 2, G2 phase is.
G2, recall, is the time between the

365
00:24:36,000 --> 00:24:41,000
ending of DNA replication
and the beginning of mitosis.

366
00:24:41,000 --> 00:24:46,000
And how do we do that? Well,
we can do an inhibitor of DNA

367
00:24:46,000 --> 00:24:52,000
synthesis. So,
let's say, here is,

368
00:24:52,000 --> 00:24:57,000
I'm going to redraw the cell cycle.
Here's S phase, G2, I'm drawing it

369
00:24:57,000 --> 00:25:02,000
again, here's M, and here's G1.
And now what we can do is we'll add

370
00:25:02,000 --> 00:25:06,000
a DNA synthesis inhibitor. An
inhibitor of DNA synthesis has

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00:25:06,000 --> 00:25:10,000
no effect, as you could imagine,
on M phase. This is one of those

372
00:25:10,000 --> 00:25:14,000
things, it's called hydroxyurea.
It actually blocks the biosynthesis

373
00:25:14,000 --> 00:25:19,000
of the precursors, the,
the oxyribonucleoside

374
00:25:19,000 --> 00:25:23,000
triphosphate precursors of DNA.
And, therefore, you add hydroxyurea

375
00:25:23,000 --> 00:25:27,000
to cells and they, their,
their DNA replication grinds

376
00:25:27,000 --> 00:25:31,000
to a halt. OK.
So, let's do that.

377
00:25:31,000 --> 00:25:35,000
And we'll, we'll, we'll add
hydroxyurea to the cells for 24

378
00:25:35,000 --> 00:25:39,000
hours. What's going to happen?
What will the distribution of cells

379
00:25:39,000 --> 00:25:42,000
be afterwards?
Well, in fact,

380
00:25:42,000 --> 00:25:46,000
some of the cells, when we
first added the hydroxyurea

381
00:25:46,000 --> 00:25:49,000
the cells were scattered all around,
they were asynchronous. We'll add

382
00:25:49,000 --> 00:25:53,000
the hydroxyurea and what
we'll find is the following.

383
00:25:53,000 --> 00:25:57,000
The cells that were in the middle
of S phase when we added the

384
00:25:57,000 --> 00:26:00,000
hydroxyurea will be stuck dead in
the water. They won't be able to

385
00:26:00,000 --> 00:26:04,000
move anymore. So, these
cells that were in S phase,

386
00:26:04,000 --> 00:26:08,000
the moment we add the hydroxyurea
will be trapped right here.

387
00:26:08,000 --> 00:26:12,000
They cannot make anymore DNA.
And, therefore, they'll be frozen

388
00:26:12,000 --> 00:26:16,000
at many points, times,
points in time in S phase.

389
00:26:16,000 --> 00:26:20,000
The cells that are outside of S
phase, they can continue to advance

390
00:26:20,000 --> 00:26:24,000
all the way around the cell cycle.
And, accordingly, there'll be lots

391
00:26:24,000 --> 00:26:28,000
of cells over the next 24 hours
that are going to just pile up at the

392
00:26:28,000 --> 00:26:32,000
G1-S transition. Why? Well,
the hydroxyurea has no effect

393
00:26:32,000 --> 00:26:36,000
at all on these other points of
the cell cycle. And when these cells

394
00:26:36,000 --> 00:26:41,000
try to go, go from G1 into S, they
cannot get into S phase because

395
00:26:41,000 --> 00:26:46,000
they can't make any DNA so
they're trapped right over there.

396
00:26:46,000 --> 00:26:50,000
And now we'll take away
the hydroxyurea, which is,

397
00:26:50,000 --> 00:26:55,000
obviously is, to use the notation
I used before, an inhibitor of DNA

398
00:26:55,000 --> 00:27:00,000
synthesis. And what we can
do now is the following.

399
00:27:00,000 --> 00:27:04,000
We're going to ask, after
we take out the hydroxyurea

400
00:27:04,000 --> 00:27:08,000
we're going to put in colcemid.
Why do we want to do that? Well,

401
00:27:08,000 --> 00:27:12,000
what we really want to do is to ask,
after the cells have reached, have

402
00:27:12,000 --> 00:27:16,000
gone through S phase, how
soon does a labeled cell move

403
00:27:16,000 --> 00:27:20,000
from S phase all the way into
M phase? How long does it take?

404
00:27:20,000 --> 00:27:24,000
How long is G2? And so what
we'll do is the following.

405
00:27:24,000 --> 00:27:28,000
We take away the hydroxyurea.
This allows all these cells to

406
00:27:28,000 --> 00:27:32,000
begin to move out. Is this
a fully synchronous culture?

407
00:27:32,000 --> 00:27:36,000
Well, actually, no, because some
of these cells are over here.

408
00:27:36,000 --> 00:27:39,000
There's a whole bunch over here.
These are synchronous, but the rest

409
00:27:39,000 --> 00:27:42,000
of the ones they're
already scattered out,

410
00:27:42,000 --> 00:27:46,000
so there's going to be some pioneers
over here moving ahead of the

411
00:27:46,000 --> 00:27:49,000
phalanx, and they'll be some
straggles and then there's going to

412
00:27:49,000 --> 00:27:53,000
be a big slug of these cells that
are moving as the rear guard ahead.

413
00:27:53,000 --> 00:27:56,000
Now, what we're going to do, after
we take away the hydroxyurea we're

414
00:27:56,000 --> 00:28:00,000
going to add our
old friend colcemid.

415
00:28:00,000 --> 00:28:03,000
And colcemid, I tried to
spell it right this time,

416
00:28:03,000 --> 00:28:06,000
colcemid is going to block cells,
as we obviously said before, right

417
00:28:06,000 --> 00:28:09,000
over here. And then what we're
going to do is the following.

418
00:28:09,000 --> 00:28:12,000
We're going to look, every hour
we're going to take some cells and

419
00:28:12,000 --> 00:28:16,000
put them in colcemid, or we'll
leave them in colcemid the

420
00:28:16,000 --> 00:28:19,000
whole time and every hour we'll
take cells out of the Petri dish,

421
00:28:19,000 --> 00:28:22,000
which have been in colcemid since
they were released from here,

422
00:28:22,000 --> 00:28:25,000
and we're going to look
at the metaphase cells.

423
00:28:25,000 --> 00:28:29,000
And how do we look at
the metaphase cells?

424
00:28:29,000 --> 00:28:33,000
Well, here's a metaphase cell.
Here will be its chromosomes.

425
00:28:33,000 --> 00:28:37,000
Obviously, they're chromatids
at this point. They're under the

426
00:28:37,000 --> 00:28:41,000
microscope. And they
start accumulating up here.

427
00:28:41,000 --> 00:28:45,000
And each and every hour we're going
to take some of these metaphase

428
00:28:45,000 --> 00:28:49,000
cells, we'll take them out of the
Petri dish, and after we've put them

429
00:28:49,000 --> 00:28:53,000
in, or we'll leave them in
the Petri dish if you want,

430
00:28:53,000 --> 00:28:57,000
and after they're there and they've,
they've come over here we'll fix

431
00:28:57,000 --> 00:29:02,000
them onto the plate. We'll
add some alcohol or something

432
00:29:02,000 --> 00:29:06,000
which causes them to stick to
the plate, they can't swim away,

433
00:29:06,000 --> 00:29:11,000
and then after that we'll
add some radioactive emulsion.

434
00:29:11,000 --> 00:29:15,000
So, here's, let's imagine here is
a cell at the bottom of the plate in

435
00:29:15,000 --> 00:29:20,000
metaphase, we're going to add some
photographic emulsion on top of that

436
00:29:20,000 --> 00:29:24,000
like that. And then what we're
going to see is how soon we can

437
00:29:24,000 --> 00:29:29,000
detect radioactive metaphase
chromosomes. How can we do that?

438
00:29:29,000 --> 00:29:33,000
Because each time an electron
leaves the tritiated thymidine,

439
00:29:33,000 --> 00:29:38,000
each time a beta particle leaves
it's going to cause a grain of

440
00:29:38,000 --> 00:29:42,000
emulsion, of silver to form in the,
in the photographic emulsion that

441
00:29:42,000 --> 00:29:47,000
we've layered above the cell. So,
this photo emulsion is only not

442
00:29:47,000 --> 00:29:53,000
a way of detecting light but
a way of detecting when there,

443
00:29:53,000 --> 00:29:58,000
whenever there's radioactivity
in, that's being emitted. And what

444
00:29:58,000 --> 00:30:04,000
we're going to look for are grains,
silver grains that are located above

445
00:30:04,000 --> 00:30:08,000
the chromosomes in the microscope.
We could look down through this

446
00:30:08,000 --> 00:30:12,000
plate, through the emulsion, on
the chromosomes we can see them

447
00:30:12,000 --> 00:30:16,000
clearly. I've shown you that before,
and we're going to ask ourselves the

448
00:30:16,000 --> 00:30:20,000
question, when can we begin to
associate silver grains with the

449
00:30:20,000 --> 00:30:24,000
chromosomes? Because those silver
grains must have been incorporated

450
00:30:24,000 --> 00:30:28,000
into the chromosomes
during the previous S phase,

451
00:30:28,000 --> 00:30:32,000
that is when cells were over here.
If a cell was over here and we added

452
00:30:32,000 --> 00:30:36,000
colcemid and there won't be any,
it won't be radioactively labeled.

453
00:30:36,000 --> 00:30:40,000
So, what we're really going
to ask now is the following.

454
00:30:40,000 --> 00:30:44,000
These cells are all
advancing through like this,

455
00:30:44,000 --> 00:30:48,000
they're advancing into M phase,
when does the first radiolabeled

456
00:30:48,000 --> 00:30:53,000
cell get, after we release them
with hydroxyurea, when does it get,

457
00:30:53,000 --> 00:30:57,000
when do we first see cells like this?
And the fact is after five or six

458
00:30:57,000 --> 00:31:01,000
hours after we've released them from
S phase then we begin to see cells

459
00:31:01,000 --> 00:31:05,000
like this which have chromosomes
on which there is radio,

460
00:31:05,000 --> 00:31:10,000
with which there's
radioactivity associated.

461
00:31:10,000 --> 00:31:15,000
Well, keep in mind that when cells
move through the S phase here,

462
00:31:15,000 --> 00:31:20,000
this is when they incorporated
tritiated thymidine to their DNA.

463
00:31:20,000 --> 00:31:25,000
Cells can't incorporate
tritiated thymidine here.

464
00:31:25,000 --> 00:31:30,000
They can't incorporate it here.
They can't incorporate it here.

465
00:31:30,000 --> 00:31:33,000
OK? Right. So, we can
allow cells to incorporate a

466
00:31:33,000 --> 00:31:37,000
little bit of tritiated thymidine,
we can freeze them in here, we'll

467
00:31:37,000 --> 00:31:40,000
allow, give them a little bit,
bit of tritiated thymidine, let it

468
00:31:40,000 --> 00:31:44,000
get into the DNA here, and
then we'll add hydroxyurea very

469
00:31:44,000 --> 00:31:47,000
shortly thereafter and freeze
the cells right like this.

470
00:31:47,000 --> 00:31:51,000
So, they'll be, they'll be
radioactive by virtue of having

471
00:31:51,000 --> 00:31:55,000
dwelled here at different times
in the S phase. And when we add

472
00:31:55,000 --> 00:31:58,000
hydroxyurea then, the
cells that were already in S

473
00:31:58,000 --> 00:32:02,000
phase and incorporated a bit
of tritiated thymidine will

474
00:32:02,000 --> 00:32:05,000
remain in S phase. All of
the other cells which were,

475
00:32:05,000 --> 00:32:08,000
when we added the hydroxyurea
over here, they will go all the way

476
00:32:08,000 --> 00:32:11,000
around here. They will move all
the way around the cell cycle.

477
00:32:11,000 --> 00:32:14,000
They might have happened to
incorporated a little bit of

478
00:32:14,000 --> 00:32:17,000
tritiated thymidine before
we added the hydroxyurea,

479
00:32:17,000 --> 00:32:21,000
but they'll get trapped over here.
So the only, so the cells that have

480
00:32:21,000 --> 00:32:24,000
tritiated thymidine, they
will be scattered at various

481
00:32:24,000 --> 00:32:27,000
points in the S phase. We'll
remove the hydroxyurea that

482
00:32:27,000 --> 00:32:30,000
allows the cells to escape
from S phase and move all the

483
00:32:30,000 --> 00:32:34,000
way around to here. And then
we begin to look for seeing

484
00:32:34,000 --> 00:32:40,000
when cells get radiolabeled
chromosomes. Is that clearer now?

485
00:32:40,000 --> 00:32:45,000
A little clear. OK. Now, let's
begin to ask how this is all

486
00:32:45,000 --> 00:32:51,000
coordinated. What determines how
cells, when cells will grow and when

487
00:32:51,000 --> 00:32:57,000
cells with not grow? So,
let's go back to our depiction

488
00:32:57,000 --> 00:33:03,000
of the cell cycle. How
do, what controls all this?

489
00:33:03,000 --> 00:33:09,000
What determines when a cell is
going to move around the cell cycle?

490
00:33:09,000 --> 00:33:15,000
In our bodies we have roughly three
times ten to the thirteenth cells.

491
00:33:15,000 --> 00:33:21,000
I don't know if that's a number
I mentioned to you before.

492
00:33:21,000 --> 00:33:27,000
It's a pretty interesting
number. It's probably not the most

493
00:33:27,000 --> 00:33:32,000
interesting number. By
now far more people are

494
00:33:32,000 --> 00:33:36,000
interested in the number of how many
games the Red Sox have won or lost,

495
00:33:36,000 --> 00:33:41,000
but it's a number anyhow. How many
cell divisions do we go through in a

496
00:33:41,000 --> 00:33:46,000
lifetime? Did we ever talk about
that? How many times do we have a

497
00:33:46,000 --> 00:33:50,000
cell growth and division
cycle in a human lifetime,

498
00:33:50,000 --> 00:33:55,000
lifetime? And the answer is in
human lifetime there are about ten

499
00:33:55,000 --> 00:34:00,000
to the sixth cell divisions.
It's a staggering number.

500
00:34:00,000 --> 00:34:03,000
Ten to the sixteenth in one
human body in a human lifetime.

501
00:34:03,000 --> 00:34:07,000
And if you figure out what that
amounts to, I think it's something

502
00:34:07,000 --> 00:34:11,000
like, and you can figure it out
yourself, I think it amounts to

503
00:34:11,000 --> 00:34:15,000
something like ten to the seventh
cell divisions in each second.

504
00:34:15,000 --> 00:34:18,000
So, each time I'm talking with you
I'm going, I've already gone through

505
00:34:18,000 --> 00:34:22,000
any one of my sentences.
And by the time my longwinded

506
00:34:22,000 --> 00:34:26,000
sentence is finished, I've
probably already had 100

507
00:34:26,000 --> 00:34:30,000
million mitoses happening
inside of me. That's --

508
00:34:30,000 --> 00:34:33,000
Maybe even more because some
of my sentences are really long.

509
00:34:33,000 --> 00:34:37,000
So, imagine that, ten to
the seventh for a second.

510
00:34:37,000 --> 00:34:40,000
You can figure out the number.
This is a lifetime of let's say you

511
00:34:40,000 --> 00:34:44,000
live 70 years, you can
do the math of how many it

512
00:34:44,000 --> 00:34:48,000
is per second. A lot
of that's going on,

513
00:34:48,000 --> 00:34:51,000
by the way, in the bone marrow
and in the gut, because in the bone

514
00:34:51,000 --> 00:34:55,000
marrow you're constantly replacing
a lot of your red blood cells.

515
00:34:55,000 --> 00:34:59,000
You have lots of them, but
they only last for 120 days.

516
00:34:59,000 --> 00:35:02,000
So, after 120 days a red blood cell
is broken down and you get a new red

517
00:35:02,000 --> 00:35:06,000
blood cell in its place. But
obviously you have more than

518
00:35:06,000 --> 00:35:10,000
one blood cell in your body. The
same thing happens in the gut.

519
00:35:10,000 --> 00:35:14,000
The cells lining the gut, those
are called epithelial cells,

520
00:35:14,000 --> 00:35:18,000
they're lining the surface of the
gut, and they are constantly being

521
00:35:18,000 --> 00:35:22,000
sloughed off the wall of the gut.
Why? Because it's not so nice to

522
00:35:22,000 --> 00:35:26,000
live on the wall of the
gut. It's not a very pleasant

523
00:35:26,000 --> 00:35:29,000
environment. And the
cells are therefore

524
00:35:29,000 --> 00:35:33,000
constantly gotten, gotten
rid of because they have a

525
00:35:33,000 --> 00:35:37,000
hard time surviving for
extended periods of time.

526
00:35:37,000 --> 00:35:41,000
Those cells live only three or
four days on the wall of the gut.

527
00:35:41,000 --> 00:35:44,000
So, when you go to the bathroom,
I don't mean to be too graphic, but

528
00:35:44,000 --> 00:35:48,000
number two, a significant proportion
of what comes out the bottom is

529
00:35:48,000 --> 00:35:52,000
actually cells that have been
sloughed off from the epithelial

530
00:35:52,000 --> 00:35:56,000
lining of the gut. I don't
know what the proportion is.

531
00:35:56,000 --> 00:36:00,000
It's something like 20% or 30%.
About half of what comes out is

532
00:36:00,000 --> 00:36:04,000
actually bacterial in the,
in the feces. We're not having

533
00:36:04,000 --> 00:36:08,000
lunch now so you can,
you can absorb all this,

534
00:36:08,000 --> 00:36:12,000
right? About half of it.
Almost none of the bulk of the

535
00:36:12,000 --> 00:36:16,000
feces actually ends up being what
you ate because most of what you ate

536
00:36:16,000 --> 00:36:20,000
ended up, ends up getting compressed
to a very small amount of solid

537
00:36:20,000 --> 00:36:24,000
matter. So, most of what
comes out is, is not what,

538
00:36:24,000 --> 00:36:28,000
what came in, is not the
processing of food but other things,

539
00:36:28,000 --> 00:36:32,000
as I've just mentioned.
And, by the way,

540
00:36:32,000 --> 00:36:37,000
here's another unsettling statistic.
It's not so graphic but it's

541
00:36:37,000 --> 00:36:42,000
unsettling if you think about it.
There are more bacterial cells in

542
00:36:42,000 --> 00:36:46,000
your gut than in the rest, than
there are mammalian eukaryotic

543
00:36:46,000 --> 00:36:51,000
cells in the rest of your body.
They're taking over. There are

544
00:36:51,000 --> 00:36:56,000
more of them than there are
us in each one of our bodies.

545
00:36:56,000 --> 00:37:01,000
Go figure. Anyhow. Why
am I going on this long

546
00:37:01,000 --> 00:37:05,000
excursion? Well, only he
knows and he's not telling.

547
00:37:05,000 --> 00:37:09,000
But there was a reason behind it.
The reason why I'm going into this

548
00:37:09,000 --> 00:37:13,000
is to impress on you the fact
that it's really important that the

549
00:37:13,000 --> 00:37:17,000
advance of cells through their
growth and division cycle that we've

550
00:37:17,000 --> 00:37:21,000
just talked about here is
very carefully controlled,

551
00:37:21,000 --> 00:37:25,000
because each one of these growth,
cell growth and division cycles is

552
00:37:25,000 --> 00:37:30,000
the opportunity for disaster.
What kind of disaster?

553
00:37:30,000 --> 00:37:33,000
Well, if the cell makes a mistake
in its chromosomes it might die.

554
00:37:33,000 --> 00:37:37,000
The death of a cell in
our body is not a disaster.

555
00:37:37,000 --> 00:37:40,000
I just indicated to you that we're,
we're making ten to the seventh new

556
00:37:40,000 --> 00:37:44,000
cells every second, but
hopefully we're making exactly

557
00:37:44,000 --> 00:37:48,000
the same number of cells that die.
So, ten to the seventh cells are

558
00:37:48,000 --> 00:37:51,000
made for every ten to the
seventh cells that die.

559
00:37:51,000 --> 00:37:55,000
If you have an excess of new cells
compared with dying cells you're in

560
00:37:55,000 --> 00:37:59,000
bad shape because that's
really the disease of cancer.

561
00:37:59,000 --> 00:38:02,000
And that implies, even
with our knowing any,

562
00:38:02,000 --> 00:38:05,000
without our knowing anything else
about cancer, that there have to be

563
00:38:05,000 --> 00:38:09,000
very careful controls over the
decision on the part of a cell to

564
00:38:09,000 --> 00:38:12,000
whether, as to whether it can grow
or divide, as to whether or not it

565
00:38:12,000 --> 00:38:16,000
should grow and divide.
Now, what does that mean?

566
00:38:16,000 --> 00:38:19,000
Well, part of the decisions come
from the following situation.

567
00:38:19,000 --> 00:38:22,000
Let's imagine here we're growing,
we're talking about a human tissue.

568
00:38:22,000 --> 00:38:26,000
Each one of these things is a cell.
And, as usual, my art is not that

569
00:38:26,000 --> 00:38:30,000
splendid. But one of
the things I want to

570
00:38:30,000 --> 00:38:34,000
impress on you is that the body
cannot give license to this cell or

571
00:38:34,000 --> 00:38:38,000
to this cell or to this cell to
make the decision about growing and

572
00:38:38,000 --> 00:38:42,000
dividing on its own. A cell
cannot go and say I think I

573
00:38:42,000 --> 00:38:46,000
feel like dividing,
I'm going to go do it.

574
00:38:46,000 --> 00:38:50,000
That's a no-no. Why is it a no-no?
Because the moment one grants

575
00:38:50,000 --> 00:38:54,000
autonomy to such a cell one's
in a very dangerous situation.

576
00:38:54,000 --> 00:38:58,000
Instead, what has to happen, what
must happen is that cells talk

577
00:38:58,000 --> 00:39:03,000
to one another and
cells read a consensus.

578
00:39:03,000 --> 00:39:06,000
For instance, maybe there's a cell
that's missing right over here in

579
00:39:06,000 --> 00:39:10,000
this tissue. And keep in mind an
architecturally complex tissue has a

580
00:39:10,000 --> 00:39:13,000
precise number of cells in it
organized in a very precise pattern.

581
00:39:13,000 --> 00:39:17,000
And the way that pattern is
maintained is that the cells are

582
00:39:17,000 --> 00:39:21,000
constantly looking around and
seeing is there a gap here?

583
00:39:21,000 --> 00:39:24,000
Do we need to make a new cell or
not? It's not as if this cell says,

584
00:39:24,000 --> 00:39:28,000
well, I'm happy here and all
my neighbors are happy but,

585
00:39:28,000 --> 00:39:32,000
still, I'm going to grow and
divide because it seems like

586
00:39:32,000 --> 00:39:36,000
a nice thing to do. Again,
that's an invitation for

587
00:39:36,000 --> 00:39:40,000
disaster. And I'm saying that if
only to impress upon you the fact

588
00:39:40,000 --> 00:39:44,000
that cells only will make the
decision to grow and divide and to

589
00:39:44,000 --> 00:39:48,000
go into this cell cycle on the
basis of consultations with their

590
00:39:48,000 --> 00:39:52,000
neighbors. They're constantly
talking to one another.

591
00:39:52,000 --> 00:39:56,000
The signals, the intercellular
signals within a living tissue

592
00:39:56,000 --> 00:40:00,000
represent constant
ceaseless chatter.

593
00:40:00,000 --> 00:40:03,000
It's like everybody is on a cell
phone talking with six or eight or

594
00:40:03,000 --> 00:40:07,000
ten of his or her neighbors all
the time. And if you could hear it,

595
00:40:07,000 --> 00:40:11,000
it would be a real din. So, that
raised the question how do cells

596
00:40:11,000 --> 00:40:15,000
talk with one another? What
messages do they exchange,

597
00:40:15,000 --> 00:40:18,000
one with the other, so that they can
establish some kind of consensus as

598
00:40:18,000 --> 00:40:22,000
to whether or not it's inappropriate
for one of their number to grow and

599
00:40:22,000 --> 00:40:26,000
divide? All right. This
is an unauthorized ad.

600
00:40:26,000 --> 00:40:30,000
You can interpret
it the way you want.

601
00:40:30,000 --> 00:40:34,000
Anyhow. So how do they do that?
Well, what they do, one of the most

602
00:40:34,000 --> 00:40:38,000
important ways they communicate
is they exchange proteins with one

603
00:40:38,000 --> 00:40:42,000
another called growth factors.
And a growth factor goes from one

604
00:40:42,000 --> 00:40:46,000
cell, here's a cell, it
releases a growth factor and it

605
00:40:46,000 --> 00:40:51,000
goes to a second cell and induces
the second cell to begin to divide.

606
00:40:51,000 --> 00:40:55,000
That's a mechanism of transferring
the signal or conveying the signal.

607
00:40:55,000 --> 00:40:59,000
And a growth factor itself is a
relatively low molecular weight

608
00:40:59,000 --> 00:41:03,000
protein which travels
through interstellular,

609
00:41:03,000 --> 00:41:08,000
interstellar, intercellular space.
It moves from one cell to the other.

610
00:41:08,000 --> 00:41:12,000
So, it's secreted by one cell, it
moves here, and then it goes over

611
00:41:12,000 --> 00:41:16,000
here to a second cell. In
fact, if we want to blow up the

612
00:41:16,000 --> 00:41:20,000
second cell, the second cell has on
its surface a specific protein which

613
00:41:20,000 --> 00:41:25,000
is known as a receptor or, in
this case, we can say a growth

614
00:41:25,000 --> 00:41:29,000
factor receptor, which
represents a means by which

615
00:41:29,000 --> 00:41:33,000
that cell senses the presence of a
growth factor in the extracellular

616
00:41:33,000 --> 00:41:38,000
space. And, as is
suggested in this very

617
00:41:38,000 --> 00:41:43,000
crude and poorly drawn cartoon,
this receptor is a transmembrane

618
00:41:43,000 --> 00:41:48,000
protein. It has an extra cellular
domain, it has an intracellular

619
00:41:48,000 --> 00:41:54,000
domain, and it moves through,
it protrudes through the lipid

620
00:41:54,000 --> 00:41:59,000
bilayer of the plasma membrane.
We talked very briefly about these

621
00:41:59,000 --> 00:42:04,000
at the beginning of the semester.
And what happens is these receptors

622
00:42:04,000 --> 00:42:08,000
are so configured that in the event
that a growth factor comes over and

623
00:42:08,000 --> 00:42:13,000
is sent off by a neighboring cell,
the growth factor, I'll draw it here

624
00:42:13,000 --> 00:42:18,000
as a, as a square, this
growth factor binds to the

625
00:42:18,000 --> 00:42:22,000
receptor in a very specific fashion.
And, as a consequence, the receptor

626
00:42:22,000 --> 00:42:27,000
responds in its intracellular
or cytoplasmic domain by emitting

627
00:42:27,000 --> 00:42:31,000
signals into the cell informing the
cell that an encounter has been had

628
00:42:31,000 --> 00:42:36,000
in the extracellular space with
this growth factor which has been

629
00:42:36,000 --> 00:42:41,000
released ostensibly
by another cell.

630
00:42:41,000 --> 00:42:45,000
Now, the fact of the matter is there
are many different kinds of growth

631
00:42:45,000 --> 00:42:49,000
factors. Here I've drawn, drawn,
drawn a square one. Here's a

632
00:42:49,000 --> 00:42:53,000
triangular one.
Here's a circular one.

633
00:42:53,000 --> 00:42:57,000
And each one of these growth
factors has its own cognate receptor.

634
00:42:57,000 --> 00:43:02,000
So, here's a receptor that
binds circular growth factors.

635
00:43:02,000 --> 00:43:06,000
Here's a receptor that binds
triangular growth factors.

636
00:43:06,000 --> 00:43:10,000
Well, obviously I'm very schematic.
The point I wish to make is that

637
00:43:10,000 --> 00:43:14,000
there are multiple distinct kinds of
receptors, and each growth factor we

638
00:43:14,000 --> 00:43:19,000
can, I'll abbreviate it as a,
as a GF, each growth factor is

639
00:43:19,000 --> 00:43:23,000
what's called a ligand, or a
ligand depending on where you

640
00:43:23,000 --> 00:43:27,000
live, a ligand for the receptor.
So, in this case the square thing

641
00:43:27,000 --> 00:43:32,000
is the ligand for the receptor.
It binds to the receptor and,

642
00:43:32,000 --> 00:43:36,000
in so doing, it provokes the
receptor to emit a signal which his

643
00:43:36,000 --> 00:43:40,000
transferred in through the plasma
membrane into the cell cytoplasm.

644
00:43:40,000 --> 00:43:44,000
Clearly, what I also wish to
indicate by this is that growth

645
00:43:44,000 --> 00:43:49,000
factors don't bind to inappropriate
receptors. They only bind to their

646
00:43:49,000 --> 00:43:53,000
appropriate receptors or
their cognate receptors.

647
00:43:53,000 --> 00:43:57,000
So, for example, there's a
growth factor that we'll talk

648
00:43:57,000 --> 00:44:00,000
about shortly. It's
called epidermal

649
00:44:00,000 --> 00:44:04,000
growth factor.

650
00:44:04,000 --> 00:44:07,000
And I'll just abbreviate
GF for growth factor.

651
00:44:07,000 --> 00:44:10,000
There's another growth factor
that called platelet-derived

652
00:44:10,000 --> 00:44:16,000
growth factor.

653
00:44:16,000 --> 00:44:20,000
So, we have EGF, we have
PDGF, platelet-derived

654
00:44:20,000 --> 00:44:24,000
growth factor. And, in
fact, there's actually 30

655
00:44:24,000 --> 00:44:28,000
or 40 other kinds of growth factors.
And each one binds specifically to

656
00:44:28,000 --> 00:44:32,000
its own receptors. And so
now we begin to understand

657
00:44:32,000 --> 00:44:36,000
that the signaling channels
are actually very multiplexed.

658
00:44:36,000 --> 00:44:40,000
It's not just at one, one kind of
growth factor can travel through

659
00:44:40,000 --> 00:44:44,000
intercellular space. And,
again, this cell will only

660
00:44:44,000 --> 00:44:48,000
receive, will only make a decision
to grow and divide if its growth

661
00:44:48,000 --> 00:44:52,000
factor receptors have send an
adequate number of stimulatory

662
00:44:52,000 --> 00:44:56,000
signals into the cell which then
pursued the signaling circuitry

663
00:44:56,000 --> 00:45:00,000
which processes these signals that
it is indeed time to make the binary

664
00:45:00,000 --> 00:45:04,000
decision, growth versus non-growth
on the basis of these signals

665
00:45:04,000 --> 00:45:09,000
received from the
extracellular space.

666
00:45:09,000 --> 00:45:13,000
From that we can already intuit
something interesting about cancer

667
00:45:13,000 --> 00:45:18,000
cells. Cancer cells make a decision
to grow even without having received

668
00:45:18,000 --> 00:45:22,000
the signal from their neighbors.
In other words, cancer cells become

669
00:45:22,000 --> 00:45:27,000
quasi or totally, they
become growth factor

670
00:45:27,000 --> 00:45:32,000
independent. That is
to say somehow they have

671
00:45:32,000 --> 00:45:37,000
usurped and perturbed this signaling
mechanism and they have no longer

672
00:45:37,000 --> 00:45:41,000
become dependent on these
extracellular signals in order to

673
00:45:41,000 --> 00:45:46,000
make the decision to grow and divide.
They will just grow in essentially

674
00:45:46,000 --> 00:45:51,000
an autonomous or independent fashion.
And we know a little bit about this

675
00:45:51,000 --> 00:45:56,000
from the following
kind of experiment.

676
00:45:56,000 --> 00:46:00,000
If you put a human connective tissue
cell, a connective tissue cell is

677
00:46:00,000 --> 00:46:05,000
called a fibroblast in culture or
even a mouse, fibroblast in culture,

678
00:46:05,000 --> 00:46:09,000
you put it in a Petri dish like
this, it'll sit on the bottom.

679
00:46:09,000 --> 00:46:14,000
You can provide this fibroblast or
fibroblasts with all the nutrients

680
00:46:14,000 --> 00:46:18,000
it needs, with glucose and
with vitamins and vital amino,

681
00:46:18,000 --> 00:46:23,000
essential amino acids, whatever
you want, give it all those low

682
00:46:23,000 --> 00:46:27,000
molecular weight compounds and the,
and the fibroblasts will sit down on

683
00:46:27,000 --> 00:46:32,000
the dish. And it will
look up at you and smile.

684
00:46:32,000 --> 00:46:37,000
And it will do so for days and
weeks. It won't do anything.

685
00:46:37,000 --> 00:46:42,000
It won't grow at all. Why?
Because you haven't given it the

686
00:46:42,000 --> 00:46:46,000
requisite signals to induce it,
to persuade it, to proliferate. And

687
00:46:46,000 --> 00:46:51,000
what are those requisite signals?
They are growth factors. So, you

688
00:46:51,000 --> 00:46:56,000
need to add growth factors in
addition to the nutrients in order

689
00:46:56,000 --> 00:47:01,000
to persuade a cell to emerge from
its quiescent state into the active

690
00:47:01,000 --> 00:47:07,000
growth and division cycle. And
so in my last minutes or my last

691
00:47:07,000 --> 00:47:13,000
minute, hopefully not on this
earth but in this lecture,

692
00:47:13,000 --> 00:47:19,000
let me just say that when you, I
mean nothing is sure, but if you

693
00:47:19,000 --> 00:47:25,000
take a cell and you deprive it,
you starve it of growth factors it

694
00:47:25,000 --> 00:47:31,000
exits from the active growth and
division cycle and it will go out

695
00:47:31,000 --> 00:47:36,000
into another phase called G zero.
And it will sit there like a bump on

696
00:47:36,000 --> 00:47:40,000
a log for an extended period of
time. It will be metabolically active,

697
00:47:40,000 --> 00:47:45,000
viable, but it won't proliferate.
It will be like the cells you put in

698
00:47:45,000 --> 00:47:50,000
the Petri dish without growth
factors. And if you give it growth

699
00:47:50,000 --> 00:47:54,000
factors then the cell will go back
into the G1 phase of the cell cycle

700
00:47:54,000 --> 00:47:59,000
and begin active growth cycling.
It will pass through a successive

701
00:47:59,000 --> 00:48:04,000
cycles of growth and division as
long as you give it growth factors.

702
00:48:04,000 --> 00:48:08,000
And each time the cell emerges from
M it's going to ask the question,

703
00:48:08,000 --> 00:48:13,000
are there any growth factors around
me? If so, I might commit myself to

704
00:48:13,000 --> 00:48:17,000
doing this over again. And
if there aren't, I'm getting

705
00:48:17,000 --> 00:48:22,000
out of this racetrack and going over
to G zero where I may stay for days,

706
00:48:22,000 --> 00:48:26,000
weeks or years. On that
dramatic and tense note, see

707
00:48:26,000 --> 00:48:31,000
you on Wednesday.