Biology 122 Week8.Lecture2.Part3: Introduction to the Cell Cycle.

where we left off was just introducing the
eukaryotic cell cycle
and finding out that it is divided into
several phases
3 of which are called, are locked together
into a
grouping we call interphase which
includes a gap
in which the primary growth phase of the cell occurs
this usually is the longest phase of the
cell cycle
the S phase in which replication of DNA
takes place
S stands for synthesis so it is here
that replication of DNA takes place
and gap 2 in which the organelles are replicating
and the microtubules are beginning to
organize
into what will eventually be a mitonic
spindle that will separate the
chromatids and in mitosis
then mitosis proceeds in which the
chromosomes are partitioned to
prospective daughter cells and mitosis is
actually divided into five different
phases that we'll cover shortly
and finally cytokinesis which is the
cell division the actual cell division
which
the plasma membrane or cell walls of
your plant
are pinched off
or formed in which divides the
original parental cell into two daughter
cells and
you can calculate what the different
durations of these
phases are, for example, let's look at mitosis
if we look at the
duration of the cell cycle it takes
there's actually quite a bit of variability across
cell type in a given organism
or across different organisms
for example, in fruit fly embryos the
nuclear divisions that take place
earlier in embryogenesis only
take eight minutes
very very rapid cell division cycle
in fruit fly embryos and embryos in
general the
cell cycles tend to be shortened
somewhat during the
early stages in embryogenesis in which
masses of cells are being produced
which will form
which will later be morphed into
embryonic form
now in adults, mature cells,
take longer to grow, for example a
typical adult mammalian cell
takes about 24 hours to complete a cell
cycle liver is an exception it takes
more than a year to complete the cell
cycle
growth as we pointed out occurs during
all
interphase stages no growth
takes place during that mitonic
phase
and usually that takes about an hour in
an eukaryotic cell
and when we talk about variation in length of the cell cycle were mostly looking at
the variation that takes place in
the length of the G1 phase
now when cells exit the cell cycle they stop dividing
they enter a phase which we call G0 or the resting phase
and cells can exit permanently from the
cell cycle never divide again and enter
into a permanent G0 or they can rest
G0 and resume the cell cycle start dividing
later if the appropriate signals arise
and the cell will then respond by
my reinitiating the cell cycle
so the resting phase is G0 and
that can be entered into or
left depending here's an example of
a wonderful preparation of onion root tip
which stain for DNA in
purple you can recognize, allows you to
recognize cells in different stages of
the cell cycle so
for example right here is a cell in
metaphase
mitosis here is a the cell in anaphase of mitosis
most of the cells you will see are in
interphase in which the chromatin
is diffused and not condensed into
chromosomes
as is the case with mitosis so let's take a
higher
mag view of some of those here's a metaphase
mitotic cell most of the cells again
we can see are in
interphase with the chromatin diffuse
throughout the nucleus
here's even higher mag view here's
anaphase
of mitosis here is interphase
cell interphase this cell is in
looks like late anaphase so
you can take a preparation like this or you
could take a preparation of
any organism cells grown in culture and
stained
those cells with the dye that binds to
DNA and
look at the percentage of cells that are
in different stages of the cell cycle
so for example if you were to take a
culture of human cells grown in culture with a cell cycle of about 24
hours
lets say and you found that the
percentage of cells that were in mitosis was
approximately 4 percent of
the cells well then you can determine
how long mitosis going to last
it would be .04, 4 percent
times 24 hours equals approximately
equals
one hour so by looking at the
percentage of cells that were in a given
phase of the cell cycle you can if you
know the total length the cell cycle
you can determine then the time it
takes to complete
any different given phase because on
average
then if the
mitosis for example only took
one hour out of 24
that approximately 4 percent of the
cells would be in mitosis at any
one time
like this cell that metaphase of mitosis or this cell over here
anaphase of mitosis
so that's a nice way to calculate those things
now during interphase the G1s and G2
phases
we look at what happens during
those stages previously and
0.2 emphasize here that in the
centromere region
of chromosomes which is
characterized by given DNA sequences
the kinetochore is assembled and the kinetochore is a complex
of proteins the kinetochore
is a protein complex
is a protein complex
that assembles at the centromere and it is to the kinetochore
that microtubules attach so
chromosome a replicated chromosome looks like
this
there are two kinetochores one for each chromatid
and it is there that mitotic spindles
attach… I'm sorry microtubules
which are part of the mitotic spindle
attach
so they attachment at the kinetachores
and the chromatids are held together
by cohesion
or a protein called codensin
in metazones that
hold the chromatids together
until they separate in anaphase of
mitosis
so here is a schematic at the cell cycle
and
we will talk about these
restrictions points which represent points
which the cells asking questions of
itself
it's asking as the cell proceeds through
the cell cycle
on the route to cell division during
cytokines
are things okay for example at the G1/S
checkpoint
this is called the start or the
restriction point is also asking
itself
am I ready to synthesize new DNA is my
DNA intact has been damaged in any way
if so I should stop here and not begin
synthesis of my DNA
replicating my DNA in preparation for mitosis
likewise at the G2 mitosis
checkpoint the cell
asking itself molecularly speaking of
course am I ready to enter mitosis

and condense my chromosomes and separate
them
has there again has been damage to my
DNA are is everything okay
and at the spindle checkpoint its kind of the anaphase checkpoint it is asking
have all my chromatids been attached to
microtubules in preparation for
segregation of chromatids
to opposite poles of the cell which will enable cytokinesis
material
into equal parts to two daughter cells
so this is spindle checkpoint
asks whether we're ready to separate
the chromatids
so then let's look at this
molecular mechanisms
in at least superficial detail that
control
the progression of the cell cycle
through these
various checkpoints
and let's make sure we have some
terminology down first
before we go forward and the main one you
need to know
of course is what kinase enzymes do
kinase enzymes catalyze the reaction
on taking a target protein
plus ATP and taking one of the phosphate
groups from ATP
and phosphorylating target residues on
the
target protein if it's a serine
threanine kinase then it
attacked as phosphates to serine
threanine
amino acids on the target protein if it's a
tyrosine
kinase then it attaches the phosphate to
tyrosine amino acids on the target
protein
we've covered this already in terms of
cell signaling
i think is very important in
remember the kinases
are enzymes that phosphorylate target
proteins
and so lets look at our molecular players here, one of our molecular players
you'll see our Cdks and they have
different number Cdk2 Cdk4 Cdk1
Cdk2 and these Cdks
are cyclin-dependent kinases
and as the name implies they are kinase enzymes that phosphorylate target
enzymes
but they are dependent upon another
protein called cyclin
and just as there are different
cyclin-dependent kinases there are also
different cyclins
Cyclin D Cyclin E Cyclin A Cyclin B
and these cyclins are proteins that are,
as their name implies
these proteins are synthesized
and degraded
at various points during the cell cycle
so unlike most proteins which might have
a fairly long half-life in the cell
cyclins have very short half-lives
they exist transiently throughout the
cell cycle
and we'll look at the synthesis of cyclins shortly
but just remember that they're present only in
for short times during the cell cycle
and if that's true then their cyclin
depend kinases are only
active during certain parts of the cell
cycle because after all these kinases
these cyclin-dependent kinases
are dependent on the presense of their respective cyclins
so these cyclin-dependent kinases are only active and
can only phosphorylate proteins during
certain stages of the cell cycle
when their corresponding cyclin is
present because they're dependent
upon binding the cyclin for their
activity
now another player retinoblastoma
protein
retinoblastoma protein is we'll see is a
key regulator
of whether or not the cell will progress
to the S phase
from the G1 phase and it is
therefore a regulator at this restriction
point
the G1 S restriction point which
determines whether the cell will
continue through the cell cycle
excuse me, another player is the
e2f transcription factor
which turns on genes that are needed for
progression through the cell cycle and
cell division so
think I've active E2F as being a
transcription factor that will
cause the expression of genes the
transcription of genes that will drive
the
cell through the cell cycle and his cell
division genes
and E2F can be either an active
free form
in which it can activate genes or it can be bound to the retinoblastoma
protein
so this is where we're going to pick up
with in the
next part this lecture but keep that key
players in mind here
cyclin-dependent kinases cyclins
retinoblastoma
and E2F and we'll get into p21
a little bit
in the next section as well

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